At update of non-LP_NORMAL TID, fail instead of corrupting page header.

[pgsql.git] / contrib / postgres_fdw / connection.c

/*-------------------------------------------------------------------------
 *
 * connection.c
 *                Connection management functions for postgres_fdw
 *
 * Portions Copyright (c) 2012-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *                contrib/postgres_fdw/connection.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#if HAVE_POLL_H
#include <poll.h>
#endif

#include "access/xact.h"
#include "catalog/pg_user_mapping.h"
#include "commands/defrem.h"
#include "common/base64.h"
#include "funcapi.h"
#include "libpq/libpq-be.h"
#include "libpq/libpq-be-fe-helpers.h"
#include "mb/pg_wchar.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postgres_fdw.h"
#include "storage/latch.h"
#include "utils/builtins.h"
#include "utils/hsearch.h"
#include "utils/inval.h"
#include "utils/syscache.h"

/*
 * Connection cache hash table entry
 *
 * The lookup key in this hash table is the user mapping OID. We use just one
 * connection per user mapping ID, which ensures that all the scans use the
 * same snapshot during a query.  Using the user mapping OID rather than
 * the foreign server OID + user OID avoids creating multiple connections when
 * the public user mapping applies to all user OIDs.
 *
 * The "conn" pointer can be NULL if we don't currently have a live connection.
 * When we do have a connection, xact_depth tracks the current depth of
 * transactions and subtransactions open on the remote side.  We need to issue
 * commands at the same nesting depth on the remote as we're executing at
 * ourselves, so that rolling back a subtransaction will kill the right
 * queries and not the wrong ones.
 */
typedef Oid ConnCacheKey;

typedef struct ConnCacheEntry
{
        ConnCacheKey key;                       /* hash key (must be first) */
        PGconn     *conn;                       /* connection to foreign server, or NULL */
        /* Remaining fields are invalid when conn is NULL: */
        int                     xact_depth;             /* 0 = no xact open, 1 = main xact open, 2 =
                                                                 * one level of subxact open, etc */
        bool            have_prep_stmt; /* have we prepared any stmts in this xact? */
        bool            have_error;             /* have any subxacts aborted in this xact? */
        bool            changing_xact_state;    /* xact state change in process */
        bool            parallel_commit;        /* do we commit (sub)xacts in parallel? */
        bool            parallel_abort; /* do we abort (sub)xacts in parallel? */
        bool            invalidated;    /* true if reconnect is pending */
        bool            keep_connections;       /* setting value of keep_connections
                                                                         * server option */
        Oid                     serverid;               /* foreign server OID used to get server name */
        uint32          server_hashvalue;       /* hash value of foreign server OID */
        uint32          mapping_hashvalue;      /* hash value of user mapping OID */
        PgFdwConnState state;           /* extra per-connection state */
} ConnCacheEntry;

/*
 * Connection cache (initialized on first use)
 */
static HTAB *ConnectionHash = NULL;

/* for assigning cursor numbers and prepared statement numbers */
static unsigned int cursor_number = 0;
static unsigned int prep_stmt_number = 0;

/* tracks whether any work is needed in callback functions */
static bool xact_got_connection = false;

/* custom wait event values, retrieved from shared memory */
static uint32 pgfdw_we_cleanup_result = 0;
static uint32 pgfdw_we_connect = 0;
static uint32 pgfdw_we_get_result = 0;

/*
 * Milliseconds to wait to cancel an in-progress query or execute a cleanup
 * query; if it takes longer than 30 seconds to do these, we assume the
 * connection is dead.
 */
#define CONNECTION_CLEANUP_TIMEOUT      30000

/*
 * Milliseconds to wait before issuing another cancel request.  This covers
 * the race condition where the remote session ignored our cancel request
 * because it arrived while idle.
 */
#define RETRY_CANCEL_TIMEOUT    1000

/* Macro for constructing abort command to be sent */
#define CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel) \
        do { \
                if (toplevel) \
                        snprintf((sql), sizeof(sql), \
                                         "ABORT TRANSACTION"); \
                else \
                        snprintf((sql), sizeof(sql), \
                                         "ROLLBACK TO SAVEPOINT s%d; RELEASE SAVEPOINT s%d", \
                                         (entry)->xact_depth, (entry)->xact_depth); \
        } while(0)

/*
 * Extension version number, for supporting older extension versions' objects
 */
enum pgfdwVersion
{
        PGFDW_V1_1 = 0,
        PGFDW_V1_2,
};

/*
 * SQL functions
 */
PG_FUNCTION_INFO_V1(postgres_fdw_get_connections);
PG_FUNCTION_INFO_V1(postgres_fdw_get_connections_1_2);
PG_FUNCTION_INFO_V1(postgres_fdw_disconnect);
PG_FUNCTION_INFO_V1(postgres_fdw_disconnect_all);

/* prototypes of private functions */
static void make_new_connection(ConnCacheEntry *entry, UserMapping *user);
static PGconn *connect_pg_server(ForeignServer *server, UserMapping *user);
static void disconnect_pg_server(ConnCacheEntry *entry);
static void check_conn_params(const char **keywords, const char **values, UserMapping *user);
static void configure_remote_session(PGconn *conn);
static void do_sql_command_begin(PGconn *conn, const char *sql);
static void do_sql_command_end(PGconn *conn, const char *sql,
                                                           bool consume_input);
static void begin_remote_xact(ConnCacheEntry *entry);
static void pgfdw_xact_callback(XactEvent event, void *arg);
static void pgfdw_subxact_callback(SubXactEvent event,
                                                                   SubTransactionId mySubid,
                                                                   SubTransactionId parentSubid,
                                                                   void *arg);
static void pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue);
static void pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry);
static void pgfdw_reset_xact_state(ConnCacheEntry *entry, bool toplevel);
static bool pgfdw_cancel_query(PGconn *conn);
static bool pgfdw_cancel_query_begin(PGconn *conn, TimestampTz endtime);
static bool pgfdw_cancel_query_end(PGconn *conn, TimestampTz endtime,
                                                                   TimestampTz retrycanceltime,
                                                                   bool consume_input);
static bool pgfdw_exec_cleanup_query(PGconn *conn, const char *query,
                                                                         bool ignore_errors);
static bool pgfdw_exec_cleanup_query_begin(PGconn *conn, const char *query);
static bool pgfdw_exec_cleanup_query_end(PGconn *conn, const char *query,
                                                                                 TimestampTz endtime,
                                                                                 bool consume_input,
                                                                                 bool ignore_errors);
static bool pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime,
                                                                         TimestampTz retrycanceltime,
                                                                         PGresult **result, bool *timed_out);
static void pgfdw_abort_cleanup(ConnCacheEntry *entry, bool toplevel);
static bool pgfdw_abort_cleanup_begin(ConnCacheEntry *entry, bool toplevel,
                                                                          List **pending_entries,
                                                                          List **cancel_requested);
static void pgfdw_finish_pre_commit_cleanup(List *pending_entries);
static void pgfdw_finish_pre_subcommit_cleanup(List *pending_entries,
                                                                                           int curlevel);
static void pgfdw_finish_abort_cleanup(List *pending_entries,
                                                                           List *cancel_requested,
                                                                           bool toplevel);
static void pgfdw_security_check(const char **keywords, const char **values,
                                                                 UserMapping *user, PGconn *conn);
static bool UserMappingPasswordRequired(UserMapping *user);
static bool UseScramPassthrough(ForeignServer *server, UserMapping *user);
static bool disconnect_cached_connections(Oid serverid);
static void postgres_fdw_get_connections_internal(FunctionCallInfo fcinfo,
                                                                                                  enum pgfdwVersion api_version);
static int      pgfdw_conn_check(PGconn *conn);
static bool pgfdw_conn_checkable(void);

/*
 * Get a PGconn which can be used to execute queries on the remote PostgreSQL
 * server with the user's authorization.  A new connection is established
 * if we don't already have a suitable one, and a transaction is opened at
 * the right subtransaction nesting depth if we didn't do that already.
 *
 * will_prep_stmt must be true if caller intends to create any prepared
 * statements.  Since those don't go away automatically at transaction end
 * (not even on error), we need this flag to cue manual cleanup.
 *
 * If state is not NULL, *state receives the per-connection state associated
 * with the PGconn.
 */
PGconn *
GetConnection(UserMapping *user, bool will_prep_stmt, PgFdwConnState **state)
{
        bool            found;
        bool            retry = false;
        ConnCacheEntry *entry;
        ConnCacheKey key;
        MemoryContext ccxt = CurrentMemoryContext;

        /* First time through, initialize connection cache hashtable */
        if (ConnectionHash == NULL)
        {
                HASHCTL         ctl;

                if (pgfdw_we_get_result == 0)
                        pgfdw_we_get_result =
                                WaitEventExtensionNew("PostgresFdwGetResult");

                ctl.keysize = sizeof(ConnCacheKey);
                ctl.entrysize = sizeof(ConnCacheEntry);
                ConnectionHash = hash_create("postgres_fdw connections", 8,
                                                                         &ctl,
                                                                         HASH_ELEM | HASH_BLOBS);

                /*
                 * Register some callback functions that manage connection cleanup.
                 * This should be done just once in each backend.
                 */
                RegisterXactCallback(pgfdw_xact_callback, NULL);
                RegisterSubXactCallback(pgfdw_subxact_callback, NULL);
                CacheRegisterSyscacheCallback(FOREIGNSERVEROID,
                                                                          pgfdw_inval_callback, (Datum) 0);
                CacheRegisterSyscacheCallback(USERMAPPINGOID,
                                                                          pgfdw_inval_callback, (Datum) 0);
        }

        /* Set flag that we did GetConnection during the current transaction */
        xact_got_connection = true;

        /* Create hash key for the entry.  Assume no pad bytes in key struct */
        key = user->umid;

        /*
         * Find or create cached entry for requested connection.
         */
        entry = hash_search(ConnectionHash, &key, HASH_ENTER, &found);
        if (!found)
        {
                /*
                 * We need only clear "conn" here; remaining fields will be filled
                 * later when "conn" is set.
                 */
                entry->conn = NULL;
        }

        /* Reject further use of connections which failed abort cleanup. */
        pgfdw_reject_incomplete_xact_state_change(entry);

        /*
         * If the connection needs to be remade due to invalidation, disconnect as
         * soon as we're out of all transactions.
         */
        if (entry->conn != NULL && entry->invalidated && entry->xact_depth == 0)
        {
                elog(DEBUG3, "closing connection %p for option changes to take effect",
                         entry->conn);
                disconnect_pg_server(entry);
        }

        /*
         * If cache entry doesn't have a connection, we have to establish a new
         * connection.  (If connect_pg_server throws an error, the cache entry
         * will remain in a valid empty state, ie conn == NULL.)
         */
        if (entry->conn == NULL)
                make_new_connection(entry, user);

        /*
         * We check the health of the cached connection here when using it.  In
         * cases where we're out of all transactions, if a broken connection is
         * detected, we try to reestablish a new connection later.
         */
        PG_TRY();
        {
                /* Process a pending asynchronous request if any. */
                if (entry->state.pendingAreq)
                        process_pending_request(entry->state.pendingAreq);
                /* Start a new transaction or subtransaction if needed. */
                begin_remote_xact(entry);
        }
        PG_CATCH();
        {
                MemoryContext ecxt = MemoryContextSwitchTo(ccxt);
                ErrorData  *errdata = CopyErrorData();

                /*
                 * Determine whether to try to reestablish the connection.
                 *
                 * After a broken connection is detected in libpq, any error other
                 * than connection failure (e.g., out-of-memory) can be thrown
                 * somewhere between return from libpq and the expected ereport() call
                 * in pgfdw_report_error(). In this case, since PQstatus() indicates
                 * CONNECTION_BAD, checking only PQstatus() causes the false detection
                 * of connection failure. To avoid this, we also verify that the
                 * error's sqlstate is ERRCODE_CONNECTION_FAILURE. Note that also
                 * checking only the sqlstate can cause another false detection
                 * because pgfdw_report_error() may report ERRCODE_CONNECTION_FAILURE
                 * for any libpq-originated error condition.
                 */
                if (errdata->sqlerrcode != ERRCODE_CONNECTION_FAILURE ||
                        PQstatus(entry->conn) != CONNECTION_BAD ||
                        entry->xact_depth > 0)
                {
                        MemoryContextSwitchTo(ecxt);
                        PG_RE_THROW();
                }

                /* Clean up the error state */
                FlushErrorState();
                FreeErrorData(errdata);
                errdata = NULL;

                retry = true;
        }
        PG_END_TRY();

        /*
         * If a broken connection is detected, disconnect it, reestablish a new
         * connection and retry a new remote transaction. If connection failure is
         * reported again, we give up getting a connection.
         */
        if (retry)
        {
                Assert(entry->xact_depth == 0);

                ereport(DEBUG3,
                                (errmsg_internal("could not start remote transaction on connection %p",
                                                                 entry->conn)),
                                errdetail_internal("%s", pchomp(PQerrorMessage(entry->conn))));

                elog(DEBUG3, "closing connection %p to reestablish a new one",
                         entry->conn);
                disconnect_pg_server(entry);

                make_new_connection(entry, user);

                begin_remote_xact(entry);
        }

        /* Remember if caller will prepare statements */
        entry->have_prep_stmt |= will_prep_stmt;

        /* If caller needs access to the per-connection state, return it. */
        if (state)
                *state = &entry->state;

        return entry->conn;
}

/*
 * Reset all transient state fields in the cached connection entry and
 * establish new connection to the remote server.
 */
static void
make_new_connection(ConnCacheEntry *entry, UserMapping *user)
{
        ForeignServer *server = GetForeignServer(user->serverid);
        ListCell   *lc;

        Assert(entry->conn == NULL);

        /* Reset all transient state fields, to be sure all are clean */
        entry->xact_depth = 0;
        entry->have_prep_stmt = false;
        entry->have_error = false;
        entry->changing_xact_state = false;
        entry->invalidated = false;
        entry->serverid = server->serverid;
        entry->server_hashvalue =
                GetSysCacheHashValue1(FOREIGNSERVEROID,
                                                          ObjectIdGetDatum(server->serverid));
        entry->mapping_hashvalue =
                GetSysCacheHashValue1(USERMAPPINGOID,
                                                          ObjectIdGetDatum(user->umid));
        memset(&entry->state, 0, sizeof(entry->state));

        /*
         * Determine whether to keep the connection that we're about to make here
         * open even after the transaction using it ends, so that the subsequent
         * transactions can re-use it.
         *
         * By default, all the connections to any foreign servers are kept open.
         *
         * Also determine whether to commit/abort (sub)transactions opened on the
         * remote server in parallel at (sub)transaction end, which is disabled by
         * default.
         *
         * Note: it's enough to determine these only when making a new connection
         * because if these settings for it are changed, it will be closed and
         * re-made later.
         */
        entry->keep_connections = true;
        entry->parallel_commit = false;
        entry->parallel_abort = false;
        foreach(lc, server->options)
        {
                DefElem    *def = (DefElem *) lfirst(lc);

                if (strcmp(def->defname, "keep_connections") == 0)
                        entry->keep_connections = defGetBoolean(def);
                else if (strcmp(def->defname, "parallel_commit") == 0)
                        entry->parallel_commit = defGetBoolean(def);
                else if (strcmp(def->defname, "parallel_abort") == 0)
                        entry->parallel_abort = defGetBoolean(def);
        }

        /* Now try to make the connection */
        entry->conn = connect_pg_server(server, user);

        elog(DEBUG3, "new postgres_fdw connection %p for server \"%s\" (user mapping oid %u, userid %u)",
                 entry->conn, server->servername, user->umid, user->userid);
}

/*
 * Check that non-superuser has used password or delegated credentials
 * to establish connection; otherwise, he's piggybacking on the
 * postgres server's user identity. See also dblink_security_check()
 * in contrib/dblink and check_conn_params.
 */
static void
pgfdw_security_check(const char **keywords, const char **values, UserMapping *user, PGconn *conn)
{
        /* Superusers bypass the check */
        if (superuser_arg(user->userid))
                return;

#ifdef ENABLE_GSS
        /* Connected via GSSAPI with delegated credentials- all good. */
        if (PQconnectionUsedGSSAPI(conn) && be_gssapi_get_delegation(MyProcPort))
                return;
#endif

        /* Ok if superuser set PW required false. */
        if (!UserMappingPasswordRequired(user))
                return;

        /* Connected via PW, with PW required true, and provided non-empty PW. */
        if (PQconnectionUsedPassword(conn))
        {
                /* ok if params contain a non-empty password */
                for (int i = 0; keywords[i] != NULL; i++)
                {
                        if (strcmp(keywords[i], "password") == 0 && values[i][0] != '\0')
                                return;
                }
        }

        ereport(ERROR,
                        (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
                         errmsg("password or GSSAPI delegated credentials required"),
                         errdetail("Non-superuser cannot connect if the server does not request a password or use GSSAPI with delegated credentials."),
                         errhint("Target server's authentication method must be changed or password_required=false set in the user mapping attributes.")));
}

/*
 * Connect to remote server using specified server and user mapping properties.
 */
static PGconn *
connect_pg_server(ForeignServer *server, UserMapping *user)
{
        PGconn     *volatile conn = NULL;

        /*
         * Use PG_TRY block to ensure closing connection on error.
         */
        PG_TRY();
        {
                const char **keywords;
                const char **values;
                char       *appname = NULL;
                int                     n;

                /*
                 * Construct connection params from generic options of ForeignServer
                 * and UserMapping.  (Some of them might not be libpq options, in
                 * which case we'll just waste a few array slots.)  Add 4 extra slots
                 * for application_name, fallback_application_name, client_encoding,
                 * end marker.
                 */
                n = list_length(server->options) + list_length(user->options) + 4 + 2;
                keywords = (const char **) palloc(n * sizeof(char *));
                values = (const char **) palloc(n * sizeof(char *));

                n = 0;
                n += ExtractConnectionOptions(server->options,
                                                                          keywords + n, values + n);
                n += ExtractConnectionOptions(user->options,
                                                                          keywords + n, values + n);

                /*
                 * Use pgfdw_application_name as application_name if set.
                 *
                 * PQconnectdbParams() processes the parameter arrays from start to
                 * end. If any key word is repeated, the last value is used. Therefore
                 * note that pgfdw_application_name must be added to the arrays after
                 * options of ForeignServer are, so that it can override
                 * application_name set in ForeignServer.
                 */
                if (pgfdw_application_name && *pgfdw_application_name != '\0')
                {
                        keywords[n] = "application_name";
                        values[n] = pgfdw_application_name;
                        n++;
                }

                /*
                 * Search the parameter arrays to find application_name setting, and
                 * replace escape sequences in it with status information if found.
                 * The arrays are searched backwards because the last value is used if
                 * application_name is repeatedly set.
                 */
                for (int i = n - 1; i >= 0; i--)
                {
                        if (strcmp(keywords[i], "application_name") == 0 &&
                                *(values[i]) != '\0')
                        {
                                /*
                                 * Use this application_name setting if it's not empty string
                                 * even after any escape sequences in it are replaced.
                                 */
                                appname = process_pgfdw_appname(values[i]);
                                if (appname[0] != '\0')
                                {
                                        values[i] = appname;
                                        break;
                                }

                                /*
                                 * This empty application_name is not used, so we set
                                 * values[i] to NULL and keep searching the array to find the
                                 * next one.
                                 */
                                values[i] = NULL;
                                pfree(appname);
                                appname = NULL;
                        }
                }

                /* Use "postgres_fdw" as fallback_application_name */
                keywords[n] = "fallback_application_name";
                values[n] = "postgres_fdw";
                n++;

                /* Set client_encoding so that libpq can convert encoding properly. */
                keywords[n] = "client_encoding";
                values[n] = GetDatabaseEncodingName();
                n++;

                if (MyProcPort->has_scram_keys && UseScramPassthrough(server, user))
                {
                        int                     len;
                        int                     encoded_len;

                        keywords[n] = "scram_client_key";
                        len = pg_b64_enc_len(sizeof(MyProcPort->scram_ClientKey));
                        /* don't forget the zero-terminator */
                        values[n] = palloc0(len + 1);
                        encoded_len = pg_b64_encode((const char *) MyProcPort->scram_ClientKey,
                                                                                sizeof(MyProcPort->scram_ClientKey),
                                                                                (char *) values[n], len);
                        if (encoded_len < 0)
                                elog(ERROR, "could not encode SCRAM client key");
                        n++;

                        keywords[n] = "scram_server_key";
                        len = pg_b64_enc_len(sizeof(MyProcPort->scram_ServerKey));
                        /* don't forget the zero-terminator */
                        values[n] = palloc0(len + 1);
                        encoded_len = pg_b64_encode((const char *) MyProcPort->scram_ServerKey,
                                                                                sizeof(MyProcPort->scram_ServerKey),
                                                                                (char *) values[n], len);
                        if (encoded_len < 0)
                                elog(ERROR, "could not encode SCRAM server key");
                        n++;
                }

                keywords[n] = values[n] = NULL;

                /*
                 * Verify the set of connection parameters only if scram pass-through
                 * is not being used because the password is not necessary.
                 */
                if (!(MyProcPort->has_scram_keys && UseScramPassthrough(server, user)))
                        check_conn_params(keywords, values, user);

                /* first time, allocate or get the custom wait event */
                if (pgfdw_we_connect == 0)
                        pgfdw_we_connect = WaitEventExtensionNew("PostgresFdwConnect");

                /* OK to make connection */
                conn = libpqsrv_connect_params(keywords, values,
                                                                           false,       /* expand_dbname */
                                                                           pgfdw_we_connect);

                if (!conn || PQstatus(conn) != CONNECTION_OK)
                        ereport(ERROR,
                                        (errcode(ERRCODE_SQLCLIENT_UNABLE_TO_ESTABLISH_SQLCONNECTION),
                                         errmsg("could not connect to server \"%s\"",
                                                        server->servername),
                                         errdetail_internal("%s", pchomp(PQerrorMessage(conn)))));

                /*
                 * Perform post-connection security checks only if scram pass-through
                 * is not being used because the password is not necessary.
                 */
                if (!(MyProcPort->has_scram_keys && UseScramPassthrough(server, user)))
                        pgfdw_security_check(keywords, values, user, conn);

                /* Prepare new session for use */
                configure_remote_session(conn);

                if (appname != NULL)
                        pfree(appname);
                pfree(keywords);
                pfree(values);
        }
        PG_CATCH();
        {
                libpqsrv_disconnect(conn);
                PG_RE_THROW();
        }
        PG_END_TRY();

        return conn;
}

/*
 * Disconnect any open connection for a connection cache entry.
 */
static void
disconnect_pg_server(ConnCacheEntry *entry)
{
        if (entry->conn != NULL)
        {
                libpqsrv_disconnect(entry->conn);
                entry->conn = NULL;
        }
}

/*
 * Return true if the password_required is defined and false for this user
 * mapping, otherwise false. The mapping has been pre-validated.
 */
static bool
UserMappingPasswordRequired(UserMapping *user)
{
        ListCell   *cell;

        foreach(cell, user->options)
        {
                DefElem    *def = (DefElem *) lfirst(cell);

                if (strcmp(def->defname, "password_required") == 0)
                        return defGetBoolean(def);
        }

        return true;
}

static bool
UseScramPassthrough(ForeignServer *server, UserMapping *user)
{
        ListCell   *cell;

        foreach(cell, server->options)
        {
                DefElem    *def = (DefElem *) lfirst(cell);

                if (strcmp(def->defname, "use_scram_passthrough") == 0)
                        return defGetBoolean(def);
        }

        foreach(cell, user->options)
        {
                DefElem    *def = (DefElem *) lfirst(cell);

                if (strcmp(def->defname, "use_scram_passthrough") == 0)
                        return defGetBoolean(def);
        }

        return false;
}

/*
 * For non-superusers, insist that the connstr specify a password or that the
 * user provided their own GSSAPI delegated credentials.  This
 * prevents a password from being picked up from .pgpass, a service file, the
 * environment, etc.  We don't want the postgres user's passwords,
 * certificates, etc to be accessible to non-superusers.  (See also
 * dblink_connstr_check in contrib/dblink.)
 */
static void
check_conn_params(const char **keywords, const char **values, UserMapping *user)
{
        int                     i;

        /* no check required if superuser */
        if (superuser_arg(user->userid))
                return;

#ifdef ENABLE_GSS
        /* ok if the user provided their own delegated credentials */
        if (be_gssapi_get_delegation(MyProcPort))
                return;
#endif

        /* ok if params contain a non-empty password */
        for (i = 0; keywords[i] != NULL; i++)
        {
                if (strcmp(keywords[i], "password") == 0 && values[i][0] != '\0')
                        return;
        }

        /* ok if the superuser explicitly said so at user mapping creation time */
        if (!UserMappingPasswordRequired(user))
                return;

        ereport(ERROR,
                        (errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
                         errmsg("password or GSSAPI delegated credentials required"),
                         errdetail("Non-superusers must delegate GSSAPI credentials, provide a password, or enable SCRAM pass-through in user mapping.")));
}

/*
 * Issue SET commands to make sure remote session is configured properly.
 *
 * We do this just once at connection, assuming nothing will change the
 * values later.  Since we'll never send volatile function calls to the
 * remote, there shouldn't be any way to break this assumption from our end.
 * It's possible to think of ways to break it at the remote end, eg making
 * a foreign table point to a view that includes a set_config call ---
 * but once you admit the possibility of a malicious view definition,
 * there are any number of ways to break things.
 */
static void
configure_remote_session(PGconn *conn)
{
        int                     remoteversion = PQserverVersion(conn);

        /* Force the search path to contain only pg_catalog (see deparse.c) */
        do_sql_command(conn, "SET search_path = pg_catalog");

        /*
         * Set remote timezone; this is basically just cosmetic, since all
         * transmitted and returned timestamptzs should specify a zone explicitly
         * anyway.  However it makes the regression test outputs more predictable.
         *
         * We don't risk setting remote zone equal to ours, since the remote
         * server might use a different timezone database.  Instead, use GMT
         * (quoted, because very old servers are picky about case).  That's
         * guaranteed to work regardless of the remote's timezone database,
         * because pg_tzset() hard-wires it (at least in PG 9.2 and later).
         */
        do_sql_command(conn, "SET timezone = 'GMT'");

        /*
         * Set values needed to ensure unambiguous data output from remote.  (This
         * logic should match what pg_dump does.  See also set_transmission_modes
         * in postgres_fdw.c.)
         */
        do_sql_command(conn, "SET datestyle = ISO");
        if (remoteversion >= 80400)
                do_sql_command(conn, "SET intervalstyle = postgres");
        if (remoteversion >= 90000)
                do_sql_command(conn, "SET extra_float_digits = 3");
        else
                do_sql_command(conn, "SET extra_float_digits = 2");
}

/*
 * Convenience subroutine to issue a non-data-returning SQL command to remote
 */
void
do_sql_command(PGconn *conn, const char *sql)
{
        do_sql_command_begin(conn, sql);
        do_sql_command_end(conn, sql, false);
}

static void
do_sql_command_begin(PGconn *conn, const char *sql)
{
        if (!PQsendQuery(conn, sql))
                pgfdw_report_error(ERROR, NULL, conn, false, sql);
}

static void
do_sql_command_end(PGconn *conn, const char *sql, bool consume_input)
{
        PGresult   *res;

        /*
         * If requested, consume whatever data is available from the socket. (Note
         * that if all data is available, this allows pgfdw_get_result to call
         * PQgetResult without forcing the overhead of WaitLatchOrSocket, which
         * would be large compared to the overhead of PQconsumeInput.)
         */
        if (consume_input && !PQconsumeInput(conn))
                pgfdw_report_error(ERROR, NULL, conn, false, sql);
        res = pgfdw_get_result(conn);
        if (PQresultStatus(res) != PGRES_COMMAND_OK)
                pgfdw_report_error(ERROR, res, conn, true, sql);
        PQclear(res);
}

/*
 * Start remote transaction or subtransaction, if needed.
 *
 * Note that we always use at least REPEATABLE READ in the remote session.
 * This is so that, if a query initiates multiple scans of the same or
 * different foreign tables, we will get snapshot-consistent results from
 * those scans.  A disadvantage is that we can't provide sane emulation of
 * READ COMMITTED behavior --- it would be nice if we had some other way to
 * control which remote queries share a snapshot.
 */
static void
begin_remote_xact(ConnCacheEntry *entry)
{
        int                     curlevel = GetCurrentTransactionNestLevel();

        /* Start main transaction if we haven't yet */
        if (entry->xact_depth <= 0)
        {
                const char *sql;

                elog(DEBUG3, "starting remote transaction on connection %p",
                         entry->conn);

                if (IsolationIsSerializable())
                        sql = "START TRANSACTION ISOLATION LEVEL SERIALIZABLE";
                else
                        sql = "START TRANSACTION ISOLATION LEVEL REPEATABLE READ";
                entry->changing_xact_state = true;
                do_sql_command(entry->conn, sql);
                entry->xact_depth = 1;
                entry->changing_xact_state = false;
        }

        /*
         * If we're in a subtransaction, stack up savepoints to match our level.
         * This ensures we can rollback just the desired effects when a
         * subtransaction aborts.
         */
        while (entry->xact_depth < curlevel)
        {
                char            sql[64];

                snprintf(sql, sizeof(sql), "SAVEPOINT s%d", entry->xact_depth + 1);
                entry->changing_xact_state = true;
                do_sql_command(entry->conn, sql);
                entry->xact_depth++;
                entry->changing_xact_state = false;
        }
}

/*
 * Release connection reference count created by calling GetConnection.
 */
void
ReleaseConnection(PGconn *conn)
{
        /*
         * Currently, we don't actually track connection references because all
         * cleanup is managed on a transaction or subtransaction basis instead. So
         * there's nothing to do here.
         */
}

/*
 * Assign a "unique" number for a cursor.
 *
 * These really only need to be unique per connection within a transaction.
 * For the moment we ignore the per-connection point and assign them across
 * all connections in the transaction, but we ask for the connection to be
 * supplied in case we want to refine that.
 *
 * Note that even if wraparound happens in a very long transaction, actual
 * collisions are highly improbable; just be sure to use %u not %d to print.
 */
unsigned int
GetCursorNumber(PGconn *conn)
{
        return ++cursor_number;
}

/*
 * Assign a "unique" number for a prepared statement.
 *
 * This works much like GetCursorNumber, except that we never reset the counter
 * within a session.  That's because we can't be 100% sure we've gotten rid
 * of all prepared statements on all connections, and it's not really worth
 * increasing the risk of prepared-statement name collisions by resetting.
 */
unsigned int
GetPrepStmtNumber(PGconn *conn)
{
        return ++prep_stmt_number;
}

/*
 * Submit a query and wait for the result.
 *
 * Since we don't use non-blocking mode, this can't process interrupts while
 * pushing the query text to the server.  That risk is relatively small, so we
 * ignore that for now.
 *
 * Caller is responsible for the error handling on the result.
 */
PGresult *
pgfdw_exec_query(PGconn *conn, const char *query, PgFdwConnState *state)
{
        /* First, process a pending asynchronous request, if any. */
        if (state && state->pendingAreq)
                process_pending_request(state->pendingAreq);

        if (!PQsendQuery(conn, query))
                return NULL;
        return pgfdw_get_result(conn);
}

/*
 * Wrap libpqsrv_get_result_last(), adding wait event.
 *
 * Caller is responsible for the error handling on the result.
 */
PGresult *
pgfdw_get_result(PGconn *conn)
{
        return libpqsrv_get_result_last(conn, pgfdw_we_get_result);
}

/*
 * Report an error we got from the remote server.
 *
 * elevel: error level to use (typically ERROR, but might be less)
 * res: PGresult containing the error
 * conn: connection we did the query on
 * clear: if true, PQclear the result (otherwise caller will handle it)
 * sql: NULL, or text of remote command we tried to execute
 *
 * Note: callers that choose not to throw ERROR for a remote error are
 * responsible for making sure that the associated ConnCacheEntry gets
 * marked with have_error = true.
 */
void
pgfdw_report_error(int elevel, PGresult *res, PGconn *conn,
                                   bool clear, const char *sql)
{
        /* If requested, PGresult must be released before leaving this function. */
        PG_TRY();
        {
                char       *diag_sqlstate = PQresultErrorField(res, PG_DIAG_SQLSTATE);
                char       *message_primary = PQresultErrorField(res, PG_DIAG_MESSAGE_PRIMARY);
                char       *message_detail = PQresultErrorField(res, PG_DIAG_MESSAGE_DETAIL);
                char       *message_hint = PQresultErrorField(res, PG_DIAG_MESSAGE_HINT);
                char       *message_context = PQresultErrorField(res, PG_DIAG_CONTEXT);
                int                     sqlstate;

                if (diag_sqlstate)
                        sqlstate = MAKE_SQLSTATE(diag_sqlstate[0],
                                                                         diag_sqlstate[1],
                                                                         diag_sqlstate[2],
                                                                         diag_sqlstate[3],
                                                                         diag_sqlstate[4]);
                else
                        sqlstate = ERRCODE_CONNECTION_FAILURE;

                /*
                 * If we don't get a message from the PGresult, try the PGconn.  This
                 * is needed because for connection-level failures, PQgetResult may
                 * just return NULL, not a PGresult at all.
                 */
                if (message_primary == NULL)
                        message_primary = pchomp(PQerrorMessage(conn));

                ereport(elevel,
                                (errcode(sqlstate),
                                 (message_primary != NULL && message_primary[0] != '\0') ?
                                 errmsg_internal("%s", message_primary) :
                                 errmsg("could not obtain message string for remote error"),
                                 message_detail ? errdetail_internal("%s", message_detail) : 0,
                                 message_hint ? errhint("%s", message_hint) : 0,
                                 message_context ? errcontext("%s", message_context) : 0,
                                 sql ? errcontext("remote SQL command: %s", sql) : 0));
        }
        PG_FINALLY();
        {
                if (clear)
                        PQclear(res);
        }
        PG_END_TRY();
}

/*
 * pgfdw_xact_callback --- cleanup at main-transaction end.
 *
 * This runs just late enough that it must not enter user-defined code
 * locally.  (Entering such code on the remote side is fine.  Its remote
 * COMMIT TRANSACTION may run deferred triggers.)
 */
static void
pgfdw_xact_callback(XactEvent event, void *arg)
{
        HASH_SEQ_STATUS scan;
        ConnCacheEntry *entry;
        List       *pending_entries = NIL;
        List       *cancel_requested = NIL;

        /* Quick exit if no connections were touched in this transaction. */
        if (!xact_got_connection)
                return;

        /*
         * Scan all connection cache entries to find open remote transactions, and
         * close them.
         */
        hash_seq_init(&scan, ConnectionHash);
        while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
        {
                PGresult   *res;

                /* Ignore cache entry if no open connection right now */
                if (entry->conn == NULL)
                        continue;

                /* If it has an open remote transaction, try to close it */
                if (entry->xact_depth > 0)
                {
                        elog(DEBUG3, "closing remote transaction on connection %p",
                                 entry->conn);

                        switch (event)
                        {
                                case XACT_EVENT_PARALLEL_PRE_COMMIT:
                                case XACT_EVENT_PRE_COMMIT:

                                        /*
                                         * If abort cleanup previously failed for this connection,
                                         * we can't issue any more commands against it.
                                         */
                                        pgfdw_reject_incomplete_xact_state_change(entry);

                                        /* Commit all remote transactions during pre-commit */
                                        entry->changing_xact_state = true;
                                        if (entry->parallel_commit)
                                        {
                                                do_sql_command_begin(entry->conn, "COMMIT TRANSACTION");
                                                pending_entries = lappend(pending_entries, entry);
                                                continue;
                                        }
                                        do_sql_command(entry->conn, "COMMIT TRANSACTION");
                                        entry->changing_xact_state = false;

                                        /*
                                         * If there were any errors in subtransactions, and we
                                         * made prepared statements, do a DEALLOCATE ALL to make
                                         * sure we get rid of all prepared statements. This is
                                         * annoying and not terribly bulletproof, but it's
                                         * probably not worth trying harder.
                                         *
                                         * DEALLOCATE ALL only exists in 8.3 and later, so this
                                         * constrains how old a server postgres_fdw can
                                         * communicate with.  We intentionally ignore errors in
                                         * the DEALLOCATE, so that we can hobble along to some
                                         * extent with older servers (leaking prepared statements
                                         * as we go; but we don't really support update operations
                                         * pre-8.3 anyway).
                                         */
                                        if (entry->have_prep_stmt && entry->have_error)
                                        {
                                                res = pgfdw_exec_query(entry->conn, "DEALLOCATE ALL",
                                                                                           NULL);
                                                PQclear(res);
                                        }
                                        entry->have_prep_stmt = false;
                                        entry->have_error = false;
                                        break;
                                case XACT_EVENT_PRE_PREPARE:

                                        /*
                                         * We disallow any remote transactions, since it's not
                                         * very reasonable to hold them open until the prepared
                                         * transaction is committed.  For the moment, throw error
                                         * unconditionally; later we might allow read-only cases.
                                         * Note that the error will cause us to come right back
                                         * here with event == XACT_EVENT_ABORT, so we'll clean up
                                         * the connection state at that point.
                                         */
                                        ereport(ERROR,
                                                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                                                         errmsg("cannot PREPARE a transaction that has operated on postgres_fdw foreign tables")));
                                        break;
                                case XACT_EVENT_PARALLEL_COMMIT:
                                case XACT_EVENT_COMMIT:
                                case XACT_EVENT_PREPARE:
                                        /* Pre-commit should have closed the open transaction */
                                        elog(ERROR, "missed cleaning up connection during pre-commit");
                                        break;
                                case XACT_EVENT_PARALLEL_ABORT:
                                case XACT_EVENT_ABORT:
                                        /* Rollback all remote transactions during abort */
                                        if (entry->parallel_abort)
                                        {
                                                if (pgfdw_abort_cleanup_begin(entry, true,
                                                                                                          &pending_entries,
                                                                                                          &cancel_requested))
                                                        continue;
                                        }
                                        else
                                                pgfdw_abort_cleanup(entry, true);
                                        break;
                        }
                }

                /* Reset state to show we're out of a transaction */
                pgfdw_reset_xact_state(entry, true);
        }

        /* If there are any pending connections, finish cleaning them up */
        if (pending_entries || cancel_requested)
        {
                if (event == XACT_EVENT_PARALLEL_PRE_COMMIT ||
                        event == XACT_EVENT_PRE_COMMIT)
                {
                        Assert(cancel_requested == NIL);
                        pgfdw_finish_pre_commit_cleanup(pending_entries);
                }
                else
                {
                        Assert(event == XACT_EVENT_PARALLEL_ABORT ||
                                   event == XACT_EVENT_ABORT);
                        pgfdw_finish_abort_cleanup(pending_entries, cancel_requested,
                                                                           true);
                }
        }

        /*
         * Regardless of the event type, we can now mark ourselves as out of the
         * transaction.  (Note: if we are here during PRE_COMMIT or PRE_PREPARE,
         * this saves a useless scan of the hashtable during COMMIT or PREPARE.)
         */
        xact_got_connection = false;

        /* Also reset cursor numbering for next transaction */
        cursor_number = 0;
}

/*
 * pgfdw_subxact_callback --- cleanup at subtransaction end.
 */
static void
pgfdw_subxact_callback(SubXactEvent event, SubTransactionId mySubid,
                                           SubTransactionId parentSubid, void *arg)
{
        HASH_SEQ_STATUS scan;
        ConnCacheEntry *entry;
        int                     curlevel;
        List       *pending_entries = NIL;
        List       *cancel_requested = NIL;

        /* Nothing to do at subxact start, nor after commit. */
        if (!(event == SUBXACT_EVENT_PRE_COMMIT_SUB ||
                  event == SUBXACT_EVENT_ABORT_SUB))
                return;

        /* Quick exit if no connections were touched in this transaction. */
        if (!xact_got_connection)
                return;

        /*
         * Scan all connection cache entries to find open remote subtransactions
         * of the current level, and close them.
         */
        curlevel = GetCurrentTransactionNestLevel();
        hash_seq_init(&scan, ConnectionHash);
        while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
        {
                char            sql[100];

                /*
                 * We only care about connections with open remote subtransactions of
                 * the current level.
                 */
                if (entry->conn == NULL || entry->xact_depth < curlevel)
                        continue;

                if (entry->xact_depth > curlevel)
                        elog(ERROR, "missed cleaning up remote subtransaction at level %d",
                                 entry->xact_depth);

                if (event == SUBXACT_EVENT_PRE_COMMIT_SUB)
                {
                        /*
                         * If abort cleanup previously failed for this connection, we
                         * can't issue any more commands against it.
                         */
                        pgfdw_reject_incomplete_xact_state_change(entry);

                        /* Commit all remote subtransactions during pre-commit */
                        snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
                        entry->changing_xact_state = true;
                        if (entry->parallel_commit)
                        {
                                do_sql_command_begin(entry->conn, sql);
                                pending_entries = lappend(pending_entries, entry);
                                continue;
                        }
                        do_sql_command(entry->conn, sql);
                        entry->changing_xact_state = false;
                }
                else
                {
                        /* Rollback all remote subtransactions during abort */
                        if (entry->parallel_abort)
                        {
                                if (pgfdw_abort_cleanup_begin(entry, false,
                                                                                          &pending_entries,
                                                                                          &cancel_requested))
                                        continue;
                        }
                        else
                                pgfdw_abort_cleanup(entry, false);
                }

                /* OK, we're outta that level of subtransaction */
                pgfdw_reset_xact_state(entry, false);
        }

        /* If there are any pending connections, finish cleaning them up */
        if (pending_entries || cancel_requested)
        {
                if (event == SUBXACT_EVENT_PRE_COMMIT_SUB)
                {
                        Assert(cancel_requested == NIL);
                        pgfdw_finish_pre_subcommit_cleanup(pending_entries, curlevel);
                }
                else
                {
                        Assert(event == SUBXACT_EVENT_ABORT_SUB);
                        pgfdw_finish_abort_cleanup(pending_entries, cancel_requested,
                                                                           false);
                }
        }
}

/*
 * Connection invalidation callback function
 *
 * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
 * close connections depending on that entry immediately if current transaction
 * has not used those connections yet. Otherwise, mark those connections as
 * invalid and then make pgfdw_xact_callback() close them at the end of current
 * transaction, since they cannot be closed in the midst of the transaction
 * using them. Closed connections will be remade at the next opportunity if
 * necessary.
 *
 * Although most cache invalidation callbacks blow away all the related stuff
 * regardless of the given hashvalue, connections are expensive enough that
 * it's worth trying to avoid that.
 *
 * NB: We could avoid unnecessary disconnection more strictly by examining
 * individual option values, but it seems too much effort for the gain.
 */
static void
pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue)
{
        HASH_SEQ_STATUS scan;
        ConnCacheEntry *entry;

        Assert(cacheid == FOREIGNSERVEROID || cacheid == USERMAPPINGOID);

        /* ConnectionHash must exist already, if we're registered */
        hash_seq_init(&scan, ConnectionHash);
        while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
        {
                /* Ignore invalid entries */
                if (entry->conn == NULL)
                        continue;

                /* hashvalue == 0 means a cache reset, must clear all state */
                if (hashvalue == 0 ||
                        (cacheid == FOREIGNSERVEROID &&
                         entry->server_hashvalue == hashvalue) ||
                        (cacheid == USERMAPPINGOID &&
                         entry->mapping_hashvalue == hashvalue))
                {
                        /*
                         * Close the connection immediately if it's not used yet in this
                         * transaction. Otherwise mark it as invalid so that
                         * pgfdw_xact_callback() can close it at the end of this
                         * transaction.
                         */
                        if (entry->xact_depth == 0)
                        {
                                elog(DEBUG3, "discarding connection %p", entry->conn);
                                disconnect_pg_server(entry);
                        }
                        else
                                entry->invalidated = true;
                }
        }
}

/*
 * Raise an error if the given connection cache entry is marked as being
 * in the middle of an xact state change.  This should be called at which no
 * such change is expected to be in progress; if one is found to be in
 * progress, it means that we aborted in the middle of a previous state change
 * and now don't know what the remote transaction state actually is.
 * Such connections can't safely be further used.  Re-establishing the
 * connection would change the snapshot and roll back any writes already
 * performed, so that's not an option, either. Thus, we must abort.
 */
static void
pgfdw_reject_incomplete_xact_state_change(ConnCacheEntry *entry)
{
        ForeignServer *server;

        /* nothing to do for inactive entries and entries of sane state */
        if (entry->conn == NULL || !entry->changing_xact_state)
                return;

        /* make sure this entry is inactive */
        disconnect_pg_server(entry);

        /* find server name to be shown in the message below */
        server = GetForeignServer(entry->serverid);

        ereport(ERROR,
                        (errcode(ERRCODE_CONNECTION_EXCEPTION),
                         errmsg("connection to server \"%s\" was lost",
                                        server->servername)));
}

/*
 * Reset state to show we're out of a (sub)transaction.
 */
static void
pgfdw_reset_xact_state(ConnCacheEntry *entry, bool toplevel)
{
        if (toplevel)
        {
                /* Reset state to show we're out of a transaction */
                entry->xact_depth = 0;

                /*
                 * If the connection isn't in a good idle state, it is marked as
                 * invalid or keep_connections option of its server is disabled, then
                 * discard it to recover. Next GetConnection will open a new
                 * connection.
                 */
                if (PQstatus(entry->conn) != CONNECTION_OK ||
                        PQtransactionStatus(entry->conn) != PQTRANS_IDLE ||
                        entry->changing_xact_state ||
                        entry->invalidated ||
                        !entry->keep_connections)
                {
                        elog(DEBUG3, "discarding connection %p", entry->conn);
                        disconnect_pg_server(entry);
                }
        }
        else
        {
                /* Reset state to show we're out of a subtransaction */
                entry->xact_depth--;
        }
}

/*
 * Cancel the currently-in-progress query (whose query text we do not have)
 * and ignore the result.  Returns true if we successfully cancel the query
 * and discard any pending result, and false if not.
 *
 * It's not a huge problem if we throw an ERROR here, but if we get into error
 * recursion trouble, we'll end up slamming the connection shut, which will
 * necessitate failing the entire toplevel transaction even if subtransactions
 * were used.  Try to use WARNING where we can.
 *
 * XXX: if the query was one sent by fetch_more_data_begin(), we could get the
 * query text from the pendingAreq saved in the per-connection state, then
 * report the query using it.
 */
static bool
pgfdw_cancel_query(PGconn *conn)
{
        TimestampTz now = GetCurrentTimestamp();
        TimestampTz endtime;
        TimestampTz retrycanceltime;

        /*
         * If it takes too long to cancel the query and discard the result, assume
         * the connection is dead.
         */
        endtime = TimestampTzPlusMilliseconds(now, CONNECTION_CLEANUP_TIMEOUT);

        /*
         * Also, lose patience and re-issue the cancel request after a little bit.
         * (This serves to close some race conditions.)
         */
        retrycanceltime = TimestampTzPlusMilliseconds(now, RETRY_CANCEL_TIMEOUT);

        if (!pgfdw_cancel_query_begin(conn, endtime))
                return false;
        return pgfdw_cancel_query_end(conn, endtime, retrycanceltime, false);
}

/*
 * Submit a cancel request to the given connection, waiting only until
 * the given time.
 *
 * We sleep interruptibly until we receive confirmation that the cancel
 * request has been accepted, and if it is, return true; if the timeout
 * lapses without that, or the request fails for whatever reason, return
 * false.
 */
static bool
pgfdw_cancel_query_begin(PGconn *conn, TimestampTz endtime)
{
        const char *errormsg = libpqsrv_cancel(conn, endtime);

        if (errormsg != NULL)
                ereport(WARNING,
                                errcode(ERRCODE_CONNECTION_FAILURE),
                                errmsg("could not send cancel request: %s", errormsg));

        return errormsg == NULL;
}

static bool
pgfdw_cancel_query_end(PGconn *conn, TimestampTz endtime,
                                           TimestampTz retrycanceltime, bool consume_input)
{
        PGresult   *result;
        bool            timed_out;

        /*
         * If requested, consume whatever data is available from the socket. (Note
         * that if all data is available, this allows pgfdw_get_cleanup_result to
         * call PQgetResult without forcing the overhead of WaitLatchOrSocket,
         * which would be large compared to the overhead of PQconsumeInput.)
         */
        if (consume_input && !PQconsumeInput(conn))
        {
                ereport(WARNING,
                                (errcode(ERRCODE_CONNECTION_FAILURE),
                                 errmsg("could not get result of cancel request: %s",
                                                pchomp(PQerrorMessage(conn)))));
                return false;
        }

        /* Get and discard the result of the query. */
        if (pgfdw_get_cleanup_result(conn, endtime, retrycanceltime,
                                                                 &result, &timed_out))
        {
                if (timed_out)
                        ereport(WARNING,
                                        (errmsg("could not get result of cancel request due to timeout")));
                else
                        ereport(WARNING,
                                        (errcode(ERRCODE_CONNECTION_FAILURE),
                                         errmsg("could not get result of cancel request: %s",
                                                        pchomp(PQerrorMessage(conn)))));

                return false;
        }
        PQclear(result);

        return true;
}

/*
 * Submit a query during (sub)abort cleanup and wait up to 30 seconds for the
 * result.  If the query is executed without error, the return value is true.
 * If the query is executed successfully but returns an error, the return
 * value is true if and only if ignore_errors is set.  If the query can't be
 * sent or times out, the return value is false.
 *
 * It's not a huge problem if we throw an ERROR here, but if we get into error
 * recursion trouble, we'll end up slamming the connection shut, which will
 * necessitate failing the entire toplevel transaction even if subtransactions
 * were used.  Try to use WARNING where we can.
 */
static bool
pgfdw_exec_cleanup_query(PGconn *conn, const char *query, bool ignore_errors)
{
        TimestampTz endtime;

        /*
         * If it takes too long to execute a cleanup query, assume the connection
         * is dead.  It's fairly likely that this is why we aborted in the first
         * place (e.g. statement timeout, user cancel), so the timeout shouldn't
         * be too long.
         */
        endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                                                                  CONNECTION_CLEANUP_TIMEOUT);

        if (!pgfdw_exec_cleanup_query_begin(conn, query))
                return false;
        return pgfdw_exec_cleanup_query_end(conn, query, endtime,
                                                                                false, ignore_errors);
}

static bool
pgfdw_exec_cleanup_query_begin(PGconn *conn, const char *query)
{
        Assert(query != NULL);

        /*
         * Submit a query.  Since we don't use non-blocking mode, this also can
         * block.  But its risk is relatively small, so we ignore that for now.
         */
        if (!PQsendQuery(conn, query))
        {
                pgfdw_report_error(WARNING, NULL, conn, false, query);
                return false;
        }

        return true;
}

static bool
pgfdw_exec_cleanup_query_end(PGconn *conn, const char *query,
                                                         TimestampTz endtime, bool consume_input,
                                                         bool ignore_errors)
{
        PGresult   *result;
        bool            timed_out;

        Assert(query != NULL);

        /*
         * If requested, consume whatever data is available from the socket. (Note
         * that if all data is available, this allows pgfdw_get_cleanup_result to
         * call PQgetResult without forcing the overhead of WaitLatchOrSocket,
         * which would be large compared to the overhead of PQconsumeInput.)
         */
        if (consume_input && !PQconsumeInput(conn))
        {
                pgfdw_report_error(WARNING, NULL, conn, false, query);
                return false;
        }

        /* Get the result of the query. */
        if (pgfdw_get_cleanup_result(conn, endtime, endtime, &result, &timed_out))
        {
                if (timed_out)
                        ereport(WARNING,
                                        (errmsg("could not get query result due to timeout"),
                                         errcontext("remote SQL command: %s", query)));
                else
                        pgfdw_report_error(WARNING, NULL, conn, false, query);

                return false;
        }

        /* Issue a warning if not successful. */
        if (PQresultStatus(result) != PGRES_COMMAND_OK)
        {
                pgfdw_report_error(WARNING, result, conn, true, query);
                return ignore_errors;
        }
        PQclear(result);

        return true;
}

/*
 * Get, during abort cleanup, the result of a query that is in progress.
 * This might be a query that is being interrupted by a cancel request or by
 * transaction abort, or it might be a query that was initiated as part of
 * transaction abort to get the remote side back to the appropriate state.
 *
 * endtime is the time at which we should give up and assume the remote side
 * is dead.  retrycanceltime is the time at which we should issue a fresh
 * cancel request (pass the same value as endtime if this is not wanted).
 *
 * Returns true if the timeout expired or connection trouble occurred,
 * false otherwise.  Sets *result except in case of a true result.
 * Sets *timed_out to true only when the timeout expired.
 */
static bool
pgfdw_get_cleanup_result(PGconn *conn, TimestampTz endtime,
                                                 TimestampTz retrycanceltime,
                                                 PGresult **result,
                                                 bool *timed_out)
{
        volatile bool failed = false;
        PGresult   *volatile last_res = NULL;

        *result = NULL;
        *timed_out = false;

        /* In what follows, do not leak any PGresults on an error. */
        PG_TRY();
        {
                int                     canceldelta = RETRY_CANCEL_TIMEOUT * 2;

                for (;;)
                {
                        PGresult   *res;

                        while (PQisBusy(conn))
                        {
                                int                     wc;
                                TimestampTz now = GetCurrentTimestamp();
                                long            cur_timeout;

                                /* If timeout has expired, give up. */
                                if (now >= endtime)
                                {
                                        *timed_out = true;
                                        failed = true;
                                        goto exit;
                                }

                                /* If we need to re-issue the cancel request, do that. */
                                if (now >= retrycanceltime)
                                {
                                        /* We ignore failure to issue the repeated request. */
                                        (void) libpqsrv_cancel(conn, endtime);

                                        /* Recompute "now" in case that took measurable time. */
                                        now = GetCurrentTimestamp();

                                        /* Adjust re-cancel timeout in increasing steps. */
                                        retrycanceltime = TimestampTzPlusMilliseconds(now,
                                                                                                                                  canceldelta);
                                        canceldelta += canceldelta;
                                }

                                /* If timeout has expired, give up, else get sleep time. */
                                cur_timeout = TimestampDifferenceMilliseconds(now,
                                                                                                                          Min(endtime,
                                                                                                                                  retrycanceltime));
                                if (cur_timeout <= 0)
                                {
                                        *timed_out = true;
                                        failed = true;
                                        goto exit;
                                }

                                /* first time, allocate or get the custom wait event */
                                if (pgfdw_we_cleanup_result == 0)
                                        pgfdw_we_cleanup_result = WaitEventExtensionNew("PostgresFdwCleanupResult");

                                /* Sleep until there's something to do */
                                wc = WaitLatchOrSocket(MyLatch,
                                                                           WL_LATCH_SET | WL_SOCKET_READABLE |
                                                                           WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                                                                           PQsocket(conn),
                                                                           cur_timeout, pgfdw_we_cleanup_result);
                                ResetLatch(MyLatch);

                                CHECK_FOR_INTERRUPTS();

                                /* Data available in socket? */
                                if (wc & WL_SOCKET_READABLE)
                                {
                                        if (!PQconsumeInput(conn))
                                        {
                                                /* connection trouble */
                                                failed = true;
                                                goto exit;
                                        }
                                }
                        }

                        res = PQgetResult(conn);
                        if (res == NULL)
                                break;                  /* query is complete */

                        PQclear(last_res);
                        last_res = res;
                }
exit:   ;
        }
        PG_CATCH();
        {
                PQclear(last_res);
                PG_RE_THROW();
        }
        PG_END_TRY();

        if (failed)
                PQclear(last_res);
        else
                *result = last_res;
        return failed;
}

/*
 * Abort remote transaction or subtransaction.
 *
 * "toplevel" should be set to true if toplevel (main) transaction is
 * rollbacked, false otherwise.
 *
 * Set entry->changing_xact_state to false on success, true on failure.
 */
static void
pgfdw_abort_cleanup(ConnCacheEntry *entry, bool toplevel)
{
        char            sql[100];

        /*
         * Don't try to clean up the connection if we're already in error
         * recursion trouble.
         */
        if (in_error_recursion_trouble())
                entry->changing_xact_state = true;

        /*
         * If connection is already unsalvageable, don't touch it further.
         */
        if (entry->changing_xact_state)
                return;

        /*
         * Mark this connection as in the process of changing transaction state.
         */
        entry->changing_xact_state = true;

        /* Assume we might have lost track of prepared statements */
        entry->have_error = true;

        /*
         * If a command has been submitted to the remote server by using an
         * asynchronous execution function, the command might not have yet
         * completed.  Check to see if a command is still being processed by the
         * remote server, and if so, request cancellation of the command.
         */
        if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE &&
                !pgfdw_cancel_query(entry->conn))
                return;                                 /* Unable to cancel running query */

        CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
        if (!pgfdw_exec_cleanup_query(entry->conn, sql, false))
                return;                                 /* Unable to abort remote (sub)transaction */

        if (toplevel)
        {
                if (entry->have_prep_stmt && entry->have_error &&
                        !pgfdw_exec_cleanup_query(entry->conn,
                                                                          "DEALLOCATE ALL",
                                                                          true))
                        return;                         /* Trouble clearing prepared statements */

                entry->have_prep_stmt = false;
                entry->have_error = false;
        }

        /*
         * If pendingAreq of the per-connection state is not NULL, it means that
         * an asynchronous fetch begun by fetch_more_data_begin() was not done
         * successfully and thus the per-connection state was not reset in
         * fetch_more_data(); in that case reset the per-connection state here.
         */
        if (entry->state.pendingAreq)
                memset(&entry->state, 0, sizeof(entry->state));

        /* Disarm changing_xact_state if it all worked */
        entry->changing_xact_state = false;
}

/*
 * Like pgfdw_abort_cleanup, submit an abort command or cancel request, but
 * don't wait for the result.
 *
 * Returns true if the abort command or cancel request is successfully issued,
 * false otherwise.  If the abort command is successfully issued, the given
 * connection cache entry is appended to *pending_entries.  Otherwise, if the
 * cancel request is successfully issued, it is appended to *cancel_requested.
 */
static bool
pgfdw_abort_cleanup_begin(ConnCacheEntry *entry, bool toplevel,
                                                  List **pending_entries, List **cancel_requested)
{
        /*
         * Don't try to clean up the connection if we're already in error
         * recursion trouble.
         */
        if (in_error_recursion_trouble())
                entry->changing_xact_state = true;

        /*
         * If connection is already unsalvageable, don't touch it further.
         */
        if (entry->changing_xact_state)
                return false;

        /*
         * Mark this connection as in the process of changing transaction state.
         */
        entry->changing_xact_state = true;

        /* Assume we might have lost track of prepared statements */
        entry->have_error = true;

        /*
         * If a command has been submitted to the remote server by using an
         * asynchronous execution function, the command might not have yet
         * completed.  Check to see if a command is still being processed by the
         * remote server, and if so, request cancellation of the command.
         */
        if (PQtransactionStatus(entry->conn) == PQTRANS_ACTIVE)
        {
                TimestampTz endtime;

                endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                                                                          CONNECTION_CLEANUP_TIMEOUT);
                if (!pgfdw_cancel_query_begin(entry->conn, endtime))
                        return false;           /* Unable to cancel running query */
                *cancel_requested = lappend(*cancel_requested, entry);
        }
        else
        {
                char            sql[100];

                CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
                if (!pgfdw_exec_cleanup_query_begin(entry->conn, sql))
                        return false;           /* Unable to abort remote transaction */
                *pending_entries = lappend(*pending_entries, entry);
        }

        return true;
}

/*
 * Finish pre-commit cleanup of connections on each of which we've sent a
 * COMMIT command to the remote server.
 */
static void
pgfdw_finish_pre_commit_cleanup(List *pending_entries)
{
        ConnCacheEntry *entry;
        List       *pending_deallocs = NIL;
        ListCell   *lc;

        Assert(pending_entries);

        /*
         * Get the result of the COMMIT command for each of the pending entries
         */
        foreach(lc, pending_entries)
        {
                entry = (ConnCacheEntry *) lfirst(lc);

                Assert(entry->changing_xact_state);

                /*
                 * We might already have received the result on the socket, so pass
                 * consume_input=true to try to consume it first
                 */
                do_sql_command_end(entry->conn, "COMMIT TRANSACTION", true);
                entry->changing_xact_state = false;

                /* Do a DEALLOCATE ALL in parallel if needed */
                if (entry->have_prep_stmt && entry->have_error)
                {
                        /* Ignore errors (see notes in pgfdw_xact_callback) */
                        if (PQsendQuery(entry->conn, "DEALLOCATE ALL"))
                        {
                                pending_deallocs = lappend(pending_deallocs, entry);
                                continue;
                        }
                }
                entry->have_prep_stmt = false;
                entry->have_error = false;

                pgfdw_reset_xact_state(entry, true);
        }

        /* No further work if no pending entries */
        if (!pending_deallocs)
                return;

        /*
         * Get the result of the DEALLOCATE command for each of the pending
         * entries
         */
        foreach(lc, pending_deallocs)
        {
                PGresult   *res;

                entry = (ConnCacheEntry *) lfirst(lc);

                /* Ignore errors (see notes in pgfdw_xact_callback) */
                while ((res = PQgetResult(entry->conn)) != NULL)
                {
                        PQclear(res);
                        /* Stop if the connection is lost (else we'll loop infinitely) */
                        if (PQstatus(entry->conn) == CONNECTION_BAD)
                                break;
                }
                entry->have_prep_stmt = false;
                entry->have_error = false;

                pgfdw_reset_xact_state(entry, true);
        }
}

/*
 * Finish pre-subcommit cleanup of connections on each of which we've sent a
 * RELEASE command to the remote server.
 */
static void
pgfdw_finish_pre_subcommit_cleanup(List *pending_entries, int curlevel)
{
        ConnCacheEntry *entry;
        char            sql[100];
        ListCell   *lc;

        Assert(pending_entries);

        /*
         * Get the result of the RELEASE command for each of the pending entries
         */
        snprintf(sql, sizeof(sql), "RELEASE SAVEPOINT s%d", curlevel);
        foreach(lc, pending_entries)
        {
                entry = (ConnCacheEntry *) lfirst(lc);

                Assert(entry->changing_xact_state);

                /*
                 * We might already have received the result on the socket, so pass
                 * consume_input=true to try to consume it first
                 */
                do_sql_command_end(entry->conn, sql, true);
                entry->changing_xact_state = false;

                pgfdw_reset_xact_state(entry, false);
        }
}

/*
 * Finish abort cleanup of connections on each of which we've sent an abort
 * command or cancel request to the remote server.
 */
static void
pgfdw_finish_abort_cleanup(List *pending_entries, List *cancel_requested,
                                                   bool toplevel)
{
        List       *pending_deallocs = NIL;
        ListCell   *lc;

        /*
         * For each of the pending cancel requests (if any), get and discard the
         * result of the query, and submit an abort command to the remote server.
         */
        if (cancel_requested)
        {
                foreach(lc, cancel_requested)
                {
                        ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
                        TimestampTz now = GetCurrentTimestamp();
                        TimestampTz endtime;
                        TimestampTz retrycanceltime;
                        char            sql[100];

                        Assert(entry->changing_xact_state);

                        /*
                         * Set end time.  You might think we should do this before issuing
                         * cancel request like in normal mode, but that is problematic,
                         * because if, for example, it took longer than 30 seconds to
                         * process the first few entries in the cancel_requested list, it
                         * would cause a timeout error when processing each of the
                         * remaining entries in the list, leading to slamming that entry's
                         * connection shut.
                         */
                        endtime = TimestampTzPlusMilliseconds(now,
                                                                                                  CONNECTION_CLEANUP_TIMEOUT);
                        retrycanceltime = TimestampTzPlusMilliseconds(now,
                                                                                                                  RETRY_CANCEL_TIMEOUT);

                        if (!pgfdw_cancel_query_end(entry->conn, endtime,
                                                                                retrycanceltime, true))
                        {
                                /* Unable to cancel running query */
                                pgfdw_reset_xact_state(entry, toplevel);
                                continue;
                        }

                        /* Send an abort command in parallel if needed */
                        CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
                        if (!pgfdw_exec_cleanup_query_begin(entry->conn, sql))
                        {
                                /* Unable to abort remote (sub)transaction */
                                pgfdw_reset_xact_state(entry, toplevel);
                        }
                        else
                                pending_entries = lappend(pending_entries, entry);
                }
        }

        /* No further work if no pending entries */
        if (!pending_entries)
                return;

        /*
         * Get the result of the abort command for each of the pending entries
         */
        foreach(lc, pending_entries)
        {
                ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
                TimestampTz endtime;
                char            sql[100];

                Assert(entry->changing_xact_state);

                /*
                 * Set end time.  We do this now, not before issuing the command like
                 * in normal mode, for the same reason as for the cancel_requested
                 * entries.
                 */
                endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                                                                          CONNECTION_CLEANUP_TIMEOUT);

                CONSTRUCT_ABORT_COMMAND(sql, entry, toplevel);
                if (!pgfdw_exec_cleanup_query_end(entry->conn, sql, endtime,
                                                                                  true, false))
                {
                        /* Unable to abort remote (sub)transaction */
                        pgfdw_reset_xact_state(entry, toplevel);
                        continue;
                }

                if (toplevel)
                {
                        /* Do a DEALLOCATE ALL in parallel if needed */
                        if (entry->have_prep_stmt && entry->have_error)
                        {
                                if (!pgfdw_exec_cleanup_query_begin(entry->conn,
                                                                                                        "DEALLOCATE ALL"))
                                {
                                        /* Trouble clearing prepared statements */
                                        pgfdw_reset_xact_state(entry, toplevel);
                                }
                                else
                                        pending_deallocs = lappend(pending_deallocs, entry);
                                continue;
                        }
                        entry->have_prep_stmt = false;
                        entry->have_error = false;
                }

                /* Reset the per-connection state if needed */
                if (entry->state.pendingAreq)
                        memset(&entry->state, 0, sizeof(entry->state));

                /* We're done with this entry; unset the changing_xact_state flag */
                entry->changing_xact_state = false;
                pgfdw_reset_xact_state(entry, toplevel);
        }

        /* No further work if no pending entries */
        if (!pending_deallocs)
                return;
        Assert(toplevel);

        /*
         * Get the result of the DEALLOCATE command for each of the pending
         * entries
         */
        foreach(lc, pending_deallocs)
        {
                ConnCacheEntry *entry = (ConnCacheEntry *) lfirst(lc);
                TimestampTz endtime;

                Assert(entry->changing_xact_state);
                Assert(entry->have_prep_stmt);
                Assert(entry->have_error);

                /*
                 * Set end time.  We do this now, not before issuing the command like
                 * in normal mode, for the same reason as for the cancel_requested
                 * entries.
                 */
                endtime = TimestampTzPlusMilliseconds(GetCurrentTimestamp(),
                                                                                          CONNECTION_CLEANUP_TIMEOUT);

                if (!pgfdw_exec_cleanup_query_end(entry->conn, "DEALLOCATE ALL",
                                                                                  endtime, true, true))
                {
                        /* Trouble clearing prepared statements */
                        pgfdw_reset_xact_state(entry, toplevel);
                        continue;
                }
                entry->have_prep_stmt = false;
                entry->have_error = false;

                /* Reset the per-connection state if needed */
                if (entry->state.pendingAreq)
                        memset(&entry->state, 0, sizeof(entry->state));

                /* We're done with this entry; unset the changing_xact_state flag */
                entry->changing_xact_state = false;
                pgfdw_reset_xact_state(entry, toplevel);
        }
}

/* Number of output arguments (columns) for various API versions */
#define POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_1  2
#define POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_2  5
#define POSTGRES_FDW_GET_CONNECTIONS_COLS       5       /* maximum of above */

/*
 * Internal function used by postgres_fdw_get_connections variants.
 *
 * For API version 1.1, this function takes no input parameter and
 * returns a set of records with the following values:
 *
 * - server_name - server name of active connection. In case the foreign server
 *   is dropped but still the connection is active, then the server name will
 *   be NULL in output.
 * - valid - true/false representing whether the connection is valid or not.
 *   Note that connections can become invalid in pgfdw_inval_callback.
 *
 * For API version 1.2 and later, this function takes an input parameter
 * to check a connection status and returns the following
 * additional values along with the three values from version 1.1:
 *
 * - user_name - the local user name of the active connection. In case the
 *   user mapping is dropped but the connection is still active, then the
 *   user name will be NULL in the output.
 * - used_in_xact - true if the connection is used in the current transaction.
 * - closed - true if the connection is closed.
 *
 * No records are returned when there are no cached connections at all.
 */
static void
postgres_fdw_get_connections_internal(FunctionCallInfo fcinfo,
                                                                          enum pgfdwVersion api_version)
{
        ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
        HASH_SEQ_STATUS scan;
        ConnCacheEntry *entry;

        InitMaterializedSRF(fcinfo, 0);

        /* If cache doesn't exist, we return no records */
        if (!ConnectionHash)
                return;

        /* Check we have the expected number of output arguments */
        switch (rsinfo->setDesc->natts)
        {
                case POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_1:
                        if (api_version != PGFDW_V1_1)
                                elog(ERROR, "incorrect number of output arguments");
                        break;
                case POSTGRES_FDW_GET_CONNECTIONS_COLS_V1_2:
                        if (api_version != PGFDW_V1_2)
                                elog(ERROR, "incorrect number of output arguments");
                        break;
                default:
                        elog(ERROR, "incorrect number of output arguments");
        }

        hash_seq_init(&scan, ConnectionHash);
        while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
        {
                ForeignServer *server;
                Datum           values[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
                bool            nulls[POSTGRES_FDW_GET_CONNECTIONS_COLS] = {0};
                int                     i = 0;

                /* We only look for open remote connections */
                if (!entry->conn)
                        continue;

                server = GetForeignServerExtended(entry->serverid, FSV_MISSING_OK);

                /*
                 * The foreign server may have been dropped in current explicit
                 * transaction. It is not possible to drop the server from another
                 * session when the connection associated with it is in use in the
                 * current transaction, if tried so, the drop query in another session
                 * blocks until the current transaction finishes.
                 *
                 * Even though the server is dropped in the current transaction, the
                 * cache can still have associated active connection entry, say we
                 * call such connections dangling. Since we can not fetch the server
                 * name from system catalogs for dangling connections, instead we show
                 * NULL value for server name in output.
                 *
                 * We could have done better by storing the server name in the cache
                 * entry instead of server oid so that it could be used in the output.
                 * But the server name in each cache entry requires 64 bytes of
                 * memory, which is huge, when there are many cached connections and
                 * the use case i.e. dropping the foreign server within the explicit
                 * current transaction seems rare. So, we chose to show NULL value for
                 * server name in output.
                 *
                 * Such dangling connections get closed either in next use or at the
                 * end of current explicit transaction in pgfdw_xact_callback.
                 */
                if (!server)
                {
                        /*
                         * If the server has been dropped in the current explicit
                         * transaction, then this entry would have been invalidated in
                         * pgfdw_inval_callback at the end of drop server command. Note
                         * that this connection would not have been closed in
                         * pgfdw_inval_callback because it is still being used in the
                         * current explicit transaction. So, assert that here.
                         */
                        Assert(entry->conn && entry->xact_depth > 0 && entry->invalidated);

                        /* Show null, if no server name was found */
                        nulls[i++] = true;
                }
                else
                        values[i++] = CStringGetTextDatum(server->servername);

                if (api_version >= PGFDW_V1_2)
                {
                        HeapTuple       tp;

                        /* Use the system cache to obtain the user mapping */
                        tp = SearchSysCache1(USERMAPPINGOID, ObjectIdGetDatum(entry->key));

                        /*
                         * Just like in the foreign server case, user mappings can also be
                         * dropped in the current explicit transaction. Therefore, the
                         * similar check as in the server case is required.
                         */
                        if (!HeapTupleIsValid(tp))
                        {
                                /*
                                 * If we reach here, this entry must have been invalidated in
                                 * pgfdw_inval_callback, same as in the server case.
                                 */
                                Assert(entry->conn && entry->xact_depth > 0 &&
                                           entry->invalidated);

                                nulls[i++] = true;
                        }
                        else
                        {
                                Oid                     userid;

                                userid = ((Form_pg_user_mapping) GETSTRUCT(tp))->umuser;
                                values[i++] = CStringGetTextDatum(MappingUserName(userid));
                                ReleaseSysCache(tp);
                        }
                }

                values[i++] = BoolGetDatum(!entry->invalidated);

                if (api_version >= PGFDW_V1_2)
                {
                        bool            check_conn = PG_GETARG_BOOL(0);

                        /* Is this connection used in the current transaction? */
                        values[i++] = BoolGetDatum(entry->xact_depth > 0);

                        /*
                         * If a connection status check is requested and supported, return
                         * whether the connection is closed. Otherwise, return NULL.
                         */
                        if (check_conn && pgfdw_conn_checkable())
                                values[i++] = BoolGetDatum(pgfdw_conn_check(entry->conn) != 0);
                        else
                                nulls[i++] = true;
                }

                tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
        }
}

/*
 * List active foreign server connections.
 *
 * The SQL API of this function has changed multiple times, and will likely
 * do so again in future.  To support the case where a newer version of this
 * loadable module is being used with an old SQL declaration of the function,
 * we continue to support the older API versions.
 */
Datum
postgres_fdw_get_connections_1_2(PG_FUNCTION_ARGS)
{
        postgres_fdw_get_connections_internal(fcinfo, PGFDW_V1_2);

        PG_RETURN_VOID();
}

Datum
postgres_fdw_get_connections(PG_FUNCTION_ARGS)
{
        postgres_fdw_get_connections_internal(fcinfo, PGFDW_V1_1);

        PG_RETURN_VOID();
}

/*
 * Disconnect the specified cached connections.
 *
 * This function discards the open connections that are established by
 * postgres_fdw from the local session to the foreign server with
 * the given name. Note that there can be multiple connections to
 * the given server using different user mappings. If the connections
 * are used in the current local transaction, they are not disconnected
 * and warning messages are reported. This function returns true
 * if it disconnects at least one connection, otherwise false. If no
 * foreign server with the given name is found, an error is reported.
 */
Datum
postgres_fdw_disconnect(PG_FUNCTION_ARGS)
{
        ForeignServer *server;
        char       *servername;

        servername = text_to_cstring(PG_GETARG_TEXT_PP(0));
        server = GetForeignServerByName(servername, false);

        PG_RETURN_BOOL(disconnect_cached_connections(server->serverid));
}

/*
 * Disconnect all the cached connections.
 *
 * This function discards all the open connections that are established by
 * postgres_fdw from the local session to the foreign servers.
 * If the connections are used in the current local transaction, they are
 * not disconnected and warning messages are reported. This function
 * returns true if it disconnects at least one connection, otherwise false.
 */
Datum
postgres_fdw_disconnect_all(PG_FUNCTION_ARGS)
{
        PG_RETURN_BOOL(disconnect_cached_connections(InvalidOid));
}

/*
 * Workhorse to disconnect cached connections.
 *
 * This function scans all the connection cache entries and disconnects
 * the open connections whose foreign server OID matches with
 * the specified one. If InvalidOid is specified, it disconnects all
 * the cached connections.
 *
 * This function emits a warning for each connection that's used in
 * the current transaction and doesn't close it. It returns true if
 * it disconnects at least one connection, otherwise false.
 *
 * Note that this function disconnects even the connections that are
 * established by other users in the same local session using different
 * user mappings. This leads even non-superuser to be able to close
 * the connections established by superusers in the same local session.
 *
 * XXX As of now we don't see any security risk doing this. But we should
 * set some restrictions on that, for example, prevent non-superuser
 * from closing the connections established by superusers even
 * in the same session?
 */
static bool
disconnect_cached_connections(Oid serverid)
{
        HASH_SEQ_STATUS scan;
        ConnCacheEntry *entry;
        bool            all = !OidIsValid(serverid);
        bool            result = false;

        /*
         * Connection cache hashtable has not been initialized yet in this
         * session, so return false.
         */
        if (!ConnectionHash)
                return false;

        hash_seq_init(&scan, ConnectionHash);
        while ((entry = (ConnCacheEntry *) hash_seq_search(&scan)))
        {
                /* Ignore cache entry if no open connection right now. */
                if (!entry->conn)
                        continue;

                if (all || entry->serverid == serverid)
                {
                        /*
                         * Emit a warning because the connection to close is used in the
                         * current transaction and cannot be disconnected right now.
                         */
                        if (entry->xact_depth > 0)
                        {
                                ForeignServer *server;

                                server = GetForeignServerExtended(entry->serverid,
                                                                                                  FSV_MISSING_OK);

                                if (!server)
                                {
                                        /*
                                         * If the foreign server was dropped while its connection
                                         * was used in the current transaction, the connection
                                         * must have been marked as invalid by
                                         * pgfdw_inval_callback at the end of DROP SERVER command.
                                         */
                                        Assert(entry->invalidated);

                                        ereport(WARNING,
                                                        (errmsg("cannot close dropped server connection because it is still in use")));
                                }
                                else
                                        ereport(WARNING,
                                                        (errmsg("cannot close connection for server \"%s\" because it is still in use",
                                                                        server->servername)));
                        }
                        else
                        {
                                elog(DEBUG3, "discarding connection %p", entry->conn);
                                disconnect_pg_server(entry);
                                result = true;
                        }
                }
        }

        return result;
}

/*
 * Check if the remote server closed the connection.
 *
 * Returns 1 if the connection is closed, -1 if an error occurred,
 * and 0 if it's not closed or if the connection check is unavailable
 * on this platform.
 */
static int
pgfdw_conn_check(PGconn *conn)
{
        int                     sock = PQsocket(conn);

        if (PQstatus(conn) != CONNECTION_OK || sock == -1)
                return -1;

#if (defined(HAVE_POLL) && defined(POLLRDHUP))
        {
                struct pollfd input_fd;
                int                     result;

                input_fd.fd = sock;
                input_fd.events = POLLRDHUP;
                input_fd.revents = 0;

                do
                        result = poll(&input_fd, 1, 0);
                while (result < 0 && errno == EINTR);

                if (result < 0)
                        return -1;

                return (input_fd.revents &
                                (POLLRDHUP | POLLHUP | POLLERR | POLLNVAL)) ? 1 : 0;
        }
#else
        return 0;
#endif
}

/*
 * Check if connection status checking is available on this platform.
 *
 * Returns true if available, false otherwise.
 */
static bool
pgfdw_conn_checkable(void)
{
#if (defined(HAVE_POLL) && defined(POLLRDHUP))
        return true;
#else
        return false;
#endif
}