nbtree: fix read page recheck typo.

[pgsql.git] / src / backend / storage / ipc / shm_mq.c

/*-------------------------------------------------------------------------
 *
 * shm_mq.c
 *        single-reader, single-writer shared memory message queue
 *
 * Both the sender and the receiver must have a PGPROC; their respective
 * process latches are used for synchronization.  Only the sender may send,
 * and only the receiver may receive.  This is intended to allow a user
 * backend to communicate with worker backends that it has registered.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/storage/ipc/shm_mq.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "miscadmin.h"
#include "pgstat.h"
#include "port/pg_bitutils.h"
#include "postmaster/bgworker.h"
#include "storage/shm_mq.h"
#include "storage/spin.h"
#include "utils/memutils.h"

/*
 * This structure represents the actual queue, stored in shared memory.
 *
 * Some notes on synchronization:
 *
 * mq_receiver and mq_bytes_read can only be changed by the receiver; and
 * mq_sender and mq_bytes_written can only be changed by the sender.
 * mq_receiver and mq_sender are protected by mq_mutex, although, importantly,
 * they cannot change once set, and thus may be read without a lock once this
 * is known to be the case.
 *
 * mq_bytes_read and mq_bytes_written are not protected by the mutex.  Instead,
 * they are written atomically using 8 byte loads and stores.  Memory barriers
 * must be carefully used to synchronize reads and writes of these values with
 * reads and writes of the actual data in mq_ring.
 *
 * mq_detached needs no locking.  It can be set by either the sender or the
 * receiver, but only ever from false to true, so redundant writes don't
 * matter.  It is important that if we set mq_detached and then set the
 * counterparty's latch, the counterparty must be certain to see the change
 * after waking up.  Since SetLatch begins with a memory barrier and ResetLatch
 * ends with one, this should be OK.
 *
 * mq_ring_size and mq_ring_offset never change after initialization, and
 * can therefore be read without the lock.
 *
 * Importantly, mq_ring can be safely read and written without a lock.
 * At any given time, the difference between mq_bytes_read and
 * mq_bytes_written defines the number of bytes within mq_ring that contain
 * unread data, and mq_bytes_read defines the position where those bytes
 * begin.  The sender can increase the number of unread bytes at any time,
 * but only the receiver can give license to overwrite those bytes, by
 * incrementing mq_bytes_read.  Therefore, it's safe for the receiver to read
 * the unread bytes it knows to be present without the lock.  Conversely,
 * the sender can write to the unused portion of the ring buffer without
 * the lock, because nobody else can be reading or writing those bytes.  The
 * receiver could be making more bytes unused by incrementing mq_bytes_read,
 * but that's OK.  Note that it would be unsafe for the receiver to read any
 * data it's already marked as read, or to write any data; and it would be
 * unsafe for the sender to reread any data after incrementing
 * mq_bytes_written, but fortunately there's no need for any of that.
 */
struct shm_mq
{
        slock_t         mq_mutex;
        PGPROC     *mq_receiver;
        PGPROC     *mq_sender;
        pg_atomic_uint64 mq_bytes_read;
        pg_atomic_uint64 mq_bytes_written;
        Size            mq_ring_size;
        bool            mq_detached;
        uint8           mq_ring_offset;
        char            mq_ring[FLEXIBLE_ARRAY_MEMBER];
};

/*
 * This structure is a backend-private handle for access to a queue.
 *
 * mqh_queue is a pointer to the queue we've attached, and mqh_segment is
 * an optional pointer to the dynamic shared memory segment that contains it.
 * (If mqh_segment is provided, we register an on_dsm_detach callback to
 * make sure we detach from the queue before detaching from DSM.)
 *
 * If this queue is intended to connect the current process with a background
 * worker that started it, the user can pass a pointer to the worker handle
 * to shm_mq_attach(), and we'll store it in mqh_handle.  The point of this
 * is to allow us to begin sending to or receiving from that queue before the
 * process we'll be communicating with has even been started.  If it fails
 * to start, the handle will allow us to notice that and fail cleanly, rather
 * than waiting forever; see shm_mq_wait_internal.  This is mostly useful in
 * simple cases - e.g. where there are just 2 processes communicating; in
 * more complex scenarios, every process may not have a BackgroundWorkerHandle
 * available, or may need to watch for the failure of more than one other
 * process at a time.
 *
 * When a message exists as a contiguous chunk of bytes in the queue - that is,
 * it is smaller than the size of the ring buffer and does not wrap around
 * the end - we return the message to the caller as a pointer into the buffer.
 * For messages that are larger or happen to wrap, we reassemble the message
 * locally by copying the chunks into a backend-local buffer.  mqh_buffer is
 * the buffer, and mqh_buflen is the number of bytes allocated for it.
 *
 * mqh_send_pending, is number of bytes that is written to the queue but not
 * yet updated in the shared memory.  We will not update it until the written
 * data is 1/4th of the ring size or the tuple queue is full.  This will
 * prevent frequent CPU cache misses, and it will also avoid frequent
 * SetLatch() calls, which are quite expensive.
 *
 * mqh_partial_bytes, mqh_expected_bytes, and mqh_length_word_complete
 * are used to track the state of non-blocking operations.  When the caller
 * attempts a non-blocking operation that returns SHM_MQ_WOULD_BLOCK, they
 * are expected to retry the call at a later time with the same argument;
 * we need to retain enough state to pick up where we left off.
 * mqh_length_word_complete tracks whether we are done sending or receiving
 * (whichever we're doing) the entire length word.  mqh_partial_bytes tracks
 * the number of bytes read or written for either the length word or the
 * message itself, and mqh_expected_bytes - which is used only for reads -
 * tracks the expected total size of the payload.
 *
 * mqh_counterparty_attached tracks whether we know the counterparty to have
 * attached to the queue at some previous point.  This lets us avoid some
 * mutex acquisitions.
 *
 * mqh_context is the memory context in effect at the time we attached to
 * the shm_mq.  The shm_mq_handle itself is allocated in this context, and
 * we make sure any other allocations we do happen in this context as well,
 * to avoid nasty surprises.
 */
struct shm_mq_handle
{
        shm_mq     *mqh_queue;
        dsm_segment *mqh_segment;
        BackgroundWorkerHandle *mqh_handle;
        char       *mqh_buffer;
        Size            mqh_buflen;
        Size            mqh_consume_pending;
        Size            mqh_send_pending;
        Size            mqh_partial_bytes;
        Size            mqh_expected_bytes;
        bool            mqh_length_word_complete;
        bool            mqh_counterparty_attached;
        MemoryContext mqh_context;
};

static void shm_mq_detach_internal(shm_mq *mq);
static shm_mq_result shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes,
                                                                           const void *data, bool nowait, Size *bytes_written);
static shm_mq_result shm_mq_receive_bytes(shm_mq_handle *mqh,
                                                                                  Size bytes_needed, bool nowait, Size *nbytesp,
                                                                                  void **datap);
static bool shm_mq_counterparty_gone(shm_mq *mq,
                                                                         BackgroundWorkerHandle *handle);
static bool shm_mq_wait_internal(shm_mq *mq, PGPROC **ptr,
                                                                 BackgroundWorkerHandle *handle);
static void shm_mq_inc_bytes_read(shm_mq *mq, Size n);
static void shm_mq_inc_bytes_written(shm_mq *mq, Size n);
static void shm_mq_detach_callback(dsm_segment *seg, Datum arg);

/* Minimum queue size is enough for header and at least one chunk of data. */
const Size      shm_mq_minimum_size =
MAXALIGN(offsetof(shm_mq, mq_ring)) + MAXIMUM_ALIGNOF;

#define MQH_INITIAL_BUFSIZE                             8192

/*
 * Initialize a new shared message queue.
 */
shm_mq *
shm_mq_create(void *address, Size size)
{
        shm_mq     *mq = address;
        Size            data_offset = MAXALIGN(offsetof(shm_mq, mq_ring));

        /* If the size isn't MAXALIGN'd, just discard the odd bytes. */
        size = MAXALIGN_DOWN(size);

        /* Queue size must be large enough to hold some data. */
        Assert(size > data_offset);

        /* Initialize queue header. */
        SpinLockInit(&mq->mq_mutex);
        mq->mq_receiver = NULL;
        mq->mq_sender = NULL;
        pg_atomic_init_u64(&mq->mq_bytes_read, 0);
        pg_atomic_init_u64(&mq->mq_bytes_written, 0);
        mq->mq_ring_size = size - data_offset;
        mq->mq_detached = false;
        mq->mq_ring_offset = data_offset - offsetof(shm_mq, mq_ring);

        return mq;
}

/*
 * Set the identity of the process that will receive from a shared message
 * queue.
 */
void
shm_mq_set_receiver(shm_mq *mq, PGPROC *proc)
{
        PGPROC     *sender;

        SpinLockAcquire(&mq->mq_mutex);
        Assert(mq->mq_receiver == NULL);
        mq->mq_receiver = proc;
        sender = mq->mq_sender;
        SpinLockRelease(&mq->mq_mutex);

        if (sender != NULL)
                SetLatch(&sender->procLatch);
}

/*
 * Set the identity of the process that will send to a shared message queue.
 */
void
shm_mq_set_sender(shm_mq *mq, PGPROC *proc)
{
        PGPROC     *receiver;

        SpinLockAcquire(&mq->mq_mutex);
        Assert(mq->mq_sender == NULL);
        mq->mq_sender = proc;
        receiver = mq->mq_receiver;
        SpinLockRelease(&mq->mq_mutex);

        if (receiver != NULL)
                SetLatch(&receiver->procLatch);
}

/*
 * Get the configured receiver.
 */
PGPROC *
shm_mq_get_receiver(shm_mq *mq)
{
        PGPROC     *receiver;

        SpinLockAcquire(&mq->mq_mutex);
        receiver = mq->mq_receiver;
        SpinLockRelease(&mq->mq_mutex);

        return receiver;
}

/*
 * Get the configured sender.
 */
PGPROC *
shm_mq_get_sender(shm_mq *mq)
{
        PGPROC     *sender;

        SpinLockAcquire(&mq->mq_mutex);
        sender = mq->mq_sender;
        SpinLockRelease(&mq->mq_mutex);

        return sender;
}

/*
 * Attach to a shared message queue so we can send or receive messages.
 *
 * The memory context in effect at the time this function is called should
 * be one which will last for at least as long as the message queue itself.
 * We'll allocate the handle in that context, and future allocations that
 * are needed to buffer incoming data will happen in that context as well.
 *
 * If seg != NULL, the queue will be automatically detached when that dynamic
 * shared memory segment is detached.
 *
 * If handle != NULL, the queue can be read or written even before the
 * other process has attached.  We'll wait for it to do so if needed.  The
 * handle must be for a background worker initialized with bgw_notify_pid
 * equal to our PID.
 *
 * shm_mq_detach() should be called when done.  This will free the
 * shm_mq_handle and mark the queue itself as detached, so that our
 * counterpart won't get stuck waiting for us to fill or drain the queue
 * after we've already lost interest.
 */
shm_mq_handle *
shm_mq_attach(shm_mq *mq, dsm_segment *seg, BackgroundWorkerHandle *handle)
{
        shm_mq_handle *mqh = palloc(sizeof(shm_mq_handle));

        Assert(mq->mq_receiver == MyProc || mq->mq_sender == MyProc);
        mqh->mqh_queue = mq;
        mqh->mqh_segment = seg;
        mqh->mqh_handle = handle;
        mqh->mqh_buffer = NULL;
        mqh->mqh_buflen = 0;
        mqh->mqh_consume_pending = 0;
        mqh->mqh_send_pending = 0;
        mqh->mqh_partial_bytes = 0;
        mqh->mqh_expected_bytes = 0;
        mqh->mqh_length_word_complete = false;
        mqh->mqh_counterparty_attached = false;
        mqh->mqh_context = CurrentMemoryContext;

        if (seg != NULL)
                on_dsm_detach(seg, shm_mq_detach_callback, PointerGetDatum(mq));

        return mqh;
}

/*
 * Associate a BackgroundWorkerHandle with a shm_mq_handle just as if it had
 * been passed to shm_mq_attach.
 */
void
shm_mq_set_handle(shm_mq_handle *mqh, BackgroundWorkerHandle *handle)
{
        Assert(mqh->mqh_handle == NULL);
        mqh->mqh_handle = handle;
}

/*
 * Write a message into a shared message queue.
 */
shm_mq_result
shm_mq_send(shm_mq_handle *mqh, Size nbytes, const void *data, bool nowait,
                        bool force_flush)
{
        shm_mq_iovec iov;

        iov.data = data;
        iov.len = nbytes;

        return shm_mq_sendv(mqh, &iov, 1, nowait, force_flush);
}

/*
 * Write a message into a shared message queue, gathered from multiple
 * addresses.
 *
 * When nowait = false, we'll wait on our process latch when the ring buffer
 * fills up, and then continue writing once the receiver has drained some data.
 * The process latch is reset after each wait.
 *
 * When nowait = true, we do not manipulate the state of the process latch;
 * instead, if the buffer becomes full, we return SHM_MQ_WOULD_BLOCK.  In
 * this case, the caller should call this function again, with the same
 * arguments, each time the process latch is set.  (Once begun, the sending
 * of a message cannot be aborted except by detaching from the queue; changing
 * the length or payload will corrupt the queue.)
 *
 * When force_flush = true, we immediately update the shm_mq's mq_bytes_written
 * and notify the receiver (if it is already attached).  Otherwise, we don't
 * update it until we have written an amount of data greater than 1/4th of the
 * ring size.
 */
shm_mq_result
shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov, int iovcnt, bool nowait,
                         bool force_flush)
{
        shm_mq_result res;
        shm_mq     *mq = mqh->mqh_queue;
        PGPROC     *receiver;
        Size            nbytes = 0;
        Size            bytes_written;
        int                     i;
        int                     which_iov = 0;
        Size            offset;

        Assert(mq->mq_sender == MyProc);

        /* Compute total size of write. */
        for (i = 0; i < iovcnt; ++i)
                nbytes += iov[i].len;

        /* Prevent writing messages overwhelming the receiver. */
        if (nbytes > MaxAllocSize)
                ereport(ERROR,
                                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                                 errmsg("cannot send a message of size %zu via shared memory queue",
                                                nbytes)));

        /* Try to write, or finish writing, the length word into the buffer. */
        while (!mqh->mqh_length_word_complete)
        {
                Assert(mqh->mqh_partial_bytes < sizeof(Size));
                res = shm_mq_send_bytes(mqh, sizeof(Size) - mqh->mqh_partial_bytes,
                                                                ((char *) &nbytes) + mqh->mqh_partial_bytes,
                                                                nowait, &bytes_written);

                if (res == SHM_MQ_DETACHED)
                {
                        /* Reset state in case caller tries to send another message. */
                        mqh->mqh_partial_bytes = 0;
                        mqh->mqh_length_word_complete = false;
                        return res;
                }
                mqh->mqh_partial_bytes += bytes_written;

                if (mqh->mqh_partial_bytes >= sizeof(Size))
                {
                        Assert(mqh->mqh_partial_bytes == sizeof(Size));

                        mqh->mqh_partial_bytes = 0;
                        mqh->mqh_length_word_complete = true;
                }

                if (res != SHM_MQ_SUCCESS)
                        return res;

                /* Length word can't be split unless bigger than required alignment. */
                Assert(mqh->mqh_length_word_complete || sizeof(Size) > MAXIMUM_ALIGNOF);
        }

        /* Write the actual data bytes into the buffer. */
        Assert(mqh->mqh_partial_bytes <= nbytes);
        offset = mqh->mqh_partial_bytes;
        do
        {
                Size            chunksize;

                /* Figure out which bytes need to be sent next. */
                if (offset >= iov[which_iov].len)
                {
                        offset -= iov[which_iov].len;
                        ++which_iov;
                        if (which_iov >= iovcnt)
                                break;
                        continue;
                }

                /*
                 * We want to avoid copying the data if at all possible, but every
                 * chunk of bytes we write into the queue has to be MAXALIGN'd, except
                 * the last.  Thus, if a chunk other than the last one ends on a
                 * non-MAXALIGN'd boundary, we have to combine the tail end of its
                 * data with data from one or more following chunks until we either
                 * reach the last chunk or accumulate a number of bytes which is
                 * MAXALIGN'd.
                 */
                if (which_iov + 1 < iovcnt &&
                        offset + MAXIMUM_ALIGNOF > iov[which_iov].len)
                {
                        char            tmpbuf[MAXIMUM_ALIGNOF];
                        int                     j = 0;

                        for (;;)
                        {
                                if (offset < iov[which_iov].len)
                                {
                                        tmpbuf[j] = iov[which_iov].data[offset];
                                        j++;
                                        offset++;
                                        if (j == MAXIMUM_ALIGNOF)
                                                break;
                                }
                                else
                                {
                                        offset -= iov[which_iov].len;
                                        which_iov++;
                                        if (which_iov >= iovcnt)
                                                break;
                                }
                        }

                        res = shm_mq_send_bytes(mqh, j, tmpbuf, nowait, &bytes_written);

                        if (res == SHM_MQ_DETACHED)
                        {
                                /* Reset state in case caller tries to send another message. */
                                mqh->mqh_partial_bytes = 0;
                                mqh->mqh_length_word_complete = false;
                                return res;
                        }

                        mqh->mqh_partial_bytes += bytes_written;
                        if (res != SHM_MQ_SUCCESS)
                                return res;
                        continue;
                }

                /*
                 * If this is the last chunk, we can write all the data, even if it
                 * isn't a multiple of MAXIMUM_ALIGNOF.  Otherwise, we need to
                 * MAXALIGN_DOWN the write size.
                 */
                chunksize = iov[which_iov].len - offset;
                if (which_iov + 1 < iovcnt)
                        chunksize = MAXALIGN_DOWN(chunksize);
                res = shm_mq_send_bytes(mqh, chunksize, &iov[which_iov].data[offset],
                                                                nowait, &bytes_written);

                if (res == SHM_MQ_DETACHED)
                {
                        /* Reset state in case caller tries to send another message. */
                        mqh->mqh_length_word_complete = false;
                        mqh->mqh_partial_bytes = 0;
                        return res;
                }

                mqh->mqh_partial_bytes += bytes_written;
                offset += bytes_written;
                if (res != SHM_MQ_SUCCESS)
                        return res;
        } while (mqh->mqh_partial_bytes < nbytes);

        /* Reset for next message. */
        mqh->mqh_partial_bytes = 0;
        mqh->mqh_length_word_complete = false;

        /* If queue has been detached, let caller know. */
        if (mq->mq_detached)
                return SHM_MQ_DETACHED;

        /*
         * If the counterparty is known to have attached, we can read mq_receiver
         * without acquiring the spinlock.  Otherwise, more caution is needed.
         */
        if (mqh->mqh_counterparty_attached)
                receiver = mq->mq_receiver;
        else
        {
                SpinLockAcquire(&mq->mq_mutex);
                receiver = mq->mq_receiver;
                SpinLockRelease(&mq->mq_mutex);
                if (receiver != NULL)
                        mqh->mqh_counterparty_attached = true;
        }

        /*
         * If the caller has requested force flush or we have written more than
         * 1/4 of the ring size, mark it as written in shared memory and notify
         * the receiver.
         */
        if (force_flush || mqh->mqh_send_pending > (mq->mq_ring_size >> 2))
        {
                shm_mq_inc_bytes_written(mq, mqh->mqh_send_pending);
                if (receiver != NULL)
                        SetLatch(&receiver->procLatch);
                mqh->mqh_send_pending = 0;
        }

        return SHM_MQ_SUCCESS;
}

/*
 * Receive a message from a shared message queue.
 *
 * We set *nbytes to the message length and *data to point to the message
 * payload.  If the entire message exists in the queue as a single,
 * contiguous chunk, *data will point directly into shared memory; otherwise,
 * it will point to a temporary buffer.  This mostly avoids data copying in
 * the hoped-for case where messages are short compared to the buffer size,
 * while still allowing longer messages.  In either case, the return value
 * remains valid until the next receive operation is performed on the queue.
 *
 * When nowait = false, we'll wait on our process latch when the ring buffer
 * is empty and we have not yet received a full message.  The sender will
 * set our process latch after more data has been written, and we'll resume
 * processing.  Each call will therefore return a complete message
 * (unless the sender detaches the queue).
 *
 * When nowait = true, we do not manipulate the state of the process latch;
 * instead, whenever the buffer is empty and we need to read from it, we
 * return SHM_MQ_WOULD_BLOCK.  In this case, the caller should call this
 * function again after the process latch has been set.
 */
shm_mq_result
shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
{
        shm_mq     *mq = mqh->mqh_queue;
        shm_mq_result res;
        Size            rb = 0;
        Size            nbytes;
        void       *rawdata;

        Assert(mq->mq_receiver == MyProc);

        /* We can't receive data until the sender has attached. */
        if (!mqh->mqh_counterparty_attached)
        {
                if (nowait)
                {
                        int                     counterparty_gone;

                        /*
                         * We shouldn't return at this point at all unless the sender
                         * hasn't attached yet.  However, the correct return value depends
                         * on whether the sender is still attached.  If we first test
                         * whether the sender has ever attached and then test whether the
                         * sender has detached, there's a race condition: a sender that
                         * attaches and detaches very quickly might fool us into thinking
                         * the sender never attached at all.  So, test whether our
                         * counterparty is definitively gone first, and only afterwards
                         * check whether the sender ever attached in the first place.
                         */
                        counterparty_gone = shm_mq_counterparty_gone(mq, mqh->mqh_handle);
                        if (shm_mq_get_sender(mq) == NULL)
                        {
                                if (counterparty_gone)
                                        return SHM_MQ_DETACHED;
                                else
                                        return SHM_MQ_WOULD_BLOCK;
                        }
                }
                else if (!shm_mq_wait_internal(mq, &mq->mq_sender, mqh->mqh_handle)
                                 && shm_mq_get_sender(mq) == NULL)
                {
                        mq->mq_detached = true;
                        return SHM_MQ_DETACHED;
                }
                mqh->mqh_counterparty_attached = true;
        }

        /*
         * If we've consumed an amount of data greater than 1/4th of the ring
         * size, mark it consumed in shared memory.  We try to avoid doing this
         * unnecessarily when only a small amount of data has been consumed,
         * because SetLatch() is fairly expensive and we don't want to do it too
         * often.
         */
        if (mqh->mqh_consume_pending > mq->mq_ring_size / 4)
        {
                shm_mq_inc_bytes_read(mq, mqh->mqh_consume_pending);
                mqh->mqh_consume_pending = 0;
        }

        /* Try to read, or finish reading, the length word from the buffer. */
        while (!mqh->mqh_length_word_complete)
        {
                /* Try to receive the message length word. */
                Assert(mqh->mqh_partial_bytes < sizeof(Size));
                res = shm_mq_receive_bytes(mqh, sizeof(Size) - mqh->mqh_partial_bytes,
                                                                   nowait, &rb, &rawdata);
                if (res != SHM_MQ_SUCCESS)
                        return res;

                /*
                 * Hopefully, we'll receive the entire message length word at once.
                 * But if sizeof(Size) > MAXIMUM_ALIGNOF, then it might be split over
                 * multiple reads.
                 */
                if (mqh->mqh_partial_bytes == 0 && rb >= sizeof(Size))
                {
                        Size            needed;

                        nbytes = *(Size *) rawdata;

                        /* If we've already got the whole message, we're done. */
                        needed = MAXALIGN(sizeof(Size)) + MAXALIGN(nbytes);
                        if (rb >= needed)
                        {
                                mqh->mqh_consume_pending += needed;
                                *nbytesp = nbytes;
                                *datap = ((char *) rawdata) + MAXALIGN(sizeof(Size));
                                return SHM_MQ_SUCCESS;
                        }

                        /*
                         * We don't have the whole message, but we at least have the whole
                         * length word.
                         */
                        mqh->mqh_expected_bytes = nbytes;
                        mqh->mqh_length_word_complete = true;
                        mqh->mqh_consume_pending += MAXALIGN(sizeof(Size));
                        rb -= MAXALIGN(sizeof(Size));
                }
                else
                {
                        Size            lengthbytes;

                        /* Can't be split unless bigger than required alignment. */
                        Assert(sizeof(Size) > MAXIMUM_ALIGNOF);

                        /* Message word is split; need buffer to reassemble. */
                        if (mqh->mqh_buffer == NULL)
                        {
                                mqh->mqh_buffer = MemoryContextAlloc(mqh->mqh_context,
                                                                                                         MQH_INITIAL_BUFSIZE);
                                mqh->mqh_buflen = MQH_INITIAL_BUFSIZE;
                        }
                        Assert(mqh->mqh_buflen >= sizeof(Size));

                        /* Copy partial length word; remember to consume it. */
                        if (mqh->mqh_partial_bytes + rb > sizeof(Size))
                                lengthbytes = sizeof(Size) - mqh->mqh_partial_bytes;
                        else
                                lengthbytes = rb;
                        memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata,
                                   lengthbytes);
                        mqh->mqh_partial_bytes += lengthbytes;
                        mqh->mqh_consume_pending += MAXALIGN(lengthbytes);
                        rb -= lengthbytes;

                        /* If we now have the whole word, we're ready to read payload. */
                        if (mqh->mqh_partial_bytes >= sizeof(Size))
                        {
                                Assert(mqh->mqh_partial_bytes == sizeof(Size));
                                mqh->mqh_expected_bytes = *(Size *) mqh->mqh_buffer;
                                mqh->mqh_length_word_complete = true;
                                mqh->mqh_partial_bytes = 0;
                        }
                }
        }
        nbytes = mqh->mqh_expected_bytes;

        /*
         * Should be disallowed on the sending side already, but better check and
         * error out on the receiver side as well rather than trying to read a
         * prohibitively large message.
         */
        if (nbytes > MaxAllocSize)
                ereport(ERROR,
                                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                                 errmsg("invalid message size %zu in shared memory queue",
                                                nbytes)));

        if (mqh->mqh_partial_bytes == 0)
        {
                /*
                 * Try to obtain the whole message in a single chunk.  If this works,
                 * we need not copy the data and can return a pointer directly into
                 * shared memory.
                 */
                res = shm_mq_receive_bytes(mqh, nbytes, nowait, &rb, &rawdata);
                if (res != SHM_MQ_SUCCESS)
                        return res;
                if (rb >= nbytes)
                {
                        mqh->mqh_length_word_complete = false;
                        mqh->mqh_consume_pending += MAXALIGN(nbytes);
                        *nbytesp = nbytes;
                        *datap = rawdata;
                        return SHM_MQ_SUCCESS;
                }

                /*
                 * The message has wrapped the buffer.  We'll need to copy it in order
                 * to return it to the client in one chunk.  First, make sure we have
                 * a large enough buffer available.
                 */
                if (mqh->mqh_buflen < nbytes)
                {
                        Size            newbuflen;

                        /*
                         * Increase size to the next power of 2 that's >= nbytes, but
                         * limit to MaxAllocSize.
                         */
                        newbuflen = pg_nextpower2_size_t(nbytes);
                        newbuflen = Min(newbuflen, MaxAllocSize);

                        if (mqh->mqh_buffer != NULL)
                        {
                                pfree(mqh->mqh_buffer);
                                mqh->mqh_buffer = NULL;
                                mqh->mqh_buflen = 0;
                        }
                        mqh->mqh_buffer = MemoryContextAlloc(mqh->mqh_context, newbuflen);
                        mqh->mqh_buflen = newbuflen;
                }
        }

        /* Loop until we've copied the entire message. */
        for (;;)
        {
                Size            still_needed;

                /* Copy as much as we can. */
                Assert(mqh->mqh_partial_bytes + rb <= nbytes);
                if (rb > 0)
                {
                        memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata, rb);
                        mqh->mqh_partial_bytes += rb;
                }

                /*
                 * Update count of bytes that can be consumed, accounting for
                 * alignment padding.  Note that this will never actually insert any
                 * padding except at the end of a message, because the buffer size is
                 * a multiple of MAXIMUM_ALIGNOF, and each read and write is as well.
                 */
                Assert(mqh->mqh_partial_bytes == nbytes || rb == MAXALIGN(rb));
                mqh->mqh_consume_pending += MAXALIGN(rb);

                /* If we got all the data, exit the loop. */
                if (mqh->mqh_partial_bytes >= nbytes)
                        break;

                /* Wait for some more data. */
                still_needed = nbytes - mqh->mqh_partial_bytes;
                res = shm_mq_receive_bytes(mqh, still_needed, nowait, &rb, &rawdata);
                if (res != SHM_MQ_SUCCESS)
                        return res;
                if (rb > still_needed)
                        rb = still_needed;
        }

        /* Return the complete message, and reset for next message. */
        *nbytesp = nbytes;
        *datap = mqh->mqh_buffer;
        mqh->mqh_length_word_complete = false;
        mqh->mqh_partial_bytes = 0;
        return SHM_MQ_SUCCESS;
}

/*
 * Wait for the other process that's supposed to use this queue to attach
 * to it.
 *
 * The return value is SHM_MQ_DETACHED if the worker has already detached or
 * if it dies; it is SHM_MQ_SUCCESS if we detect that the worker has attached.
 * Note that we will only be able to detect that the worker has died before
 * attaching if a background worker handle was passed to shm_mq_attach().
 */
shm_mq_result
shm_mq_wait_for_attach(shm_mq_handle *mqh)
{
        shm_mq     *mq = mqh->mqh_queue;
        PGPROC    **victim;

        if (shm_mq_get_receiver(mq) == MyProc)
                victim = &mq->mq_sender;
        else
        {
                Assert(shm_mq_get_sender(mq) == MyProc);
                victim = &mq->mq_receiver;
        }

        if (shm_mq_wait_internal(mq, victim, mqh->mqh_handle))
                return SHM_MQ_SUCCESS;
        else
                return SHM_MQ_DETACHED;
}

/*
 * Detach from a shared message queue, and destroy the shm_mq_handle.
 */
void
shm_mq_detach(shm_mq_handle *mqh)
{
        /* Before detaching, notify the receiver about any already-written data. */
        if (mqh->mqh_send_pending > 0)
        {
                shm_mq_inc_bytes_written(mqh->mqh_queue, mqh->mqh_send_pending);
                mqh->mqh_send_pending = 0;
        }

        /* Notify counterparty that we're outta here. */
        shm_mq_detach_internal(mqh->mqh_queue);

        /* Cancel on_dsm_detach callback, if any. */
        if (mqh->mqh_segment)
                cancel_on_dsm_detach(mqh->mqh_segment,
                                                         shm_mq_detach_callback,
                                                         PointerGetDatum(mqh->mqh_queue));

        /* Release local memory associated with handle. */
        if (mqh->mqh_buffer != NULL)
                pfree(mqh->mqh_buffer);
        pfree(mqh);
}

/*
 * Notify counterparty that we're detaching from shared message queue.
 *
 * The purpose of this function is to make sure that the process
 * with which we're communicating doesn't block forever waiting for us to
 * fill or drain the queue once we've lost interest.  When the sender
 * detaches, the receiver can read any messages remaining in the queue;
 * further reads will return SHM_MQ_DETACHED.  If the receiver detaches,
 * further attempts to send messages will likewise return SHM_MQ_DETACHED.
 *
 * This is separated out from shm_mq_detach() because if the on_dsm_detach
 * callback fires, we only want to do this much.  We do not try to touch
 * the local shm_mq_handle, as it may have been pfree'd already.
 */
static void
shm_mq_detach_internal(shm_mq *mq)
{
        PGPROC     *victim;

        SpinLockAcquire(&mq->mq_mutex);
        if (mq->mq_sender == MyProc)
                victim = mq->mq_receiver;
        else
        {
                Assert(mq->mq_receiver == MyProc);
                victim = mq->mq_sender;
        }
        mq->mq_detached = true;
        SpinLockRelease(&mq->mq_mutex);

        if (victim != NULL)
                SetLatch(&victim->procLatch);
}

/*
 * Get the shm_mq from handle.
 */
shm_mq *
shm_mq_get_queue(shm_mq_handle *mqh)
{
        return mqh->mqh_queue;
}

/*
 * Write bytes into a shared message queue.
 */
static shm_mq_result
shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
                                  bool nowait, Size *bytes_written)
{
        shm_mq     *mq = mqh->mqh_queue;
        Size            sent = 0;
        uint64          used;
        Size            ringsize = mq->mq_ring_size;
        Size            available;

        while (sent < nbytes)
        {
                uint64          rb;
                uint64          wb;

                /* Compute number of ring buffer bytes used and available. */
                rb = pg_atomic_read_u64(&mq->mq_bytes_read);
                wb = pg_atomic_read_u64(&mq->mq_bytes_written) + mqh->mqh_send_pending;
                Assert(wb >= rb);
                used = wb - rb;
                Assert(used <= ringsize);
                available = Min(ringsize - used, nbytes - sent);

                /*
                 * Bail out if the queue has been detached.  Note that we would be in
                 * trouble if the compiler decided to cache the value of
                 * mq->mq_detached in a register or on the stack across loop
                 * iterations.  It probably shouldn't do that anyway since we'll
                 * always return, call an external function that performs a system
                 * call, or reach a memory barrier at some point later in the loop,
                 * but just to be sure, insert a compiler barrier here.
                 */
                pg_compiler_barrier();
                if (mq->mq_detached)
                {
                        *bytes_written = sent;
                        return SHM_MQ_DETACHED;
                }

                if (available == 0 && !mqh->mqh_counterparty_attached)
                {
                        /*
                         * The queue is full, so if the receiver isn't yet known to be
                         * attached, we must wait for that to happen.
                         */
                        if (nowait)
                        {
                                if (shm_mq_counterparty_gone(mq, mqh->mqh_handle))
                                {
                                        *bytes_written = sent;
                                        return SHM_MQ_DETACHED;
                                }
                                if (shm_mq_get_receiver(mq) == NULL)
                                {
                                        *bytes_written = sent;
                                        return SHM_MQ_WOULD_BLOCK;
                                }
                        }
                        else if (!shm_mq_wait_internal(mq, &mq->mq_receiver,
                                                                                   mqh->mqh_handle))
                        {
                                mq->mq_detached = true;
                                *bytes_written = sent;
                                return SHM_MQ_DETACHED;
                        }
                        mqh->mqh_counterparty_attached = true;

                        /*
                         * The receiver may have read some data after attaching, so we
                         * must not wait without rechecking the queue state.
                         */
                }
                else if (available == 0)
                {
                        /* Update the pending send bytes in the shared memory. */
                        shm_mq_inc_bytes_written(mq, mqh->mqh_send_pending);

                        /*
                         * Since mq->mqh_counterparty_attached is known to be true at this
                         * point, mq_receiver has been set, and it can't change once set.
                         * Therefore, we can read it without acquiring the spinlock.
                         */
                        Assert(mqh->mqh_counterparty_attached);
                        SetLatch(&mq->mq_receiver->procLatch);

                        /*
                         * We have just updated the mqh_send_pending bytes in the shared
                         * memory so reset it.
                         */
                        mqh->mqh_send_pending = 0;

                        /* Skip manipulation of our latch if nowait = true. */
                        if (nowait)
                        {
                                *bytes_written = sent;
                                return SHM_MQ_WOULD_BLOCK;
                        }

                        /*
                         * Wait for our latch to be set.  It might already be set for some
                         * unrelated reason, but that'll just result in one extra trip
                         * through the loop.  It's worth it to avoid resetting the latch
                         * at top of loop, because setting an already-set latch is much
                         * cheaper than setting one that has been reset.
                         */
                        (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                                                         WAIT_EVENT_MESSAGE_QUEUE_SEND);

                        /* Reset the latch so we don't spin. */
                        ResetLatch(MyLatch);

                        /* An interrupt may have occurred while we were waiting. */
                        CHECK_FOR_INTERRUPTS();
                }
                else
                {
                        Size            offset;
                        Size            sendnow;

                        offset = wb % (uint64) ringsize;
                        sendnow = Min(available, ringsize - offset);

                        /*
                         * Write as much data as we can via a single memcpy(). Make sure
                         * these writes happen after the read of mq_bytes_read, above.
                         * This barrier pairs with the one in shm_mq_inc_bytes_read.
                         * (Since we're separating the read of mq_bytes_read from a
                         * subsequent write to mq_ring, we need a full barrier here.)
                         */
                        pg_memory_barrier();
                        memcpy(&mq->mq_ring[mq->mq_ring_offset + offset],
                                   (char *) data + sent, sendnow);
                        sent += sendnow;

                        /*
                         * Update count of bytes written, with alignment padding.  Note
                         * that this will never actually insert any padding except at the
                         * end of a run of bytes, because the buffer size is a multiple of
                         * MAXIMUM_ALIGNOF, and each read is as well.
                         */
                        Assert(sent == nbytes || sendnow == MAXALIGN(sendnow));

                        /*
                         * For efficiency, we don't update the bytes written in the shared
                         * memory and also don't set the reader's latch here.  Refer to
                         * the comments atop the shm_mq_handle structure for more
                         * information.
                         */
                        mqh->mqh_send_pending += MAXALIGN(sendnow);
                }
        }

        *bytes_written = sent;
        return SHM_MQ_SUCCESS;
}

/*
 * Wait until at least *nbytesp bytes are available to be read from the
 * shared message queue, or until the buffer wraps around.  If the queue is
 * detached, returns SHM_MQ_DETACHED.  If nowait is specified and a wait
 * would be required, returns SHM_MQ_WOULD_BLOCK.  Otherwise, *datap is set
 * to the location at which data bytes can be read, *nbytesp is set to the
 * number of bytes which can be read at that address, and the return value
 * is SHM_MQ_SUCCESS.
 */
static shm_mq_result
shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait,
                                         Size *nbytesp, void **datap)
{
        shm_mq     *mq = mqh->mqh_queue;
        Size            ringsize = mq->mq_ring_size;
        uint64          used;
        uint64          written;

        for (;;)
        {
                Size            offset;
                uint64          read;

                /* Get bytes written, so we can compute what's available to read. */
                written = pg_atomic_read_u64(&mq->mq_bytes_written);

                /*
                 * Get bytes read.  Include bytes we could consume but have not yet
                 * consumed.
                 */
                read = pg_atomic_read_u64(&mq->mq_bytes_read) +
                        mqh->mqh_consume_pending;
                used = written - read;
                Assert(used <= ringsize);
                offset = read % (uint64) ringsize;

                /* If we have enough data or buffer has wrapped, we're done. */
                if (used >= bytes_needed || offset + used >= ringsize)
                {
                        *nbytesp = Min(used, ringsize - offset);
                        *datap = &mq->mq_ring[mq->mq_ring_offset + offset];

                        /*
                         * Separate the read of mq_bytes_written, above, from caller's
                         * attempt to read the data itself.  Pairs with the barrier in
                         * shm_mq_inc_bytes_written.
                         */
                        pg_read_barrier();
                        return SHM_MQ_SUCCESS;
                }

                /*
                 * Fall out before waiting if the queue has been detached.
                 *
                 * Note that we don't check for this until *after* considering whether
                 * the data already available is enough, since the receiver can finish
                 * receiving a message stored in the buffer even after the sender has
                 * detached.
                 */
                if (mq->mq_detached)
                {
                        /*
                         * If the writer advanced mq_bytes_written and then set
                         * mq_detached, we might not have read the final value of
                         * mq_bytes_written above.  Insert a read barrier and then check
                         * again if mq_bytes_written has advanced.
                         */
                        pg_read_barrier();
                        if (written != pg_atomic_read_u64(&mq->mq_bytes_written))
                                continue;

                        return SHM_MQ_DETACHED;
                }

                /*
                 * We didn't get enough data to satisfy the request, so mark any data
                 * previously-consumed as read to make more buffer space.
                 */
                if (mqh->mqh_consume_pending > 0)
                {
                        shm_mq_inc_bytes_read(mq, mqh->mqh_consume_pending);
                        mqh->mqh_consume_pending = 0;
                }

                /* Skip manipulation of our latch if nowait = true. */
                if (nowait)
                        return SHM_MQ_WOULD_BLOCK;

                /*
                 * Wait for our latch to be set.  It might already be set for some
                 * unrelated reason, but that'll just result in one extra trip through
                 * the loop.  It's worth it to avoid resetting the latch at top of
                 * loop, because setting an already-set latch is much cheaper than
                 * setting one that has been reset.
                 */
                (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                                                 WAIT_EVENT_MESSAGE_QUEUE_RECEIVE);

                /* Reset the latch so we don't spin. */
                ResetLatch(MyLatch);

                /* An interrupt may have occurred while we were waiting. */
                CHECK_FOR_INTERRUPTS();
        }
}

/*
 * Test whether a counterparty who may not even be alive yet is definitely gone.
 */
static bool
shm_mq_counterparty_gone(shm_mq *mq, BackgroundWorkerHandle *handle)
{
        pid_t           pid;

        /* If the queue has been detached, counterparty is definitely gone. */
        if (mq->mq_detached)
                return true;

        /* If there's a handle, check worker status. */
        if (handle != NULL)
        {
                BgwHandleStatus status;

                /* Check for unexpected worker death. */
                status = GetBackgroundWorkerPid(handle, &pid);
                if (status != BGWH_STARTED && status != BGWH_NOT_YET_STARTED)
                {
                        /* Mark it detached, just to make it official. */
                        mq->mq_detached = true;
                        return true;
                }
        }

        /* Counterparty is not definitively gone. */
        return false;
}

/*
 * This is used when a process is waiting for its counterpart to attach to the
 * queue.  We exit when the other process attaches as expected, or, if
 * handle != NULL, when the referenced background process or the postmaster
 * dies.  Note that if handle == NULL, and the process fails to attach, we'll
 * potentially get stuck here forever waiting for a process that may never
 * start.  We do check for interrupts, though.
 *
 * ptr is a pointer to the memory address that we're expecting to become
 * non-NULL when our counterpart attaches to the queue.
 */
static bool
shm_mq_wait_internal(shm_mq *mq, PGPROC **ptr, BackgroundWorkerHandle *handle)
{
        bool            result = false;

        for (;;)
        {
                BgwHandleStatus status;
                pid_t           pid;

                /* Acquire the lock just long enough to check the pointer. */
                SpinLockAcquire(&mq->mq_mutex);
                result = (*ptr != NULL);
                SpinLockRelease(&mq->mq_mutex);

                /* Fail if detached; else succeed if initialized. */
                if (mq->mq_detached)
                {
                        result = false;
                        break;
                }
                if (result)
                        break;

                if (handle != NULL)
                {
                        /* Check for unexpected worker death. */
                        status = GetBackgroundWorkerPid(handle, &pid);
                        if (status != BGWH_STARTED && status != BGWH_NOT_YET_STARTED)
                        {
                                result = false;
                                break;
                        }
                }

                /* Wait to be signaled. */
                (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                                                 WAIT_EVENT_MESSAGE_QUEUE_INTERNAL);

                /* Reset the latch so we don't spin. */
                ResetLatch(MyLatch);

                /* An interrupt may have occurred while we were waiting. */
                CHECK_FOR_INTERRUPTS();
        }

        return result;
}

/*
 * Increment the number of bytes read.
 */
static void
shm_mq_inc_bytes_read(shm_mq *mq, Size n)
{
        PGPROC     *sender;

        /*
         * Separate prior reads of mq_ring from the increment of mq_bytes_read
         * which follows.  This pairs with the full barrier in
         * shm_mq_send_bytes(). We only need a read barrier here because the
         * increment of mq_bytes_read is actually a read followed by a dependent
         * write.
         */
        pg_read_barrier();

        /*
         * There's no need to use pg_atomic_fetch_add_u64 here, because nobody
         * else can be changing this value.  This method should be cheaper.
         */
        pg_atomic_write_u64(&mq->mq_bytes_read,
                                                pg_atomic_read_u64(&mq->mq_bytes_read) + n);

        /*
         * We shouldn't have any bytes to read without a sender, so we can read
         * mq_sender here without a lock.  Once it's initialized, it can't change.
         */
        sender = mq->mq_sender;
        Assert(sender != NULL);
        SetLatch(&sender->procLatch);
}

/*
 * Increment the number of bytes written.
 */
static void
shm_mq_inc_bytes_written(shm_mq *mq, Size n)
{
        /*
         * Separate prior reads of mq_ring from the write of mq_bytes_written
         * which we're about to do.  Pairs with the read barrier found in
         * shm_mq_receive_bytes.
         */
        pg_write_barrier();

        /*
         * There's no need to use pg_atomic_fetch_add_u64 here, because nobody
         * else can be changing this value.  This method avoids taking the bus
         * lock unnecessarily.
         */
        pg_atomic_write_u64(&mq->mq_bytes_written,
                                                pg_atomic_read_u64(&mq->mq_bytes_written) + n);
}

/* Shim for on_dsm_detach callback. */
static void
shm_mq_detach_callback(dsm_segment *seg, Datum arg)
{
        shm_mq     *mq = (shm_mq *) DatumGetPointer(arg);

        shm_mq_detach_internal(mq);
}