shm_mq.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2019, 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 "postmaster/bgworker.h"
#include "storage/procsignal.h"
#include "storage/shm_mq.h"
#include "storage/spin.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_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_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_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)
{
    shm_mq_iovec iov;

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

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

/*
 * 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.)
 */
shm_mq_result
shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov, int iovcnt, bool nowait)
{
    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;

    /* 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 and assume it isn't NULL.  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)
            return SHM_MQ_SUCCESS;
        mqh->mqh_counterparty_attached = true;
    }

    /* Notify receiver of the newly-written data, and return. */
    SetLatch(&receiver->procLatch);
    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;

    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 = Max(mqh->mqh_buflen, MQH_INITIAL_BUFSIZE);

            while (newbuflen < nbytes)
                newbuflen *= 2;

            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);
        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)
{
    /* 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);
        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)
        {
            /*
             * 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);

            /* 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_MQ_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));
            shm_mq_inc_bytes_written(mq, MAXALIGN(sendnow));

            /*
             * For efficiency, we don't set the reader's latch here.  We'll do
             * that only when the buffer fills up or after writing an entire
             * message.
             */
        }
    }

    *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_MQ_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 signalled. */
        (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                         WAIT_EVENT_MQ_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);
}