Now that we have non-Latin1 SGML detection, restore Latin1 chars

[pgsql.git] / src / backend / replication / logical / snapbuild.c

/*-------------------------------------------------------------------------
 *
 * snapbuild.c
 *
 *        Infrastructure for building historic catalog snapshots based on contents
 *        of the WAL, for the purpose of decoding heapam.c style values in the
 *        WAL.
 *
 * NOTES:
 *
 * We build snapshots which can *only* be used to read catalog contents and we
 * do so by reading and interpreting the WAL stream. The aim is to build a
 * snapshot that behaves the same as a freshly taken MVCC snapshot would have
 * at the time the XLogRecord was generated.
 *
 * To build the snapshots we reuse the infrastructure built for Hot
 * Standby. The in-memory snapshots we build look different than HS' because
 * we have different needs. To successfully decode data from the WAL we only
 * need to access catalog tables and (sys|rel|cat)cache, not the actual user
 * tables since the data we decode is wholly contained in the WAL
 * records. Also, our snapshots need to be different in comparison to normal
 * MVCC ones because in contrast to those we cannot fully rely on the clog and
 * pg_subtrans for information about committed transactions because they might
 * commit in the future from the POV of the WAL entry we're currently
 * decoding. This definition has the advantage that we only need to prevent
 * removal of catalog rows, while normal table's rows can still be
 * removed. This is achieved by using the replication slot mechanism.
 *
 * As the percentage of transactions modifying the catalog normally is fairly
 * small in comparisons to ones only manipulating user data, we keep track of
 * the committed catalog modifying ones inside [xmin, xmax) instead of keeping
 * track of all running transactions like it's done in a normal snapshot. Note
 * that we're generally only looking at transactions that have acquired an
 * xid. That is we keep a list of transactions between snapshot->(xmin, xmax)
 * that we consider committed, everything else is considered aborted/in
 * progress. That also allows us not to care about subtransactions before they
 * have committed which means this module, in contrast to HS, doesn't have to
 * care about suboverflowed subtransactions and similar.
 *
 * One complexity of doing this is that to e.g. handle mixed DDL/DML
 * transactions we need Snapshots that see intermediate versions of the
 * catalog in a transaction. During normal operation this is achieved by using
 * CommandIds/cmin/cmax. The problem with that however is that for space
 * efficiency reasons, the cmin and cmax are not included in WAL records. We
 * cannot read the cmin/cmax from the tuple itself, either, because it is
 * reset on crash recovery. Even if we could, we could not decode combocids
 * which are only tracked in the original backend's memory. To work around
 * that, heapam writes an extra WAL record (XLOG_HEAP2_NEW_CID) every time a
 * catalog row is modified, which includes the cmin and cmax of the
 * tuple. During decoding, we insert the ctid->(cmin,cmax) mappings into the
 * reorder buffer, and use them at visibility checks instead of the cmin/cmax
 * on the tuple itself. Check the reorderbuffer.c's comment above
 * ResolveCminCmaxDuringDecoding() for details.
 *
 * To facilitate all this we need our own visibility routine, as the normal
 * ones are optimized for different usecases.
 *
 * To replace the normal catalog snapshots with decoding ones use the
 * SetupHistoricSnapshot() and TeardownHistoricSnapshot() functions.
 *
 *
 *
 * The snapbuild machinery is starting up in several stages, as illustrated
 * by the following graph describing the SnapBuild->state transitions:
 *
 *                 +-------------------------+
 *        +----|                 START                   |-------------+
 *        |    +-------------------------+                         |
 *        |                                     |                                                  |
 *        |                                     |                                                  |
 *        |                running_xacts #1                                        |
 *        |                                     |                                                  |
 *        |                                     |                                                  |
 *        |                                     v                                                  |
 *        |    +-------------------------+                         v
 *        |    |   BUILDING_SNAPSHOT     |------------>|
 *        |    +-------------------------+                         |
 *        |                                     |                                                  |
 *        |                                     |                                                  |
 *        | running_xacts #2, xacts from #1 finished   |
 *        |                                     |                                                  |
 *        |                                     |                                                  |
 *        |                                     v                                                  |
 *        |    +-------------------------+                         v
 *        |    |           FULL_SNAPSHOT         |------------>|
 *        |    +-------------------------+                         |
 *        |                                     |                                                  |
 * running_xacts                |                                          saved snapshot
 * with zero xacts              |                                 at running_xacts's lsn
 *        |                                     |                                                  |
 *        | running_xacts with xacts from #2 finished  |
 *        |                                     |                                                  |
 *        |                                     v                                                  |
 *        |    +-------------------------+                         |
 *        +--->|SNAPBUILD_CONSISTENT     |<------------+
 *                 +-------------------------+
 *
 * Initially the machinery is in the START stage. When an xl_running_xacts
 * record is read that is sufficiently new (above the safe xmin horizon),
 * there's a state transition. If there were no running xacts when the
 * xl_running_xacts record was generated, we'll directly go into CONSISTENT
 * state, otherwise we'll switch to the BUILDING_SNAPSHOT state. Having a full
 * snapshot means that all transactions that start henceforth can be decoded
 * in their entirety, but transactions that started previously can't. In
 * FULL_SNAPSHOT we'll switch into CONSISTENT once all those previously
 * running transactions have committed or aborted.
 *
 * Only transactions that commit after CONSISTENT state has been reached will
 * be replayed, even though they might have started while still in
 * FULL_SNAPSHOT. That ensures that we'll reach a point where no previous
 * changes has been exported, but all the following ones will be. That point
 * is a convenient point to initialize replication from, which is why we
 * export a snapshot at that point, which *can* be used to read normal data.
 *
 * Copyright (c) 2012-2024, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *        src/backend/replication/logical/snapbuild.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <sys/stat.h>
#include <unistd.h>

#include "access/heapam_xlog.h"
#include "access/transam.h"
#include "access/xact.h"
#include "common/file_utils.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "replication/logical.h"
#include "replication/reorderbuffer.h"
#include "replication/snapbuild.h"
#include "replication/snapbuild_internal.h"
#include "storage/fd.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/standby.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
#include "utils/snapshot.h"
/*
 * Starting a transaction -- which we need to do while exporting a snapshot --
 * removes knowledge about the previously used resowner, so we save it here.
 */
static ResourceOwner SavedResourceOwnerDuringExport = NULL;
static bool ExportInProgress = false;

/* ->committed and ->catchange manipulation */
static void SnapBuildPurgeOlderTxn(SnapBuild *builder);

/* snapshot building/manipulation/distribution functions */
static Snapshot SnapBuildBuildSnapshot(SnapBuild *builder);

static void SnapBuildFreeSnapshot(Snapshot snap);

static void SnapBuildSnapIncRefcount(Snapshot snap);

static void SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn);

static inline bool SnapBuildXidHasCatalogChanges(SnapBuild *builder, TransactionId xid,
                                                                                                 uint32 xinfo);

/* xlog reading helper functions for SnapBuildProcessRunningXacts */
static bool SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *running);
static void SnapBuildWaitSnapshot(xl_running_xacts *running, TransactionId cutoff);

/* serialization functions */
static void SnapBuildSerialize(SnapBuild *builder, XLogRecPtr lsn);
static bool SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn);
static void SnapBuildRestoreContents(int fd, char *dest, Size size, const char *path);

/*
 * Allocate a new snapshot builder.
 *
 * xmin_horizon is the xid >= which we can be sure no catalog rows have been
 * removed, start_lsn is the LSN >= we want to replay commits.
 */
SnapBuild *
AllocateSnapshotBuilder(ReorderBuffer *reorder,
                                                TransactionId xmin_horizon,
                                                XLogRecPtr start_lsn,
                                                bool need_full_snapshot,
                                                bool in_slot_creation,
                                                XLogRecPtr two_phase_at)
{
        MemoryContext context;
        MemoryContext oldcontext;
        SnapBuild  *builder;

        /* allocate memory in own context, to have better accountability */
        context = AllocSetContextCreate(CurrentMemoryContext,
                                                                        "snapshot builder context",
                                                                        ALLOCSET_DEFAULT_SIZES);
        oldcontext = MemoryContextSwitchTo(context);

        builder = palloc0(sizeof(SnapBuild));

        builder->state = SNAPBUILD_START;
        builder->context = context;
        builder->reorder = reorder;
        /* Other struct members initialized by zeroing via palloc0 above */

        builder->committed.xcnt = 0;
        builder->committed.xcnt_space = 128;    /* arbitrary number */
        builder->committed.xip =
                palloc0(builder->committed.xcnt_space * sizeof(TransactionId));
        builder->committed.includes_all_transactions = true;

        builder->catchange.xcnt = 0;
        builder->catchange.xip = NULL;

        builder->initial_xmin_horizon = xmin_horizon;
        builder->start_decoding_at = start_lsn;
        builder->in_slot_creation = in_slot_creation;
        builder->building_full_snapshot = need_full_snapshot;
        builder->two_phase_at = two_phase_at;

        MemoryContextSwitchTo(oldcontext);

        return builder;
}

/*
 * Free a snapshot builder.
 */
void
FreeSnapshotBuilder(SnapBuild *builder)
{
        MemoryContext context = builder->context;

        /* free snapshot explicitly, that contains some error checking */
        if (builder->snapshot != NULL)
        {
                SnapBuildSnapDecRefcount(builder->snapshot);
                builder->snapshot = NULL;
        }

        /* other resources are deallocated via memory context reset */
        MemoryContextDelete(context);
}

/*
 * Free an unreferenced snapshot that has previously been built by us.
 */
static void
SnapBuildFreeSnapshot(Snapshot snap)
{
        /* make sure we don't get passed an external snapshot */
        Assert(snap->snapshot_type == SNAPSHOT_HISTORIC_MVCC);

        /* make sure nobody modified our snapshot */
        Assert(snap->curcid == FirstCommandId);
        Assert(!snap->suboverflowed);
        Assert(!snap->takenDuringRecovery);
        Assert(snap->regd_count == 0);

        /* slightly more likely, so it's checked even without c-asserts */
        if (snap->copied)
                elog(ERROR, "cannot free a copied snapshot");

        if (snap->active_count)
                elog(ERROR, "cannot free an active snapshot");

        pfree(snap);
}

/*
 * In which state of snapshot building are we?
 */
SnapBuildState
SnapBuildCurrentState(SnapBuild *builder)
{
        return builder->state;
}

/*
 * Return the LSN at which the two-phase decoding was first enabled.
 */
XLogRecPtr
SnapBuildGetTwoPhaseAt(SnapBuild *builder)
{
        return builder->two_phase_at;
}

/*
 * Set the LSN at which two-phase decoding is enabled.
 */
void
SnapBuildSetTwoPhaseAt(SnapBuild *builder, XLogRecPtr ptr)
{
        builder->two_phase_at = ptr;
}

/*
 * Should the contents of transaction ending at 'ptr' be decoded?
 */
bool
SnapBuildXactNeedsSkip(SnapBuild *builder, XLogRecPtr ptr)
{
        return ptr < builder->start_decoding_at;
}

/*
 * Increase refcount of a snapshot.
 *
 * This is used when handing out a snapshot to some external resource or when
 * adding a Snapshot as builder->snapshot.
 */
static void
SnapBuildSnapIncRefcount(Snapshot snap)
{
        snap->active_count++;
}

/*
 * Decrease refcount of a snapshot and free if the refcount reaches zero.
 *
 * Externally visible, so that external resources that have been handed an
 * IncRef'ed Snapshot can adjust its refcount easily.
 */
void
SnapBuildSnapDecRefcount(Snapshot snap)
{
        /* make sure we don't get passed an external snapshot */
        Assert(snap->snapshot_type == SNAPSHOT_HISTORIC_MVCC);

        /* make sure nobody modified our snapshot */
        Assert(snap->curcid == FirstCommandId);
        Assert(!snap->suboverflowed);
        Assert(!snap->takenDuringRecovery);

        Assert(snap->regd_count == 0);

        Assert(snap->active_count > 0);

        /* slightly more likely, so it's checked even without casserts */
        if (snap->copied)
                elog(ERROR, "cannot free a copied snapshot");

        snap->active_count--;
        if (snap->active_count == 0)
                SnapBuildFreeSnapshot(snap);
}

/*
 * Build a new snapshot, based on currently committed catalog-modifying
 * transactions.
 *
 * In-progress transactions with catalog access are *not* allowed to modify
 * these snapshots; they have to copy them and fill in appropriate ->curcid
 * and ->subxip/subxcnt values.
 */
static Snapshot
SnapBuildBuildSnapshot(SnapBuild *builder)
{
        Snapshot        snapshot;
        Size            ssize;

        Assert(builder->state >= SNAPBUILD_FULL_SNAPSHOT);

        ssize = sizeof(SnapshotData)
                + sizeof(TransactionId) * builder->committed.xcnt
                + sizeof(TransactionId) * 1 /* toplevel xid */ ;

        snapshot = MemoryContextAllocZero(builder->context, ssize);

        snapshot->snapshot_type = SNAPSHOT_HISTORIC_MVCC;

        /*
         * We misuse the original meaning of SnapshotData's xip and subxip fields
         * to make the more fitting for our needs.
         *
         * In the 'xip' array we store transactions that have to be treated as
         * committed. Since we will only ever look at tuples from transactions
         * that have modified the catalog it's more efficient to store those few
         * that exist between xmin and xmax (frequently there are none).
         *
         * Snapshots that are used in transactions that have modified the catalog
         * also use the 'subxip' array to store their toplevel xid and all the
         * subtransaction xids so we can recognize when we need to treat rows as
         * visible that are not in xip but still need to be visible. Subxip only
         * gets filled when the transaction is copied into the context of a
         * catalog modifying transaction since we otherwise share a snapshot
         * between transactions. As long as a txn hasn't modified the catalog it
         * doesn't need to treat any uncommitted rows as visible, so there is no
         * need for those xids.
         *
         * Both arrays are qsort'ed so that we can use bsearch() on them.
         */
        Assert(TransactionIdIsNormal(builder->xmin));
        Assert(TransactionIdIsNormal(builder->xmax));

        snapshot->xmin = builder->xmin;
        snapshot->xmax = builder->xmax;

        /* store all transactions to be treated as committed by this snapshot */
        snapshot->xip =
                (TransactionId *) ((char *) snapshot + sizeof(SnapshotData));
        snapshot->xcnt = builder->committed.xcnt;
        memcpy(snapshot->xip,
                   builder->committed.xip,
                   builder->committed.xcnt * sizeof(TransactionId));

        /* sort so we can bsearch() */
        qsort(snapshot->xip, snapshot->xcnt, sizeof(TransactionId), xidComparator);

        /*
         * Initially, subxip is empty, i.e. it's a snapshot to be used by
         * transactions that don't modify the catalog. Will be filled by
         * ReorderBufferCopySnap() if necessary.
         */
        snapshot->subxcnt = 0;
        snapshot->subxip = NULL;

        snapshot->suboverflowed = false;
        snapshot->takenDuringRecovery = false;
        snapshot->copied = false;
        snapshot->curcid = FirstCommandId;
        snapshot->active_count = 0;
        snapshot->regd_count = 0;
        snapshot->snapXactCompletionCount = 0;

        return snapshot;
}

/*
 * Build the initial slot snapshot and convert it to a normal snapshot that
 * is understood by HeapTupleSatisfiesMVCC.
 *
 * The snapshot will be usable directly in current transaction or exported
 * for loading in different transaction.
 */
Snapshot
SnapBuildInitialSnapshot(SnapBuild *builder)
{
        Snapshot        snap;
        TransactionId xid;
        TransactionId safeXid;
        TransactionId *newxip;
        int                     newxcnt = 0;

        Assert(XactIsoLevel == XACT_REPEATABLE_READ);
        Assert(builder->building_full_snapshot);

        /* don't allow older snapshots */
        InvalidateCatalogSnapshot();    /* about to overwrite MyProc->xmin */
        if (HaveRegisteredOrActiveSnapshot())
                elog(ERROR, "cannot build an initial slot snapshot when snapshots exist");
        Assert(!HistoricSnapshotActive());

        if (builder->state != SNAPBUILD_CONSISTENT)
                elog(ERROR, "cannot build an initial slot snapshot before reaching a consistent state");

        if (!builder->committed.includes_all_transactions)
                elog(ERROR, "cannot build an initial slot snapshot, not all transactions are monitored anymore");

        /* so we don't overwrite the existing value */
        if (TransactionIdIsValid(MyProc->xmin))
                elog(ERROR, "cannot build an initial slot snapshot when MyProc->xmin already is valid");

        snap = SnapBuildBuildSnapshot(builder);

        /*
         * We know that snap->xmin is alive, enforced by the logical xmin
         * mechanism. Due to that we can do this without locks, we're only
         * changing our own value.
         *
         * Building an initial snapshot is expensive and an unenforced xmin
         * horizon would have bad consequences, therefore always double-check that
         * the horizon is enforced.
         */
        LWLockAcquire(ProcArrayLock, LW_SHARED);
        safeXid = GetOldestSafeDecodingTransactionId(false);
        LWLockRelease(ProcArrayLock);

        if (TransactionIdFollows(safeXid, snap->xmin))
                elog(ERROR, "cannot build an initial slot snapshot as oldest safe xid %u follows snapshot's xmin %u",
                         safeXid, snap->xmin);

        MyProc->xmin = snap->xmin;

        /* allocate in transaction context */
        newxip = (TransactionId *)
                palloc(sizeof(TransactionId) * GetMaxSnapshotXidCount());

        /*
         * snapbuild.c builds transactions in an "inverted" manner, which means it
         * stores committed transactions in ->xip, not ones in progress. Build a
         * classical snapshot by marking all non-committed transactions as
         * in-progress. This can be expensive.
         */
        for (xid = snap->xmin; NormalTransactionIdPrecedes(xid, snap->xmax);)
        {
                void       *test;

                /*
                 * Check whether transaction committed using the decoding snapshot
                 * meaning of ->xip.
                 */
                test = bsearch(&xid, snap->xip, snap->xcnt,
                                           sizeof(TransactionId), xidComparator);

                if (test == NULL)
                {
                        if (newxcnt >= GetMaxSnapshotXidCount())
                                ereport(ERROR,
                                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                                 errmsg("initial slot snapshot too large")));

                        newxip[newxcnt++] = xid;
                }

                TransactionIdAdvance(xid);
        }

        /* adjust remaining snapshot fields as needed */
        snap->snapshot_type = SNAPSHOT_MVCC;
        snap->xcnt = newxcnt;
        snap->xip = newxip;

        return snap;
}

/*
 * Export a snapshot so it can be set in another session with SET TRANSACTION
 * SNAPSHOT.
 *
 * For that we need to start a transaction in the current backend as the
 * importing side checks whether the source transaction is still open to make
 * sure the xmin horizon hasn't advanced since then.
 */
const char *
SnapBuildExportSnapshot(SnapBuild *builder)
{
        Snapshot        snap;
        char       *snapname;

        if (IsTransactionOrTransactionBlock())
                elog(ERROR, "cannot export a snapshot from within a transaction");

        if (SavedResourceOwnerDuringExport)
                elog(ERROR, "can only export one snapshot at a time");

        SavedResourceOwnerDuringExport = CurrentResourceOwner;
        ExportInProgress = true;

        StartTransactionCommand();

        /* There doesn't seem to a nice API to set these */
        XactIsoLevel = XACT_REPEATABLE_READ;
        XactReadOnly = true;

        snap = SnapBuildInitialSnapshot(builder);

        /*
         * now that we've built a plain snapshot, make it active and use the
         * normal mechanisms for exporting it
         */
        snapname = ExportSnapshot(snap);

        ereport(LOG,
                        (errmsg_plural("exported logical decoding snapshot: \"%s\" with %u transaction ID",
                                                   "exported logical decoding snapshot: \"%s\" with %u transaction IDs",
                                                   snap->xcnt,
                                                   snapname, snap->xcnt)));
        return snapname;
}

/*
 * Ensure there is a snapshot and if not build one for current transaction.
 */
Snapshot
SnapBuildGetOrBuildSnapshot(SnapBuild *builder)
{
        Assert(builder->state == SNAPBUILD_CONSISTENT);

        /* only build a new snapshot if we don't have a prebuilt one */
        if (builder->snapshot == NULL)
        {
                builder->snapshot = SnapBuildBuildSnapshot(builder);
                /* increase refcount for the snapshot builder */
                SnapBuildSnapIncRefcount(builder->snapshot);
        }

        return builder->snapshot;
}

/*
 * Reset a previously SnapBuildExportSnapshot()'ed snapshot if there is
 * any. Aborts the previously started transaction and resets the resource
 * owner back to its original value.
 */
void
SnapBuildClearExportedSnapshot(void)
{
        ResourceOwner tmpResOwner;

        /* nothing exported, that is the usual case */
        if (!ExportInProgress)
                return;

        if (!IsTransactionState())
                elog(ERROR, "clearing exported snapshot in wrong transaction state");

        /*
         * AbortCurrentTransaction() takes care of resetting the snapshot state,
         * so remember SavedResourceOwnerDuringExport.
         */
        tmpResOwner = SavedResourceOwnerDuringExport;

        /* make sure nothing could have ever happened */
        AbortCurrentTransaction();

        CurrentResourceOwner = tmpResOwner;
}

/*
 * Clear snapshot export state during transaction abort.
 */
void
SnapBuildResetExportedSnapshotState(void)
{
        SavedResourceOwnerDuringExport = NULL;
        ExportInProgress = false;
}

/*
 * Handle the effects of a single heap change, appropriate to the current state
 * of the snapshot builder and returns whether changes made at (xid, lsn) can
 * be decoded.
 */
bool
SnapBuildProcessChange(SnapBuild *builder, TransactionId xid, XLogRecPtr lsn)
{
        /*
         * We can't handle data in transactions if we haven't built a snapshot
         * yet, so don't store them.
         */
        if (builder->state < SNAPBUILD_FULL_SNAPSHOT)
                return false;

        /*
         * No point in keeping track of changes in transactions that we don't have
         * enough information about to decode. This means that they started before
         * we got into the SNAPBUILD_FULL_SNAPSHOT state.
         */
        if (builder->state < SNAPBUILD_CONSISTENT &&
                TransactionIdPrecedes(xid, builder->next_phase_at))
                return false;

        /*
         * If the reorderbuffer doesn't yet have a snapshot, add one now, it will
         * be needed to decode the change we're currently processing.
         */
        if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, xid))
        {
                /* only build a new snapshot if we don't have a prebuilt one */
                if (builder->snapshot == NULL)
                {
                        builder->snapshot = SnapBuildBuildSnapshot(builder);
                        /* increase refcount for the snapshot builder */
                        SnapBuildSnapIncRefcount(builder->snapshot);
                }

                /*
                 * Increase refcount for the transaction we're handing the snapshot
                 * out to.
                 */
                SnapBuildSnapIncRefcount(builder->snapshot);
                ReorderBufferSetBaseSnapshot(builder->reorder, xid, lsn,
                                                                         builder->snapshot);
        }

        return true;
}

/*
 * Do CommandId/combo CID handling after reading an xl_heap_new_cid record.
 * This implies that a transaction has done some form of write to system
 * catalogs.
 */
void
SnapBuildProcessNewCid(SnapBuild *builder, TransactionId xid,
                                           XLogRecPtr lsn, xl_heap_new_cid *xlrec)
{
        CommandId       cid;

        /*
         * we only log new_cid's if a catalog tuple was modified, so mark the
         * transaction as containing catalog modifications
         */
        ReorderBufferXidSetCatalogChanges(builder->reorder, xid, lsn);

        ReorderBufferAddNewTupleCids(builder->reorder, xlrec->top_xid, lsn,
                                                                 xlrec->target_locator, xlrec->target_tid,
                                                                 xlrec->cmin, xlrec->cmax,
                                                                 xlrec->combocid);

        /* figure out new command id */
        if (xlrec->cmin != InvalidCommandId &&
                xlrec->cmax != InvalidCommandId)
                cid = Max(xlrec->cmin, xlrec->cmax);
        else if (xlrec->cmax != InvalidCommandId)
                cid = xlrec->cmax;
        else if (xlrec->cmin != InvalidCommandId)
                cid = xlrec->cmin;
        else
        {
                cid = InvalidCommandId; /* silence compiler */
                elog(ERROR, "xl_heap_new_cid record without a valid CommandId");
        }

        ReorderBufferAddNewCommandId(builder->reorder, xid, lsn, cid + 1);
}

/*
 * Add a new Snapshot to all transactions we're decoding that currently are
 * in-progress so they can see new catalog contents made by the transaction
 * that just committed. This is necessary because those in-progress
 * transactions will use the new catalog's contents from here on (at the very
 * least everything they do needs to be compatible with newer catalog
 * contents).
 */
static void
SnapBuildDistributeNewCatalogSnapshot(SnapBuild *builder, XLogRecPtr lsn)
{
        dlist_iter      txn_i;
        ReorderBufferTXN *txn;

        /*
         * Iterate through all toplevel transactions. This can include
         * subtransactions which we just don't yet know to be that, but that's
         * fine, they will just get an unnecessary snapshot queued.
         */
        dlist_foreach(txn_i, &builder->reorder->toplevel_by_lsn)
        {
                txn = dlist_container(ReorderBufferTXN, node, txn_i.cur);

                Assert(TransactionIdIsValid(txn->xid));

                /*
                 * If we don't have a base snapshot yet, there are no changes in this
                 * transaction which in turn implies we don't yet need a snapshot at
                 * all. We'll add a snapshot when the first change gets queued.
                 *
                 * NB: This works correctly even for subtransactions because
                 * ReorderBufferAssignChild() takes care to transfer the base snapshot
                 * to the top-level transaction, and while iterating the changequeue
                 * we'll get the change from the subtxn.
                 */
                if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, txn->xid))
                        continue;

                /*
                 * We don't need to add snapshot to prepared transactions as they
                 * should not see the new catalog contents.
                 */
                if (rbtxn_prepared(txn) || rbtxn_skip_prepared(txn))
                        continue;

                elog(DEBUG2, "adding a new snapshot to %u at %X/%X",
                         txn->xid, LSN_FORMAT_ARGS(lsn));

                /*
                 * increase the snapshot's refcount for the transaction we are handing
                 * it out to
                 */
                SnapBuildSnapIncRefcount(builder->snapshot);
                ReorderBufferAddSnapshot(builder->reorder, txn->xid, lsn,
                                                                 builder->snapshot);
        }
}

/*
 * Keep track of a new catalog changing transaction that has committed.
 */
static void
SnapBuildAddCommittedTxn(SnapBuild *builder, TransactionId xid)
{
        Assert(TransactionIdIsValid(xid));

        if (builder->committed.xcnt == builder->committed.xcnt_space)
        {
                builder->committed.xcnt_space = builder->committed.xcnt_space * 2 + 1;

                elog(DEBUG1, "increasing space for committed transactions to %u",
                         (uint32) builder->committed.xcnt_space);

                builder->committed.xip = repalloc(builder->committed.xip,
                                                                                  builder->committed.xcnt_space * sizeof(TransactionId));
        }

        /*
         * TODO: It might make sense to keep the array sorted here instead of
         * doing it every time we build a new snapshot. On the other hand this
         * gets called repeatedly when a transaction with subtransactions commits.
         */
        builder->committed.xip[builder->committed.xcnt++] = xid;
}

/*
 * Remove knowledge about transactions we treat as committed or containing catalog
 * changes that are smaller than ->xmin. Those won't ever get checked via
 * the ->committed or ->catchange array, respectively. The committed xids will
 * get checked via the clog machinery.
 *
 * We can ideally remove the transaction from catchange array once it is
 * finished (committed/aborted) but that could be costly as we need to maintain
 * the xids order in the array.
 */
static void
SnapBuildPurgeOlderTxn(SnapBuild *builder)
{
        int                     off;
        TransactionId *workspace;
        int                     surviving_xids = 0;

        /* not ready yet */
        if (!TransactionIdIsNormal(builder->xmin))
                return;

        /* TODO: Neater algorithm than just copying and iterating? */
        workspace =
                MemoryContextAlloc(builder->context,
                                                   builder->committed.xcnt * sizeof(TransactionId));

        /* copy xids that still are interesting to workspace */
        for (off = 0; off < builder->committed.xcnt; off++)
        {
                if (NormalTransactionIdPrecedes(builder->committed.xip[off],
                                                                                builder->xmin))
                        ;                                       /* remove */
                else
                        workspace[surviving_xids++] = builder->committed.xip[off];
        }

        /* copy workspace back to persistent state */
        memcpy(builder->committed.xip, workspace,
                   surviving_xids * sizeof(TransactionId));

        elog(DEBUG3, "purged committed transactions from %u to %u, xmin: %u, xmax: %u",
                 (uint32) builder->committed.xcnt, (uint32) surviving_xids,
                 builder->xmin, builder->xmax);
        builder->committed.xcnt = surviving_xids;

        pfree(workspace);

        /*
         * Purge xids in ->catchange as well. The purged array must also be sorted
         * in xidComparator order.
         */
        if (builder->catchange.xcnt > 0)
        {
                /*
                 * Since catchange.xip is sorted, we find the lower bound of xids that
                 * are still interesting.
                 */
                for (off = 0; off < builder->catchange.xcnt; off++)
                {
                        if (TransactionIdFollowsOrEquals(builder->catchange.xip[off],
                                                                                         builder->xmin))
                                break;
                }

                surviving_xids = builder->catchange.xcnt - off;

                if (surviving_xids > 0)
                {
                        memmove(builder->catchange.xip, &(builder->catchange.xip[off]),
                                        surviving_xids * sizeof(TransactionId));
                }
                else
                {
                        pfree(builder->catchange.xip);
                        builder->catchange.xip = NULL;
                }

                elog(DEBUG3, "purged catalog modifying transactions from %u to %u, xmin: %u, xmax: %u",
                         (uint32) builder->catchange.xcnt, (uint32) surviving_xids,
                         builder->xmin, builder->xmax);
                builder->catchange.xcnt = surviving_xids;
        }
}

/*
 * Handle everything that needs to be done when a transaction commits
 */
void
SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
                                   int nsubxacts, TransactionId *subxacts, uint32 xinfo)
{
        int                     nxact;

        bool            needs_snapshot = false;
        bool            needs_timetravel = false;
        bool            sub_needs_timetravel = false;

        TransactionId xmax = xid;

        /*
         * Transactions preceding BUILDING_SNAPSHOT will neither be decoded, nor
         * will they be part of a snapshot.  So we don't need to record anything.
         */
        if (builder->state == SNAPBUILD_START ||
                (builder->state == SNAPBUILD_BUILDING_SNAPSHOT &&
                 TransactionIdPrecedes(xid, builder->next_phase_at)))
        {
                /* ensure that only commits after this are getting replayed */
                if (builder->start_decoding_at <= lsn)
                        builder->start_decoding_at = lsn + 1;
                return;
        }

        if (builder->state < SNAPBUILD_CONSISTENT)
        {
                /* ensure that only commits after this are getting replayed */
                if (builder->start_decoding_at <= lsn)
                        builder->start_decoding_at = lsn + 1;

                /*
                 * If building an exportable snapshot, force xid to be tracked, even
                 * if the transaction didn't modify the catalog.
                 */
                if (builder->building_full_snapshot)
                {
                        needs_timetravel = true;
                }
        }

        for (nxact = 0; nxact < nsubxacts; nxact++)
        {
                TransactionId subxid = subxacts[nxact];

                /*
                 * Add subtransaction to base snapshot if catalog modifying, we don't
                 * distinguish to toplevel transactions there.
                 */
                if (SnapBuildXidHasCatalogChanges(builder, subxid, xinfo))
                {
                        sub_needs_timetravel = true;
                        needs_snapshot = true;

                        elog(DEBUG1, "found subtransaction %u:%u with catalog changes",
                                 xid, subxid);

                        SnapBuildAddCommittedTxn(builder, subxid);

                        if (NormalTransactionIdFollows(subxid, xmax))
                                xmax = subxid;
                }

                /*
                 * If we're forcing timetravel we also need visibility information
                 * about subtransaction, so keep track of subtransaction's state, even
                 * if not catalog modifying.  Don't need to distribute a snapshot in
                 * that case.
                 */
                else if (needs_timetravel)
                {
                        SnapBuildAddCommittedTxn(builder, subxid);
                        if (NormalTransactionIdFollows(subxid, xmax))
                                xmax = subxid;
                }
        }

        /* if top-level modified catalog, it'll need a snapshot */
        if (SnapBuildXidHasCatalogChanges(builder, xid, xinfo))
        {
                elog(DEBUG2, "found top level transaction %u, with catalog changes",
                         xid);
                needs_snapshot = true;
                needs_timetravel = true;
                SnapBuildAddCommittedTxn(builder, xid);
        }
        else if (sub_needs_timetravel)
        {
                /* track toplevel txn as well, subxact alone isn't meaningful */
                elog(DEBUG2, "forced transaction %u to do timetravel due to one of its subtransactions",
                         xid);
                needs_timetravel = true;
                SnapBuildAddCommittedTxn(builder, xid);
        }
        else if (needs_timetravel)
        {
                elog(DEBUG2, "forced transaction %u to do timetravel", xid);

                SnapBuildAddCommittedTxn(builder, xid);
        }

        if (!needs_timetravel)
        {
                /* record that we cannot export a general snapshot anymore */
                builder->committed.includes_all_transactions = false;
        }

        Assert(!needs_snapshot || needs_timetravel);

        /*
         * Adjust xmax of the snapshot builder, we only do that for committed,
         * catalog modifying, transactions, everything else isn't interesting for
         * us since we'll never look at the respective rows.
         */
        if (needs_timetravel &&
                (!TransactionIdIsValid(builder->xmax) ||
                 TransactionIdFollowsOrEquals(xmax, builder->xmax)))
        {
                builder->xmax = xmax;
                TransactionIdAdvance(builder->xmax);
        }

        /* if there's any reason to build a historic snapshot, do so now */
        if (needs_snapshot)
        {
                /*
                 * If we haven't built a complete snapshot yet there's no need to hand
                 * it out, it wouldn't (and couldn't) be used anyway.
                 */
                if (builder->state < SNAPBUILD_FULL_SNAPSHOT)
                        return;

                /*
                 * Decrease the snapshot builder's refcount of the old snapshot, note
                 * that it still will be used if it has been handed out to the
                 * reorderbuffer earlier.
                 */
                if (builder->snapshot)
                        SnapBuildSnapDecRefcount(builder->snapshot);

                builder->snapshot = SnapBuildBuildSnapshot(builder);

                /* we might need to execute invalidations, add snapshot */
                if (!ReorderBufferXidHasBaseSnapshot(builder->reorder, xid))
                {
                        SnapBuildSnapIncRefcount(builder->snapshot);
                        ReorderBufferSetBaseSnapshot(builder->reorder, xid, lsn,
                                                                                 builder->snapshot);
                }

                /* refcount of the snapshot builder for the new snapshot */
                SnapBuildSnapIncRefcount(builder->snapshot);

                /* add a new catalog snapshot to all currently running transactions */
                SnapBuildDistributeNewCatalogSnapshot(builder, lsn);
        }
}

/*
 * Check the reorder buffer and the snapshot to see if the given transaction has
 * modified catalogs.
 */
static inline bool
SnapBuildXidHasCatalogChanges(SnapBuild *builder, TransactionId xid,
                                                          uint32 xinfo)
{
        if (ReorderBufferXidHasCatalogChanges(builder->reorder, xid))
                return true;

        /*
         * The transactions that have changed catalogs must have invalidation
         * info.
         */
        if (!(xinfo & XACT_XINFO_HAS_INVALS))
                return false;

        /* Check the catchange XID array */
        return ((builder->catchange.xcnt > 0) &&
                        (bsearch(&xid, builder->catchange.xip, builder->catchange.xcnt,
                                         sizeof(TransactionId), xidComparator) != NULL));
}

/* -----------------------------------
 * Snapshot building functions dealing with xlog records
 * -----------------------------------
 */

/*
 * Process a running xacts record, and use its information to first build a
 * historic snapshot and later to release resources that aren't needed
 * anymore.
 */
void
SnapBuildProcessRunningXacts(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *running)
{
        ReorderBufferTXN *txn;
        TransactionId xmin;

        /*
         * If we're not consistent yet, inspect the record to see whether it
         * allows to get closer to being consistent. If we are consistent, dump
         * our snapshot so others or we, after a restart, can use it.
         */
        if (builder->state < SNAPBUILD_CONSISTENT)
        {
                /* returns false if there's no point in performing cleanup just yet */
                if (!SnapBuildFindSnapshot(builder, lsn, running))
                        return;
        }
        else
                SnapBuildSerialize(builder, lsn);

        /*
         * Update range of interesting xids based on the running xacts
         * information. We don't increase ->xmax using it, because once we are in
         * a consistent state we can do that ourselves and much more efficiently
         * so, because we only need to do it for catalog transactions since we
         * only ever look at those.
         *
         * NB: We only increase xmax when a catalog modifying transaction commits
         * (see SnapBuildCommitTxn).  Because of this, xmax can be lower than
         * xmin, which looks odd but is correct and actually more efficient, since
         * we hit fast paths in heapam_visibility.c.
         */
        builder->xmin = running->oldestRunningXid;

        /* Remove transactions we don't need to keep track off anymore */
        SnapBuildPurgeOlderTxn(builder);

        /*
         * Advance the xmin limit for the current replication slot, to allow
         * vacuum to clean up the tuples this slot has been protecting.
         *
         * The reorderbuffer might have an xmin among the currently running
         * snapshots; use it if so.  If not, we need only consider the snapshots
         * we'll produce later, which can't be less than the oldest running xid in
         * the record we're reading now.
         */
        xmin = ReorderBufferGetOldestXmin(builder->reorder);
        if (xmin == InvalidTransactionId)
                xmin = running->oldestRunningXid;
        elog(DEBUG3, "xmin: %u, xmax: %u, oldest running: %u, oldest xmin: %u",
                 builder->xmin, builder->xmax, running->oldestRunningXid, xmin);
        LogicalIncreaseXminForSlot(lsn, xmin);

        /*
         * Also tell the slot where we can restart decoding from. We don't want to
         * do that after every commit because changing that implies an fsync of
         * the logical slot's state file, so we only do it every time we see a
         * running xacts record.
         *
         * Do so by looking for the oldest in progress transaction (determined by
         * the first LSN of any of its relevant records). Every transaction
         * remembers the last location we stored the snapshot to disk before its
         * beginning. That point is where we can restart from.
         */

        /*
         * Can't know about a serialized snapshot's location if we're not
         * consistent.
         */
        if (builder->state < SNAPBUILD_CONSISTENT)
                return;

        txn = ReorderBufferGetOldestTXN(builder->reorder);

        /*
         * oldest ongoing txn might have started when we didn't yet serialize
         * anything because we hadn't reached a consistent state yet.
         */
        if (txn != NULL && txn->restart_decoding_lsn != InvalidXLogRecPtr)
                LogicalIncreaseRestartDecodingForSlot(lsn, txn->restart_decoding_lsn);

        /*
         * No in-progress transaction, can reuse the last serialized snapshot if
         * we have one.
         */
        else if (txn == NULL &&
                         builder->reorder->current_restart_decoding_lsn != InvalidXLogRecPtr &&
                         builder->last_serialized_snapshot != InvalidXLogRecPtr)
                LogicalIncreaseRestartDecodingForSlot(lsn,
                                                                                          builder->last_serialized_snapshot);
}


/*
 * Build the start of a snapshot that's capable of decoding the catalog.
 *
 * Helper function for SnapBuildProcessRunningXacts() while we're not yet
 * consistent.
 *
 * Returns true if there is a point in performing internal maintenance/cleanup
 * using the xl_running_xacts record.
 */
static bool
SnapBuildFindSnapshot(SnapBuild *builder, XLogRecPtr lsn, xl_running_xacts *running)
{
        /* ---
         * Build catalog decoding snapshot incrementally using information about
         * the currently running transactions. There are several ways to do that:
         *
         * a) There were no running transactions when the xl_running_xacts record
         *        was inserted, jump to CONSISTENT immediately. We might find such a
         *        state while waiting on c)'s sub-states.
         *
         * b) This (in a previous run) or another decoding slot serialized a
         *        snapshot to disk that we can use. Can't use this method while finding
         *        the start point for decoding changes as the restart LSN would be an
         *        arbitrary LSN but we need to find the start point to extract changes
         *        where we won't see the data for partial transactions. Also, we cannot
         *        use this method when a slot needs a full snapshot for export or direct
         *        use, as that snapshot will only contain catalog modifying transactions.
         *
         * c) First incrementally build a snapshot for catalog tuples
         *        (BUILDING_SNAPSHOT), that requires all, already in-progress,
         *        transactions to finish.  Every transaction starting after that
         *        (FULL_SNAPSHOT state), has enough information to be decoded.  But
         *        for older running transactions no viable snapshot exists yet, so
         *        CONSISTENT will only be reached once all of those have finished.
         * ---
         */

        /*
         * xl_running_xacts record is older than what we can use, we might not
         * have all necessary catalog rows anymore.
         */
        if (TransactionIdIsNormal(builder->initial_xmin_horizon) &&
                NormalTransactionIdPrecedes(running->oldestRunningXid,
                                                                        builder->initial_xmin_horizon))
        {
                ereport(DEBUG1,
                                (errmsg_internal("skipping snapshot at %X/%X while building logical decoding snapshot, xmin horizon too low",
                                                                 LSN_FORMAT_ARGS(lsn)),
                                 errdetail_internal("initial xmin horizon of %u vs the snapshot's %u",
                                                                        builder->initial_xmin_horizon, running->oldestRunningXid)));


                SnapBuildWaitSnapshot(running, builder->initial_xmin_horizon);

                return true;
        }

        /*
         * a) No transaction were running, we can jump to consistent.
         *
         * This is not affected by races around xl_running_xacts, because we can
         * miss transaction commits, but currently not transactions starting.
         *
         * NB: We might have already started to incrementally assemble a snapshot,
         * so we need to be careful to deal with that.
         */
        if (running->oldestRunningXid == running->nextXid)
        {
                if (builder->start_decoding_at == InvalidXLogRecPtr ||
                        builder->start_decoding_at <= lsn)
                        /* can decode everything after this */
                        builder->start_decoding_at = lsn + 1;

                /* As no transactions were running xmin/xmax can be trivially set. */
                builder->xmin = running->nextXid;       /* < are finished */
                builder->xmax = running->nextXid;       /* >= are running */

                /* so we can safely use the faster comparisons */
                Assert(TransactionIdIsNormal(builder->xmin));
                Assert(TransactionIdIsNormal(builder->xmax));

                builder->state = SNAPBUILD_CONSISTENT;
                builder->next_phase_at = InvalidTransactionId;

                ereport(LOG,
                                (errmsg("logical decoding found consistent point at %X/%X",
                                                LSN_FORMAT_ARGS(lsn)),
                                 errdetail("There are no running transactions.")));

                return false;
        }

        /*
         * b) valid on disk state and while neither building full snapshot nor
         * creating a slot.
         */
        else if (!builder->building_full_snapshot &&
                         !builder->in_slot_creation &&
                         SnapBuildRestore(builder, lsn))
        {
                /* there won't be any state to cleanup */
                return false;
        }

        /*
         * c) transition from START to BUILDING_SNAPSHOT.
         *
         * In START state, and a xl_running_xacts record with running xacts is
         * encountered.  In that case, switch to BUILDING_SNAPSHOT state, and
         * record xl_running_xacts->nextXid.  Once all running xacts have finished
         * (i.e. they're all >= nextXid), we have a complete catalog snapshot.  It
         * might look that we could use xl_running_xacts's ->xids information to
         * get there quicker, but that is problematic because transactions marked
         * as running, might already have inserted their commit record - it's
         * infeasible to change that with locking.
         */
        else if (builder->state == SNAPBUILD_START)
        {
                builder->state = SNAPBUILD_BUILDING_SNAPSHOT;
                builder->next_phase_at = running->nextXid;

                /*
                 * Start with an xmin/xmax that's correct for future, when all the
                 * currently running transactions have finished. We'll update both
                 * while waiting for the pending transactions to finish.
                 */
                builder->xmin = running->nextXid;       /* < are finished */
                builder->xmax = running->nextXid;       /* >= are running */

                /* so we can safely use the faster comparisons */
                Assert(TransactionIdIsNormal(builder->xmin));
                Assert(TransactionIdIsNormal(builder->xmax));

                ereport(LOG,
                                (errmsg("logical decoding found initial starting point at %X/%X",
                                                LSN_FORMAT_ARGS(lsn)),
                                 errdetail("Waiting for transactions (approximately %d) older than %u to end.",
                                                   running->xcnt, running->nextXid)));

                SnapBuildWaitSnapshot(running, running->nextXid);
        }

        /*
         * c) transition from BUILDING_SNAPSHOT to FULL_SNAPSHOT.
         *
         * In BUILDING_SNAPSHOT state, and this xl_running_xacts' oldestRunningXid
         * is >= than nextXid from when we switched to BUILDING_SNAPSHOT.  This
         * means all transactions starting afterwards have enough information to
         * be decoded.  Switch to FULL_SNAPSHOT.
         */
        else if (builder->state == SNAPBUILD_BUILDING_SNAPSHOT &&
                         TransactionIdPrecedesOrEquals(builder->next_phase_at,
                                                                                   running->oldestRunningXid))
        {
                builder->state = SNAPBUILD_FULL_SNAPSHOT;
                builder->next_phase_at = running->nextXid;

                ereport(LOG,
                                (errmsg("logical decoding found initial consistent point at %X/%X",
                                                LSN_FORMAT_ARGS(lsn)),
                                 errdetail("Waiting for transactions (approximately %d) older than %u to end.",
                                                   running->xcnt, running->nextXid)));

                SnapBuildWaitSnapshot(running, running->nextXid);
        }

        /*
         * c) transition from FULL_SNAPSHOT to CONSISTENT.
         *
         * In FULL_SNAPSHOT state, and this xl_running_xacts' oldestRunningXid is
         * >= than nextXid from when we switched to FULL_SNAPSHOT.  This means all
         * transactions that are currently in progress have a catalog snapshot,
         * and all their changes have been collected.  Switch to CONSISTENT.
         */
        else if (builder->state == SNAPBUILD_FULL_SNAPSHOT &&
                         TransactionIdPrecedesOrEquals(builder->next_phase_at,
                                                                                   running->oldestRunningXid))
        {
                builder->state = SNAPBUILD_CONSISTENT;
                builder->next_phase_at = InvalidTransactionId;

                ereport(LOG,
                                (errmsg("logical decoding found consistent point at %X/%X",
                                                LSN_FORMAT_ARGS(lsn)),
                                 errdetail("There are no old transactions anymore.")));
        }

        /*
         * We already started to track running xacts and need to wait for all
         * in-progress ones to finish. We fall through to the normal processing of
         * records so incremental cleanup can be performed.
         */
        return true;
}

/* ---
 * Iterate through xids in record, wait for all older than the cutoff to
 * finish.  Then, if possible, log a new xl_running_xacts record.
 *
 * This isn't required for the correctness of decoding, but to:
 * a) allow isolationtester to notice that we're currently waiting for
 *        something.
 * b) log a new xl_running_xacts record where it'd be helpful, without having
 *        to wait for bgwriter or checkpointer.
 * ---
 */
static void
SnapBuildWaitSnapshot(xl_running_xacts *running, TransactionId cutoff)
{
        int                     off;

        for (off = 0; off < running->xcnt; off++)
        {
                TransactionId xid = running->xids[off];

                /*
                 * Upper layers should prevent that we ever need to wait on ourselves.
                 * Check anyway, since failing to do so would either result in an
                 * endless wait or an Assert() failure.
                 */
                if (TransactionIdIsCurrentTransactionId(xid))
                        elog(ERROR, "waiting for ourselves");

                if (TransactionIdFollows(xid, cutoff))
                        continue;

                XactLockTableWait(xid, NULL, NULL, XLTW_None);
        }

        /*
         * All transactions we needed to finish finished - try to ensure there is
         * another xl_running_xacts record in a timely manner, without having to
         * wait for bgwriter or checkpointer to log one.  During recovery we can't
         * enforce that, so we'll have to wait.
         */
        if (!RecoveryInProgress())
        {
                LogStandbySnapshot();
        }
}

#define SnapBuildOnDiskConstantSize \
        offsetof(SnapBuildOnDisk, builder)
#define SnapBuildOnDiskNotChecksummedSize \
        offsetof(SnapBuildOnDisk, version)

#define SNAPBUILD_MAGIC 0x51A1E001
#define SNAPBUILD_VERSION 6

/*
 * Store/Load a snapshot from disk, depending on the snapshot builder's state.
 *
 * Supposed to be used by external (i.e. not snapbuild.c) code that just read
 * a record that's a potential location for a serialized snapshot.
 */
void
SnapBuildSerializationPoint(SnapBuild *builder, XLogRecPtr lsn)
{
        if (builder->state < SNAPBUILD_CONSISTENT)
                SnapBuildRestore(builder, lsn);
        else
                SnapBuildSerialize(builder, lsn);
}

/*
 * Serialize the snapshot 'builder' at the location 'lsn' if it hasn't already
 * been done by another decoding process.
 */
static void
SnapBuildSerialize(SnapBuild *builder, XLogRecPtr lsn)
{
        Size            needed_length;
        SnapBuildOnDisk *ondisk = NULL;
        TransactionId *catchange_xip = NULL;
        MemoryContext old_ctx;
        size_t          catchange_xcnt;
        char       *ondisk_c;
        int                     fd;
        char            tmppath[MAXPGPATH];
        char            path[MAXPGPATH];
        int                     ret;
        struct stat stat_buf;
        Size            sz;

        Assert(lsn != InvalidXLogRecPtr);
        Assert(builder->last_serialized_snapshot == InvalidXLogRecPtr ||
                   builder->last_serialized_snapshot <= lsn);

        /*
         * no point in serializing if we cannot continue to work immediately after
         * restoring the snapshot
         */
        if (builder->state < SNAPBUILD_CONSISTENT)
                return;

        /* consistent snapshots have no next phase */
        Assert(builder->next_phase_at == InvalidTransactionId);

        /*
         * We identify snapshots by the LSN they are valid for. We don't need to
         * include timelines in the name as each LSN maps to exactly one timeline
         * unless the user used pg_resetwal or similar. If a user did so, there's
         * no hope continuing to decode anyway.
         */
        sprintf(path, "%s/%X-%X.snap",
                        PG_LOGICAL_SNAPSHOTS_DIR,
                        LSN_FORMAT_ARGS(lsn));

        /*
         * first check whether some other backend already has written the snapshot
         * for this LSN. It's perfectly fine if there's none, so we accept ENOENT
         * as a valid state. Everything else is an unexpected error.
         */
        ret = stat(path, &stat_buf);

        if (ret != 0 && errno != ENOENT)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not stat file \"%s\": %m", path)));

        else if (ret == 0)
        {
                /*
                 * somebody else has already serialized to this point, don't overwrite
                 * but remember location, so we don't need to read old data again.
                 *
                 * To be sure it has been synced to disk after the rename() from the
                 * tempfile filename to the real filename, we just repeat the fsync.
                 * That ought to be cheap because in most scenarios it should already
                 * be safely on disk.
                 */
                fsync_fname(path, false);
                fsync_fname(PG_LOGICAL_SNAPSHOTS_DIR, true);

                builder->last_serialized_snapshot = lsn;
                goto out;
        }

        /*
         * there is an obvious race condition here between the time we stat(2) the
         * file and us writing the file. But we rename the file into place
         * atomically and all files created need to contain the same data anyway,
         * so this is perfectly fine, although a bit of a resource waste. Locking
         * seems like pointless complication.
         */
        elog(DEBUG1, "serializing snapshot to %s", path);

        /* to make sure only we will write to this tempfile, include pid */
        sprintf(tmppath, "%s/%X-%X.snap.%d.tmp",
                        PG_LOGICAL_SNAPSHOTS_DIR,
                        LSN_FORMAT_ARGS(lsn), MyProcPid);

        /*
         * Unlink temporary file if it already exists, needs to have been before a
         * crash/error since we won't enter this function twice from within a
         * single decoding slot/backend and the temporary file contains the pid of
         * the current process.
         */
        if (unlink(tmppath) != 0 && errno != ENOENT)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not remove file \"%s\": %m", tmppath)));

        old_ctx = MemoryContextSwitchTo(builder->context);

        /* Get the catalog modifying transactions that are yet not committed */
        catchange_xip = ReorderBufferGetCatalogChangesXacts(builder->reorder);
        catchange_xcnt = dclist_count(&builder->reorder->catchange_txns);

        needed_length = sizeof(SnapBuildOnDisk) +
                sizeof(TransactionId) * (builder->committed.xcnt + catchange_xcnt);

        ondisk_c = palloc0(needed_length);
        ondisk = (SnapBuildOnDisk *) ondisk_c;
        ondisk->magic = SNAPBUILD_MAGIC;
        ondisk->version = SNAPBUILD_VERSION;
        ondisk->length = needed_length;
        INIT_CRC32C(ondisk->checksum);
        COMP_CRC32C(ondisk->checksum,
                                ((char *) ondisk) + SnapBuildOnDiskNotChecksummedSize,
                                SnapBuildOnDiskConstantSize - SnapBuildOnDiskNotChecksummedSize);
        ondisk_c += sizeof(SnapBuildOnDisk);

        memcpy(&ondisk->builder, builder, sizeof(SnapBuild));
        /* NULL-ify memory-only data */
        ondisk->builder.context = NULL;
        ondisk->builder.snapshot = NULL;
        ondisk->builder.reorder = NULL;
        ondisk->builder.committed.xip = NULL;
        ondisk->builder.catchange.xip = NULL;
        /* update catchange only on disk data */
        ondisk->builder.catchange.xcnt = catchange_xcnt;

        COMP_CRC32C(ondisk->checksum,
                                &ondisk->builder,
                                sizeof(SnapBuild));

        /* copy committed xacts */
        if (builder->committed.xcnt > 0)
        {
                sz = sizeof(TransactionId) * builder->committed.xcnt;
                memcpy(ondisk_c, builder->committed.xip, sz);
                COMP_CRC32C(ondisk->checksum, ondisk_c, sz);
                ondisk_c += sz;
        }

        /* copy catalog modifying xacts */
        if (catchange_xcnt > 0)
        {
                sz = sizeof(TransactionId) * catchange_xcnt;
                memcpy(ondisk_c, catchange_xip, sz);
                COMP_CRC32C(ondisk->checksum, ondisk_c, sz);
                ondisk_c += sz;
        }

        FIN_CRC32C(ondisk->checksum);

        /* we have valid data now, open tempfile and write it there */
        fd = OpenTransientFile(tmppath,
                                                   O_CREAT | O_EXCL | O_WRONLY | PG_BINARY);
        if (fd < 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", tmppath)));

        errno = 0;
        pgstat_report_wait_start(WAIT_EVENT_SNAPBUILD_WRITE);
        if ((write(fd, ondisk, needed_length)) != needed_length)
        {
                int                     save_errno = errno;

                CloseTransientFile(fd);

                /* if write didn't set errno, assume problem is no disk space */
                errno = save_errno ? save_errno : ENOSPC;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not write to file \"%s\": %m", tmppath)));
        }
        pgstat_report_wait_end();

        /*
         * fsync the file before renaming so that even if we crash after this we
         * have either a fully valid file or nothing.
         *
         * It's safe to just ERROR on fsync() here because we'll retry the whole
         * operation including the writes.
         *
         * TODO: Do the fsync() via checkpoints/restartpoints, doing it here has
         * some noticeable overhead since it's performed synchronously during
         * decoding?
         */
        pgstat_report_wait_start(WAIT_EVENT_SNAPBUILD_SYNC);
        if (pg_fsync(fd) != 0)
        {
                int                     save_errno = errno;

                CloseTransientFile(fd);
                errno = save_errno;
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not fsync file \"%s\": %m", tmppath)));
        }
        pgstat_report_wait_end();

        if (CloseTransientFile(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", tmppath)));

        fsync_fname(PG_LOGICAL_SNAPSHOTS_DIR, true);

        /*
         * We may overwrite the work from some other backend, but that's ok, our
         * snapshot is valid as well, we'll just have done some superfluous work.
         */
        if (rename(tmppath, path) != 0)
        {
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                                                tmppath, path)));
        }

        /* make sure we persist */
        fsync_fname(path, false);
        fsync_fname(PG_LOGICAL_SNAPSHOTS_DIR, true);

        /*
         * Now there's no way we can lose the dumped state anymore, remember this
         * as a serialization point.
         */
        builder->last_serialized_snapshot = lsn;

        MemoryContextSwitchTo(old_ctx);

out:
        ReorderBufferSetRestartPoint(builder->reorder,
                                                                 builder->last_serialized_snapshot);
        /* be tidy */
        if (ondisk)
                pfree(ondisk);
        if (catchange_xip)
                pfree(catchange_xip);
}

/*
 * Restore the logical snapshot file contents to 'ondisk'.
 *
 * 'context' is the memory context where the catalog modifying/committed xid
 * will live.
 * If 'missing_ok' is true, will not throw an error if the file is not found.
 */
bool
SnapBuildRestoreSnapshot(SnapBuildOnDisk *ondisk, const char *path,
                                                 MemoryContext context, bool missing_ok)
{
        int                     fd;
        pg_crc32c       checksum;
        Size            sz;

        fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);

        if (fd < 0)
        {
                if (missing_ok && errno == ENOENT)
                        return false;

                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not open file \"%s\": %m", path)));
        }

        /* ----
         * Make sure the snapshot had been stored safely to disk, that's normally
         * cheap.
         * Note that we do not need PANIC here, nobody will be able to use the
         * slot without fsyncing, and saving it won't succeed without an fsync()
         * either...
         * ----
         */
        fsync_fname(path, false);
        fsync_fname(PG_LOGICAL_SNAPSHOTS_DIR, true);

        /* read statically sized portion of snapshot */
        SnapBuildRestoreContents(fd, (char *) ondisk, SnapBuildOnDiskConstantSize, path);

        if (ondisk->magic != SNAPBUILD_MAGIC)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("snapbuild state file \"%s\" has wrong magic number: %u instead of %u",
                                                path, ondisk->magic, SNAPBUILD_MAGIC)));

        if (ondisk->version != SNAPBUILD_VERSION)
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("snapbuild state file \"%s\" has unsupported version: %u instead of %u",
                                                path, ondisk->version, SNAPBUILD_VERSION)));

        INIT_CRC32C(checksum);
        COMP_CRC32C(checksum,
                                ((char *) ondisk) + SnapBuildOnDiskNotChecksummedSize,
                                SnapBuildOnDiskConstantSize - SnapBuildOnDiskNotChecksummedSize);

        /* read SnapBuild */
        SnapBuildRestoreContents(fd, (char *) &ondisk->builder, sizeof(SnapBuild), path);
        COMP_CRC32C(checksum, &ondisk->builder, sizeof(SnapBuild));

        /* restore committed xacts information */
        if (ondisk->builder.committed.xcnt > 0)
        {
                sz = sizeof(TransactionId) * ondisk->builder.committed.xcnt;
                ondisk->builder.committed.xip = MemoryContextAllocZero(context, sz);
                SnapBuildRestoreContents(fd, (char *) ondisk->builder.committed.xip, sz, path);
                COMP_CRC32C(checksum, ondisk->builder.committed.xip, sz);
        }

        /* restore catalog modifying xacts information */
        if (ondisk->builder.catchange.xcnt > 0)
        {
                sz = sizeof(TransactionId) * ondisk->builder.catchange.xcnt;
                ondisk->builder.catchange.xip = MemoryContextAllocZero(context, sz);
                SnapBuildRestoreContents(fd, (char *) ondisk->builder.catchange.xip, sz, path);
                COMP_CRC32C(checksum, ondisk->builder.catchange.xip, sz);
        }

        if (CloseTransientFile(fd) != 0)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not close file \"%s\": %m", path)));

        FIN_CRC32C(checksum);

        /* verify checksum of what we've read */
        if (!EQ_CRC32C(checksum, ondisk->checksum))
                ereport(ERROR,
                                (errcode(ERRCODE_DATA_CORRUPTED),
                                 errmsg("checksum mismatch for snapbuild state file \"%s\": is %u, should be %u",
                                                path, checksum, ondisk->checksum)));

        return true;
}

/*
 * Restore a snapshot into 'builder' if previously one has been stored at the
 * location indicated by 'lsn'. Returns true if successful, false otherwise.
 */
static bool
SnapBuildRestore(SnapBuild *builder, XLogRecPtr lsn)
{
        SnapBuildOnDisk ondisk;
        char            path[MAXPGPATH];

        /* no point in loading a snapshot if we're already there */
        if (builder->state == SNAPBUILD_CONSISTENT)
                return false;

        sprintf(path, "%s/%X-%X.snap",
                        PG_LOGICAL_SNAPSHOTS_DIR,
                        LSN_FORMAT_ARGS(lsn));

        /* validate and restore the snapshot to 'ondisk' */
        if (!SnapBuildRestoreSnapshot(&ondisk, path, builder->context, true))
                return false;

        /*
         * ok, we now have a sensible snapshot here, figure out if it has more
         * information than we have.
         */

        /*
         * We are only interested in consistent snapshots for now, comparing
         * whether one incomplete snapshot is more "advanced" seems to be
         * unnecessarily complex.
         */
        if (ondisk.builder.state < SNAPBUILD_CONSISTENT)
                goto snapshot_not_interesting;

        /*
         * Don't use a snapshot that requires an xmin that we cannot guarantee to
         * be available.
         */
        if (TransactionIdPrecedes(ondisk.builder.xmin, builder->initial_xmin_horizon))
                goto snapshot_not_interesting;

        /*
         * Consistent snapshots have no next phase. Reset next_phase_at as it is
         * possible that an old value may remain.
         */
        Assert(ondisk.builder.next_phase_at == InvalidTransactionId);
        builder->next_phase_at = InvalidTransactionId;

        /* ok, we think the snapshot is sensible, copy over everything important */
        builder->xmin = ondisk.builder.xmin;
        builder->xmax = ondisk.builder.xmax;
        builder->state = ondisk.builder.state;

        builder->committed.xcnt = ondisk.builder.committed.xcnt;
        /* We only allocated/stored xcnt, not xcnt_space xids ! */
        /* don't overwrite preallocated xip, if we don't have anything here */
        if (builder->committed.xcnt > 0)
        {
                pfree(builder->committed.xip);
                builder->committed.xcnt_space = ondisk.builder.committed.xcnt;
                builder->committed.xip = ondisk.builder.committed.xip;
        }
        ondisk.builder.committed.xip = NULL;

        /* set catalog modifying transactions */
        if (builder->catchange.xip)
                pfree(builder->catchange.xip);
        builder->catchange.xcnt = ondisk.builder.catchange.xcnt;
        builder->catchange.xip = ondisk.builder.catchange.xip;
        ondisk.builder.catchange.xip = NULL;

        /* our snapshot is not interesting anymore, build a new one */
        if (builder->snapshot != NULL)
        {
                SnapBuildSnapDecRefcount(builder->snapshot);
        }
        builder->snapshot = SnapBuildBuildSnapshot(builder);
        SnapBuildSnapIncRefcount(builder->snapshot);

        ReorderBufferSetRestartPoint(builder->reorder, lsn);

        Assert(builder->state == SNAPBUILD_CONSISTENT);

        ereport(LOG,
                        (errmsg("logical decoding found consistent point at %X/%X",
                                        LSN_FORMAT_ARGS(lsn)),
                         errdetail("Logical decoding will begin using saved snapshot.")));
        return true;

snapshot_not_interesting:
        if (ondisk.builder.committed.xip != NULL)
                pfree(ondisk.builder.committed.xip);
        if (ondisk.builder.catchange.xip != NULL)
                pfree(ondisk.builder.catchange.xip);
        return false;
}

/*
 * Read the contents of the serialized snapshot to 'dest'.
 */
static void
SnapBuildRestoreContents(int fd, char *dest, Size size, const char *path)
{
        int                     readBytes;

        pgstat_report_wait_start(WAIT_EVENT_SNAPBUILD_READ);
        readBytes = read(fd, dest, size);
        pgstat_report_wait_end();
        if (readBytes != size)
        {
                int                     save_errno = errno;

                CloseTransientFile(fd);

                if (readBytes < 0)
                {
                        errno = save_errno;
                        ereport(ERROR,
                                        (errcode_for_file_access(),
                                         errmsg("could not read file \"%s\": %m", path)));
                }
                else
                        ereport(ERROR,
                                        (errcode(ERRCODE_DATA_CORRUPTED),
                                         errmsg("could not read file \"%s\": read %d of %zu",
                                                        path, readBytes, size)));
        }
}

/*
 * Remove all serialized snapshots that are not required anymore because no
 * slot can need them. This doesn't actually have to run during a checkpoint,
 * but it's a convenient point to schedule this.
 *
 * NB: We run this during checkpoints even if logical decoding is disabled so
 * we cleanup old slots at some point after it got disabled.
 */
void
CheckPointSnapBuild(void)
{
        XLogRecPtr      cutoff;
        XLogRecPtr      redo;
        DIR                *snap_dir;
        struct dirent *snap_de;
        char            path[MAXPGPATH + sizeof(PG_LOGICAL_SNAPSHOTS_DIR)];

        /*
         * We start off with a minimum of the last redo pointer. No new
         * replication slot will start before that, so that's a safe upper bound
         * for removal.
         */
        redo = GetRedoRecPtr();

        /* now check for the restart ptrs from existing slots */
        cutoff = ReplicationSlotsComputeLogicalRestartLSN();

        /* don't start earlier than the restart lsn */
        if (redo < cutoff)
                cutoff = redo;

        snap_dir = AllocateDir(PG_LOGICAL_SNAPSHOTS_DIR);
        while ((snap_de = ReadDir(snap_dir, PG_LOGICAL_SNAPSHOTS_DIR)) != NULL)
        {
                uint32          hi;
                uint32          lo;
                XLogRecPtr      lsn;
                PGFileType      de_type;

                if (strcmp(snap_de->d_name, ".") == 0 ||
                        strcmp(snap_de->d_name, "..") == 0)
                        continue;

                snprintf(path, sizeof(path), "%s/%s", PG_LOGICAL_SNAPSHOTS_DIR, snap_de->d_name);
                de_type = get_dirent_type(path, snap_de, false, DEBUG1);

                if (de_type != PGFILETYPE_ERROR && de_type != PGFILETYPE_REG)
                {
                        elog(DEBUG1, "only regular files expected: %s", path);
                        continue;
                }

                /*
                 * temporary filenames from SnapBuildSerialize() include the LSN and
                 * everything but are postfixed by .$pid.tmp. We can just remove them
                 * the same as other files because there can be none that are
                 * currently being written that are older than cutoff.
                 *
                 * We just log a message if a file doesn't fit the pattern, it's
                 * probably some editors lock/state file or similar...
                 */
                if (sscanf(snap_de->d_name, "%X-%X.snap", &hi, &lo) != 2)
                {
                        ereport(LOG,
                                        (errmsg("could not parse file name \"%s\"", path)));
                        continue;
                }

                lsn = ((uint64) hi) << 32 | lo;

                /* check whether we still need it */
                if (lsn < cutoff || cutoff == InvalidXLogRecPtr)
                {
                        elog(DEBUG1, "removing snapbuild snapshot %s", path);

                        /*
                         * It's not particularly harmful, though strange, if we can't
                         * remove the file here. Don't prevent the checkpoint from
                         * completing, that'd be a cure worse than the disease.
                         */
                        if (unlink(path) < 0)
                        {
                                ereport(LOG,
                                                (errcode_for_file_access(),
                                                 errmsg("could not remove file \"%s\": %m",
                                                                path)));
                                continue;
                        }
                }
        }
        FreeDir(snap_dir);
}

/*
 * Check if a logical snapshot at the specified point has been serialized.
 */
bool
SnapBuildSnapshotExists(XLogRecPtr lsn)
{
        char            path[MAXPGPATH];
        int                     ret;
        struct stat stat_buf;

        sprintf(path, "%s/%X-%X.snap",
                        PG_LOGICAL_SNAPSHOTS_DIR,
                        LSN_FORMAT_ARGS(lsn));

        ret = stat(path, &stat_buf);

        if (ret != 0 && errno != ENOENT)
                ereport(ERROR,
                                (errcode_for_file_access(),
                                 errmsg("could not stat file \"%s\": %m", path)));

        return ret == 0;
}