/src/postgres/src/backend/replication/logical/applyparallelworker.c

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/*-------------------------------------------------------------------------
 * applyparallelworker.c
 *     Support routines for applying xact by parallel apply worker
 *
 * Copyright (c) 2023-2025, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/applyparallelworker.c
 *
 * This file contains the code to launch, set up, and teardown a parallel apply
 * worker which receives the changes from the leader worker and invokes routines
 * to apply those on the subscriber database. Additionally, this file contains
 * routines that are intended to support setting up, using, and tearing down a
 * ParallelApplyWorkerInfo which is required so the leader worker and parallel
 * apply workers can communicate with each other.
 *
 * The parallel apply workers are assigned (if available) as soon as xact's
 * first stream is received for subscriptions that have set their 'streaming'
 * option as parallel. The leader apply worker will send changes to this new
 * worker via shared memory. We keep this worker assigned till the transaction
 * commit is received and also wait for the worker to finish at commit. This
 * preserves commit ordering and avoid file I/O in most cases, although we
 * still need to spill to a file if there is no worker available. See comments
 * atop logical/worker to know more about streamed xacts whose changes are
 * spilled to disk. It is important to maintain commit order to avoid failures
 * due to: (a) transaction dependencies - say if we insert a row in the first
 * transaction and update it in the second transaction on publisher then
 * allowing the subscriber to apply both in parallel can lead to failure in the
 * update; (b) deadlocks - allowing transactions that update the same set of
 * rows/tables in the opposite order to be applied in parallel can lead to
 * deadlocks.
 *
 * A worker pool is used to avoid restarting workers for each streaming
 * transaction. We maintain each worker's information (ParallelApplyWorkerInfo)
 * in the ParallelApplyWorkerPool. After successfully launching a new worker,
 * its information is added to the ParallelApplyWorkerPool. Once the worker
 * finishes applying the transaction, it is marked as available for re-use.
 * Now, before starting a new worker to apply the streaming transaction, we
 * check the list for any available worker. Note that we retain a maximum of
 * half the max_parallel_apply_workers_per_subscription workers in the pool and
 * after that, we simply exit the worker after applying the transaction.
 *
 * XXX This worker pool threshold is arbitrary and we can provide a GUC
 * variable for this in the future if required.
 *
 * The leader apply worker will create a separate dynamic shared memory segment
 * when each parallel apply worker starts. The reason for this design is that
 * we cannot predict how many workers will be needed. It may be possible to
 * allocate enough shared memory in one segment based on the maximum number of
 * parallel apply workers (max_parallel_apply_workers_per_subscription), but
 * this would waste memory if no process is actually started.
 *
 * The dynamic shared memory segment contains: (a) a shm_mq that is used to
 * send changes in the transaction from leader apply worker to parallel apply
 * worker; (b) another shm_mq that is used to send errors (and other messages
 * reported via elog/ereport) from the parallel apply worker to leader apply
 * worker; (c) necessary information to be shared among parallel apply workers
 * and the leader apply worker (i.e. members of ParallelApplyWorkerShared).
 *
 * Locking Considerations
 * ----------------------
 * We have a risk of deadlock due to concurrently applying the transactions in
 * parallel mode that were independent on the publisher side but became
 * dependent on the subscriber side due to the different database structures
 * (like schema of subscription tables, constraints, etc.) on each side. This
 * can happen even without parallel mode when there are concurrent operations
 * on the subscriber. In order to detect the deadlocks among leader (LA) and
 * parallel apply (PA) workers, we used lmgr locks when the PA waits for the
 * next stream (set of changes) and LA waits for PA to finish the transaction.
 * An alternative approach could be to not allow parallelism when the schema of
 * tables is different between the publisher and subscriber but that would be
 * too restrictive and would require the publisher to send much more
 * information than it is currently sending.
 *
 * Consider a case where the subscribed table does not have a unique key on the
 * publisher and has a unique key on the subscriber. The deadlock can happen in
 * the following ways:
 *
 * 1) Deadlock between the leader apply worker and a parallel apply worker
 *
 * Consider that the parallel apply worker (PA) is executing TX-1 and the
 * leader apply worker (LA) is executing TX-2 concurrently on the subscriber.
 * Now, LA is waiting for PA because of the unique key constraint of the
 * subscribed table while PA is waiting for LA to send the next stream of
 * changes or transaction finish command message.
 *
 * In order for lmgr to detect this, we have LA acquire a session lock on the
 * remote transaction (by pa_lock_stream()) and have PA wait on the lock before
 * trying to receive the next stream of changes. Specifically, LA will acquire
 * the lock in AccessExclusive mode before sending the STREAM_STOP and will
 * release it if already acquired after sending the STREAM_START, STREAM_ABORT
 * (for toplevel transaction), STREAM_PREPARE, and STREAM_COMMIT. The PA will
 * acquire the lock in AccessShare mode after processing STREAM_STOP and
 * STREAM_ABORT (for subtransaction) and then release the lock immediately
 * after acquiring it.
 *
 * The lock graph for the above example will look as follows:
 * LA (waiting to acquire the lock on the unique index) -> PA (waiting to
 * acquire the stream lock) -> LA
 *
 * This way, when PA is waiting for LA for the next stream of changes, we can
 * have a wait-edge from PA to LA in lmgr, which will make us detect the
 * deadlock between LA and PA.
 *
 * 2) Deadlock between the leader apply worker and parallel apply workers
 *
 * This scenario is similar to the first case but TX-1 and TX-2 are executed by
 * two parallel apply workers (PA-1 and PA-2 respectively). In this scenario,
 * PA-2 is waiting for PA-1 to complete its transaction while PA-1 is waiting
 * for subsequent input from LA. Also, LA is waiting for PA-2 to complete its
 * transaction in order to preserve the commit order. There is a deadlock among
 * the three processes.
 *
 * In order for lmgr to detect this, we have PA acquire a session lock (this is
 * a different lock than referred in the previous case, see
 * pa_lock_transaction()) on the transaction being applied and have LA wait on
 * the lock before proceeding in the transaction finish commands. Specifically,
 * PA will acquire this lock in AccessExclusive mode before executing the first
 * message of the transaction and release it at the xact end. LA will acquire
 * this lock in AccessShare mode at transaction finish commands and release it
 * immediately.
 *
 * The lock graph for the above example will look as follows:
 * LA (waiting to acquire the transaction lock) -> PA-2 (waiting to acquire the
 * lock due to unique index constraint) -> PA-1 (waiting to acquire the stream
 * lock) -> LA
 *
 * This way when LA is waiting to finish the transaction end command to preserve
 * the commit order, we will be able to detect deadlock, if any.
 *
 * One might think we can use XactLockTableWait(), but XactLockTableWait()
 * considers PREPARED TRANSACTION as still in progress which means the lock
 * won't be released even after the parallel apply worker has prepared the
 * transaction.
 *
 * 3) Deadlock when the shm_mq buffer is full
 *
 * In the previous scenario (ie. PA-1 and PA-2 are executing transactions
 * concurrently), if the shm_mq buffer between LA and PA-2 is full, LA has to
 * wait to send messages, and this wait doesn't appear in lmgr.
 *
 * To avoid this wait, we use a non-blocking write and wait with a timeout. If
 * the timeout is exceeded, the LA will serialize all the pending messages to
 * a file and indicate PA-2 that it needs to read that file for the remaining
 * messages. Then LA will start waiting for commit as in the previous case
 * which will detect deadlock if any. See pa_send_data() and
 * enum TransApplyAction.
 *
 * Lock types
 * ----------
 * Both the stream lock and the transaction lock mentioned above are
 * session-level locks because both locks could be acquired outside the
 * transaction, and the stream lock in the leader needs to persist across
 * transaction boundaries i.e. until the end of the streaming transaction.
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "libpq/pqformat.h"
#include "libpq/pqmq.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/logicallauncher.h"
#include "replication/logicalworker.h"
#include "replication/origin.h"
#include "replication/worker_internal.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "tcop/tcopprot.h"
#include "utils/inval.h"
#include "utils/memutils.h"
#include "utils/syscache.h"

#define PG_LOGICAL_APPLY_SHM_MAGIC 0x787ca067

/*
 * DSM keys for parallel apply worker. Unlike other parallel execution code,
 * since we don't need to worry about DSM keys conflicting with plan_node_id we
 * can use small integers.
 */
#define PARALLEL_APPLY_KEY_SHARED   1
#define PARALLEL_APPLY_KEY_MQ     2
#define PARALLEL_APPLY_KEY_ERROR_QUEUE  3

/* Queue size of DSM, 16 MB for now. */
#define DSM_QUEUE_SIZE  (16 * 1024 * 1024)

/*
 * Error queue size of DSM. It is desirable to make it large enough that a
 * typical ErrorResponse can be sent without blocking. That way, a worker that
 * errors out can write the whole message into the queue and terminate without
 * waiting for the user backend.
 */
#define DSM_ERROR_QUEUE_SIZE      (16 * 1024)

/*
 * There are three fields in each message received by the parallel apply
 * worker: start_lsn, end_lsn and send_time. Because we have updated these
 * statistics in the leader apply worker, we can ignore these fields in the
 * parallel apply worker (see function LogicalRepApplyLoop).
 */
#define SIZE_STATS_MESSAGE (2 * sizeof(XLogRecPtr) + sizeof(TimestampTz))

/*
 * The type of session-level lock on a transaction being applied on a logical
 * replication subscriber.
 */
#define PARALLEL_APPLY_LOCK_STREAM  0
#define PARALLEL_APPLY_LOCK_XACT  1

/*
 * Hash table entry to map xid to the parallel apply worker state.
 */
typedef struct ParallelApplyWorkerEntry
{
  TransactionId xid;      /* Hash key -- must be first */
  ParallelApplyWorkerInfo *winfo;
} ParallelApplyWorkerEntry;

/*
 * A hash table used to cache the state of streaming transactions being applied
 * by the parallel apply workers.
 */
static HTAB *ParallelApplyTxnHash = NULL;

/*
* A list (pool) of active parallel apply workers. The information for
* the new worker is added to the list after successfully launching it. The
* list entry is removed if there are already enough workers in the worker
* pool at the end of the transaction. For more information about the worker
* pool, see comments atop this file.
 */
static List *ParallelApplyWorkerPool = NIL;

/*
 * Information shared between leader apply worker and parallel apply worker.
 */
ParallelApplyWorkerShared *MyParallelShared = NULL;

/*
 * Is there a message sent by a parallel apply worker that the leader apply
 * worker needs to receive?
 */
volatile sig_atomic_t ParallelApplyMessagePending = false;

/*
 * Cache the parallel apply worker information required for applying the
 * current streaming transaction. It is used to save the cost of searching the
 * hash table when applying the changes between STREAM_START and STREAM_STOP.
 */
static ParallelApplyWorkerInfo *stream_apply_worker = NULL;

/* A list to maintain subtransactions, if any. */
static List *subxactlist = NIL;

static void pa_free_worker_info(ParallelApplyWorkerInfo *winfo);
static ParallelTransState pa_get_xact_state(ParallelApplyWorkerShared *wshared);
static PartialFileSetState pa_get_fileset_state(void);

/*
 * Returns true if it is OK to start a parallel apply worker, false otherwise.
 */
static bool
pa_can_start(void)
{
  /* Only leader apply workers can start parallel apply workers. */
  if (!am_leader_apply_worker())
    return false;

  /*
   * It is good to check for any change in the subscription parameter to
   * avoid the case where for a very long time the change doesn't get
   * reflected. This can happen when there is a constant flow of streaming
   * transactions that are handled by parallel apply workers.
   *
   * It is better to do it before the below checks so that the latest values
   * of subscription can be used for the checks.
   */
  maybe_reread_subscription();

  /*
   * Don't start a new parallel apply worker if the subscription is not
   * using parallel streaming mode, or if the publisher does not support
   * parallel apply.
   */
  if (!MyLogicalRepWorker->parallel_apply)
    return false;

  /*
   * Don't start a new parallel worker if user has set skiplsn as it's
   * possible that they want to skip the streaming transaction. For
   * streaming transactions, we need to serialize the transaction to a file
   * so that we can get the last LSN of the transaction to judge whether to
   * skip before starting to apply the change.
   *
   * One might think that we could allow parallelism if the first lsn of the
   * transaction is greater than skiplsn, but we don't send it with the
   * STREAM START message, and it doesn't seem worth sending the extra eight
   * bytes with the STREAM START to enable parallelism for this case.
   */
  if (!XLogRecPtrIsInvalid(MySubscription->skiplsn))
    return false;

  /*
   * For streaming transactions that are being applied using a parallel
   * apply worker, we cannot decide whether to apply the change for a
   * relation that is not in the READY state (see
   * should_apply_changes_for_rel) as we won't know remote_final_lsn by that
   * time. So, we don't start the new parallel apply worker in this case.
   */
  if (!AllTablesyncsReady())
    return false;

  return true;
}

/*
 * Set up a dynamic shared memory segment.
 *
 * We set up a control region that contains a fixed-size worker info
 * (ParallelApplyWorkerShared), a message queue, and an error queue.
 *
 * Returns true on success, false on failure.
 */
static bool
pa_setup_dsm(ParallelApplyWorkerInfo *winfo)
{
  shm_toc_estimator e;
  Size    segsize;
  dsm_segment *seg;
  shm_toc    *toc;
  ParallelApplyWorkerShared *shared;
  shm_mq     *mq;
  Size    queue_size = DSM_QUEUE_SIZE;
  Size    error_queue_size = DSM_ERROR_QUEUE_SIZE;

  /*
   * Estimate how much shared memory we need.
   *
   * Because the TOC machinery may choose to insert padding of oddly-sized
   * requests, we must estimate each chunk separately.
   *
   * We need one key to register the location of the header, and two other
   * keys to track the locations of the message queue and the error message
   * queue.
   */
  shm_toc_initialize_estimator(&e);
  shm_toc_estimate_chunk(&e, sizeof(ParallelApplyWorkerShared));
  shm_toc_estimate_chunk(&e, queue_size);
  shm_toc_estimate_chunk(&e, error_queue_size);

  shm_toc_estimate_keys(&e, 3);
  segsize = shm_toc_estimate(&e);

  /* Create the shared memory segment and establish a table of contents. */
  seg = dsm_create(shm_toc_estimate(&e), 0);
  if (!seg)
    return false;

  toc = shm_toc_create(PG_LOGICAL_APPLY_SHM_MAGIC, dsm_segment_address(seg),
             segsize);

  /* Set up the header region. */
  shared = shm_toc_allocate(toc, sizeof(ParallelApplyWorkerShared));
  SpinLockInit(&shared->mutex);

  shared->xact_state = PARALLEL_TRANS_UNKNOWN;
  pg_atomic_init_u32(&(shared->pending_stream_count), 0);
  shared->last_commit_end = InvalidXLogRecPtr;
  shared->fileset_state = FS_EMPTY;

  shm_toc_insert(toc, PARALLEL_APPLY_KEY_SHARED, shared);

  /* Set up message queue for the worker. */
  mq = shm_mq_create(shm_toc_allocate(toc, queue_size), queue_size);
  shm_toc_insert(toc, PARALLEL_APPLY_KEY_MQ, mq);
  shm_mq_set_sender(mq, MyProc);

  /* Attach the queue. */
  winfo->mq_handle = shm_mq_attach(mq, seg, NULL);

  /* Set up error queue for the worker. */
  mq = shm_mq_create(shm_toc_allocate(toc, error_queue_size),
             error_queue_size);
  shm_toc_insert(toc, PARALLEL_APPLY_KEY_ERROR_QUEUE, mq);
  shm_mq_set_receiver(mq, MyProc);

  /* Attach the queue. */
  winfo->error_mq_handle = shm_mq_attach(mq, seg, NULL);

  /* Return results to caller. */
  winfo->dsm_seg = seg;
  winfo->shared = shared;

  return true;
}

/*
 * Try to get a parallel apply worker from the pool. If none is available then
 * start a new one.
 */
static ParallelApplyWorkerInfo *
pa_launch_parallel_worker(void)
{
  MemoryContext oldcontext;
  bool    launched;
  ParallelApplyWorkerInfo *winfo;
  ListCell   *lc;

  /* Try to get an available parallel apply worker from the worker pool. */
  foreach(lc, ParallelApplyWorkerPool)
  {
    winfo = (ParallelApplyWorkerInfo *) lfirst(lc);

    if (!winfo->in_use)
      return winfo;
  }

  /*
   * Start a new parallel apply worker.
   *
   * The worker info can be used for the lifetime of the worker process, so
   * create it in a permanent context.
   */
  oldcontext = MemoryContextSwitchTo(ApplyContext);

  winfo = (ParallelApplyWorkerInfo *) palloc0(sizeof(ParallelApplyWorkerInfo));

  /* Setup shared memory. */
  if (!pa_setup_dsm(winfo))
  {
    MemoryContextSwitchTo(oldcontext);
    pfree(winfo);
    return NULL;
  }

  launched = logicalrep_worker_launch(WORKERTYPE_PARALLEL_APPLY,
                    MyLogicalRepWorker->dbid,
                    MySubscription->oid,
                    MySubscription->name,
                    MyLogicalRepWorker->userid,
                    InvalidOid,
                    dsm_segment_handle(winfo->dsm_seg));

  if (launched)
  {
    ParallelApplyWorkerPool = lappend(ParallelApplyWorkerPool, winfo);
  }
  else
  {
    pa_free_worker_info(winfo);
    winfo = NULL;
  }

  MemoryContextSwitchTo(oldcontext);

  return winfo;
}

/*
 * Allocate a parallel apply worker that will be used for the specified xid.
 *
 * We first try to get an available worker from the pool, if any and then try
 * to launch a new worker. On successful allocation, remember the worker
 * information in the hash table so that we can get it later for processing the
 * streaming changes.
 */
void
pa_allocate_worker(TransactionId xid)
{
  bool    found;
  ParallelApplyWorkerInfo *winfo = NULL;
  ParallelApplyWorkerEntry *entry;

  if (!pa_can_start())
    return;

  winfo = pa_launch_parallel_worker();
  if (!winfo)
    return;

  /* First time through, initialize parallel apply worker state hashtable. */
  if (!ParallelApplyTxnHash)
  {
    HASHCTL   ctl;

    MemSet(&ctl, 0, sizeof(ctl));
    ctl.keysize = sizeof(TransactionId);
    ctl.entrysize = sizeof(ParallelApplyWorkerEntry);
    ctl.hcxt = ApplyContext;

    ParallelApplyTxnHash = hash_create("logical replication parallel apply workers hash",
                       16, &ctl,
                       HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  }

  /* Create an entry for the requested transaction. */
  entry = hash_search(ParallelApplyTxnHash, &xid, HASH_ENTER, &found);
  if (found)
    elog(ERROR, "hash table corrupted");

  /* Update the transaction information in shared memory. */
  SpinLockAcquire(&winfo->shared->mutex);
  winfo->shared->xact_state = PARALLEL_TRANS_UNKNOWN;
  winfo->shared->xid = xid;
  SpinLockRelease(&winfo->shared->mutex);

  winfo->in_use = true;
  winfo->serialize_changes = false;
  entry->winfo = winfo;
}

/*
 * Find the assigned worker for the given transaction, if any.
 */
ParallelApplyWorkerInfo *
pa_find_worker(TransactionId xid)
{
  bool    found;
  ParallelApplyWorkerEntry *entry;

  if (!TransactionIdIsValid(xid))
    return NULL;

  if (!ParallelApplyTxnHash)
    return NULL;

  /* Return the cached parallel apply worker if valid. */
  if (stream_apply_worker)
    return stream_apply_worker;

  /* Find an entry for the requested transaction. */
  entry = hash_search(ParallelApplyTxnHash, &xid, HASH_FIND, &found);
  if (found)
  {
    /* The worker must not have exited.  */
    Assert(entry->winfo->in_use);
    return entry->winfo;
  }

  return NULL;
}

/*
 * Makes the worker available for reuse.
 *
 * This removes the parallel apply worker entry from the hash table so that it
 * can't be used. If there are enough workers in the pool, it stops the worker
 * and frees the corresponding info. Otherwise it just marks the worker as
 * available for reuse.
 *
 * For more information about the worker pool, see comments atop this file.
 */
static void
pa_free_worker(ParallelApplyWorkerInfo *winfo)
{
  Assert(!am_parallel_apply_worker());
  Assert(winfo->in_use);
  Assert(pa_get_xact_state(winfo->shared) == PARALLEL_TRANS_FINISHED);

  if (!hash_search(ParallelApplyTxnHash, &winfo->shared->xid, HASH_REMOVE, NULL))
    elog(ERROR, "hash table corrupted");

  /*
   * Stop the worker if there are enough workers in the pool.
   *
   * XXX Additionally, we also stop the worker if the leader apply worker
   * serialize part of the transaction data due to a send timeout. This is
   * because the message could be partially written to the queue and there
   * is no way to clean the queue other than resending the message until it
   * succeeds. Instead of trying to send the data which anyway would have
   * been serialized and then letting the parallel apply worker deal with
   * the spurious message, we stop the worker.
   */
  if (winfo->serialize_changes ||
    list_length(ParallelApplyWorkerPool) >
    (max_parallel_apply_workers_per_subscription / 2))
  {
    logicalrep_pa_worker_stop(winfo);
    pa_free_worker_info(winfo);

    return;
  }

  winfo->in_use = false;
  winfo->serialize_changes = false;
}

/*
 * Free the parallel apply worker information and unlink the files with
 * serialized changes if any.
 */
static void
pa_free_worker_info(ParallelApplyWorkerInfo *winfo)
{
  Assert(winfo);

  if (winfo->mq_handle)
    shm_mq_detach(winfo->mq_handle);

  if (winfo->error_mq_handle)
    shm_mq_detach(winfo->error_mq_handle);

  /* Unlink the files with serialized changes. */
  if (winfo->serialize_changes)
    stream_cleanup_files(MyLogicalRepWorker->subid, winfo->shared->xid);

  if (winfo->dsm_seg)
    dsm_detach(winfo->dsm_seg);

  /* Remove from the worker pool. */
  ParallelApplyWorkerPool = list_delete_ptr(ParallelApplyWorkerPool, winfo);

  pfree(winfo);
}

/*
 * Detach the error queue for all parallel apply workers.
 */
void
pa_detach_all_error_mq(void)
{
  ListCell   *lc;

  foreach(lc, ParallelApplyWorkerPool)
  {
    ParallelApplyWorkerInfo *winfo = (ParallelApplyWorkerInfo *) lfirst(lc);

    if (winfo->error_mq_handle)
    {
      shm_mq_detach(winfo->error_mq_handle);
      winfo->error_mq_handle = NULL;
    }
  }
}

/*
 * Check if there are any pending spooled messages.
 */
static bool
pa_has_spooled_message_pending()
{
  PartialFileSetState fileset_state;

  fileset_state = pa_get_fileset_state();

  return (fileset_state != FS_EMPTY);
}

/*
 * Replay the spooled messages once the leader apply worker has finished
 * serializing changes to the file.
 *
 * Returns false if there aren't any pending spooled messages, true otherwise.
 */
static bool
pa_process_spooled_messages_if_required(void)
{
  PartialFileSetState fileset_state;

  fileset_state = pa_get_fileset_state();

  if (fileset_state == FS_EMPTY)
    return false;

  /*
   * If the leader apply worker is busy serializing the partial changes then
   * acquire the stream lock now and wait for the leader worker to finish
   * serializing the changes. Otherwise, the parallel apply worker won't get
   * a chance to receive a STREAM_STOP (and acquire the stream lock) until
   * the leader had serialized all changes which can lead to undetected
   * deadlock.
   *
   * Note that the fileset state can be FS_SERIALIZE_DONE once the leader
   * worker has finished serializing the changes.
   */
  if (fileset_state == FS_SERIALIZE_IN_PROGRESS)
  {
    pa_lock_stream(MyParallelShared->xid, AccessShareLock);
    pa_unlock_stream(MyParallelShared->xid, AccessShareLock);

    fileset_state = pa_get_fileset_state();
  }

  /*
   * We cannot read the file immediately after the leader has serialized all
   * changes to the file because there may still be messages in the memory
   * queue. We will apply all spooled messages the next time we call this
   * function and that will ensure there are no messages left in the memory
   * queue.
   */
  if (fileset_state == FS_SERIALIZE_DONE)
  {
    pa_set_fileset_state(MyParallelShared, FS_READY);
  }
  else if (fileset_state == FS_READY)
  {
    apply_spooled_messages(&MyParallelShared->fileset,
                 MyParallelShared->xid,
                 InvalidXLogRecPtr);
    pa_set_fileset_state(MyParallelShared, FS_EMPTY);
  }

  return true;
}

/*
 * Interrupt handler for main loop of parallel apply worker.
 */
static void
ProcessParallelApplyInterrupts(void)
{
  CHECK_FOR_INTERRUPTS();

  if (ShutdownRequestPending)
  {
    ereport(LOG,
        (errmsg("logical replication parallel apply worker for subscription \"%s\" has finished",
            MySubscription->name)));

    proc_exit(0);
  }

  if (ConfigReloadPending)
  {
    ConfigReloadPending = false;
    ProcessConfigFile(PGC_SIGHUP);
  }
}

/* Parallel apply worker main loop. */
static void
LogicalParallelApplyLoop(shm_mq_handle *mqh)
{
  shm_mq_result shmq_res;
  ErrorContextCallback errcallback;
  MemoryContext oldcxt = CurrentMemoryContext;

  /*
   * Init the ApplyMessageContext which we clean up after each replication
   * protocol message.
   */
  ApplyMessageContext = AllocSetContextCreate(ApplyContext,
                        "ApplyMessageContext",
                        ALLOCSET_DEFAULT_SIZES);

  /*
   * Push apply error context callback. Fields will be filled while applying
   * a change.
   */
  errcallback.callback = apply_error_callback;
  errcallback.previous = error_context_stack;
  error_context_stack = &errcallback;

  for (;;)
  {
    void     *data;
    Size    len;

    ProcessParallelApplyInterrupts();

    /* Ensure we are reading the data into our memory context. */
    MemoryContextSwitchTo(ApplyMessageContext);

    shmq_res = shm_mq_receive(mqh, &len, &data, true);

    if (shmq_res == SHM_MQ_SUCCESS)
    {
      StringInfoData s;
      int     c;

      if (len == 0)
        elog(ERROR, "invalid message length");

      initReadOnlyStringInfo(&s, data, len);

      /*
       * The first byte of messages sent from leader apply worker to
       * parallel apply workers can only be 'w'.
       */
      c = pq_getmsgbyte(&s);
      if (c != 'w')
        elog(ERROR, "unexpected message \"%c\"", c);

      /*
       * Ignore statistics fields that have been updated by the leader
       * apply worker.
       *
       * XXX We can avoid sending the statistics fields from the leader
       * apply worker but for that, it needs to rebuild the entire
       * message by removing these fields which could be more work than
       * simply ignoring these fields in the parallel apply worker.
       */
      s.cursor += SIZE_STATS_MESSAGE;

      apply_dispatch(&s);
    }
    else if (shmq_res == SHM_MQ_WOULD_BLOCK)
    {
      /* Replay the changes from the file, if any. */
      if (!pa_process_spooled_messages_if_required())
      {
        int     rc;

        /* Wait for more work. */
        rc = WaitLatch(MyLatch,
                 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                 1000L,
                 WAIT_EVENT_LOGICAL_PARALLEL_APPLY_MAIN);

        if (rc & WL_LATCH_SET)
          ResetLatch(MyLatch);
      }
    }
    else
    {
      Assert(shmq_res == SHM_MQ_DETACHED);

      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("lost connection to the logical replication apply worker")));
    }

    MemoryContextReset(ApplyMessageContext);
    MemoryContextSwitchTo(oldcxt);
  }

  /* Pop the error context stack. */
  error_context_stack = errcallback.previous;

  MemoryContextSwitchTo(oldcxt);
}

/*
 * Make sure the leader apply worker tries to read from our error queue one more
 * time. This guards against the case where we exit uncleanly without sending
 * an ErrorResponse, for example because some code calls proc_exit directly.
 *
 * Also explicitly detach from dsm segment to invoke on_dsm_detach callbacks,
 * if any. See ParallelWorkerShutdown for details.
 */
static void
pa_shutdown(int code, Datum arg)
{
  SendProcSignal(MyLogicalRepWorker->leader_pid,
           PROCSIG_PARALLEL_APPLY_MESSAGE,
           INVALID_PROC_NUMBER);

  dsm_detach((dsm_segment *) DatumGetPointer(arg));
}

/*
 * Parallel apply worker entry point.
 */
void
ParallelApplyWorkerMain(Datum main_arg)
{
  ParallelApplyWorkerShared *shared;
  dsm_handle  handle;
  dsm_segment *seg;
  shm_toc    *toc;
  shm_mq     *mq;
  shm_mq_handle *mqh;
  shm_mq_handle *error_mqh;
  RepOriginId originid;
  int     worker_slot = DatumGetInt32(main_arg);
  char    originname[NAMEDATALEN];

  InitializingApplyWorker = true;

  /* Setup signal handling. */
  pqsignal(SIGHUP, SignalHandlerForConfigReload);
  pqsignal(SIGINT, SignalHandlerForShutdownRequest);
  pqsignal(SIGTERM, die);
  BackgroundWorkerUnblockSignals();

  /*
   * Attach to the dynamic shared memory segment for the parallel apply, and
   * find its table of contents.
   *
   * Like parallel query, we don't need resource owner by this time. See
   * ParallelWorkerMain.
   */
  memcpy(&handle, MyBgworkerEntry->bgw_extra, sizeof(dsm_handle));
  seg = dsm_attach(handle);
  if (!seg)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("could not map dynamic shared memory segment")));

  toc = shm_toc_attach(PG_LOGICAL_APPLY_SHM_MAGIC, dsm_segment_address(seg));
  if (!toc)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("invalid magic number in dynamic shared memory segment")));

  /* Look up the shared information. */
  shared = shm_toc_lookup(toc, PARALLEL_APPLY_KEY_SHARED, false);
  MyParallelShared = shared;

  /*
   * Attach to the message queue.
   */
  mq = shm_toc_lookup(toc, PARALLEL_APPLY_KEY_MQ, false);
  shm_mq_set_receiver(mq, MyProc);
  mqh = shm_mq_attach(mq, seg, NULL);

  /*
   * Primary initialization is complete. Now, we can attach to our slot.
   * This is to ensure that the leader apply worker does not write data to
   * the uninitialized memory queue.
   */
  logicalrep_worker_attach(worker_slot);

  /*
   * Register the shutdown callback after we are attached to the worker
   * slot. This is to ensure that MyLogicalRepWorker remains valid when this
   * callback is invoked.
   */
  before_shmem_exit(pa_shutdown, PointerGetDatum(seg));

  SpinLockAcquire(&MyParallelShared->mutex);
  MyParallelShared->logicalrep_worker_generation = MyLogicalRepWorker->generation;
  MyParallelShared->logicalrep_worker_slot_no = worker_slot;
  SpinLockRelease(&MyParallelShared->mutex);

  /*
   * Attach to the error queue.
   */
  mq = shm_toc_lookup(toc, PARALLEL_APPLY_KEY_ERROR_QUEUE, false);
  shm_mq_set_sender(mq, MyProc);
  error_mqh = shm_mq_attach(mq, seg, NULL);

  pq_redirect_to_shm_mq(seg, error_mqh);
  pq_set_parallel_leader(MyLogicalRepWorker->leader_pid,
               INVALID_PROC_NUMBER);

  MyLogicalRepWorker->last_send_time = MyLogicalRepWorker->last_recv_time =
    MyLogicalRepWorker->reply_time = 0;

  InitializeLogRepWorker();

  InitializingApplyWorker = false;

  /* Setup replication origin tracking. */
  StartTransactionCommand();
  ReplicationOriginNameForLogicalRep(MySubscription->oid, InvalidOid,
                     originname, sizeof(originname));
  originid = replorigin_by_name(originname, false);

  /*
   * The parallel apply worker doesn't need to monopolize this replication
   * origin which was already acquired by its leader process.
   */
  replorigin_session_setup(originid, MyLogicalRepWorker->leader_pid);
  replorigin_session_origin = originid;
  CommitTransactionCommand();

  /*
   * Setup callback for syscache so that we know when something changes in
   * the subscription relation state.
   */
  CacheRegisterSyscacheCallback(SUBSCRIPTIONRELMAP,
                  invalidate_syncing_table_states,
                  (Datum) 0);

  set_apply_error_context_origin(originname);

  LogicalParallelApplyLoop(mqh);

  /*
   * The parallel apply worker must not get here because the parallel apply
   * worker will only stop when it receives a SIGTERM or SIGINT from the
   * leader, or when there is an error. None of these cases will allow the
   * code to reach here.
   */
  Assert(false);
}

/*
 * Handle receipt of an interrupt indicating a parallel apply worker message.
 *
 * Note: this is called within a signal handler! All we can do is set a flag
 * that will cause the next CHECK_FOR_INTERRUPTS() to invoke
 * ProcessParallelApplyMessages().
 */
void
HandleParallelApplyMessageInterrupt(void)
{
  InterruptPending = true;
  ParallelApplyMessagePending = true;
  SetLatch(MyLatch);
}

/*
 * Process a single protocol message received from a single parallel apply
 * worker.
 */
static void
ProcessParallelApplyMessage(StringInfo msg)
{
  char    msgtype;

  msgtype = pq_getmsgbyte(msg);

  switch (msgtype)
  {
    case 'E':       /* ErrorResponse */
      {
        ErrorData edata;

        /* Parse ErrorResponse. */
        pq_parse_errornotice(msg, &edata);

        /*
         * If desired, add a context line to show that this is a
         * message propagated from a parallel apply worker. Otherwise,
         * it can sometimes be confusing to understand what actually
         * happened.
         */
        if (edata.context)
          edata.context = psprintf("%s\n%s", edata.context,
                       _("logical replication parallel apply worker"));
        else
          edata.context = pstrdup(_("logical replication parallel apply worker"));

        /*
         * Context beyond that should use the error context callbacks
         * that were in effect in LogicalRepApplyLoop().
         */
        error_context_stack = apply_error_context_stack;

        /*
         * The actual error must have been reported by the parallel
         * apply worker.
         */
        ereport(ERROR,
            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
             errmsg("logical replication parallel apply worker exited due to error"),
             errcontext("%s", edata.context)));
      }

      /*
       * Don't need to do anything about NoticeResponse and
       * NotifyResponse as the logical replication worker doesn't need
       * to send messages to the client.
       */
    case 'N':
    case 'A':
      break;

    default:
      elog(ERROR, "unrecognized message type received from logical replication parallel apply worker: %c (message length %d bytes)",
         msgtype, msg->len);
  }
}

/*
 * Handle any queued protocol messages received from parallel apply workers.
 */
void
ProcessParallelApplyMessages(void)
{
  ListCell   *lc;
  MemoryContext oldcontext;

  static MemoryContext hpam_context = NULL;

  /*
   * This is invoked from ProcessInterrupts(), and since some of the
   * functions it calls contain CHECK_FOR_INTERRUPTS(), there is a potential
   * for recursive calls if more signals are received while this runs. It's
   * unclear that recursive entry would be safe, and it doesn't seem useful
   * even if it is safe, so let's block interrupts until done.
   */
  HOLD_INTERRUPTS();

  /*
   * Moreover, CurrentMemoryContext might be pointing almost anywhere. We
   * don't want to risk leaking data into long-lived contexts, so let's do
   * our work here in a private context that we can reset on each use.
   */
  if (!hpam_context)     /* first time through? */
    hpam_context = AllocSetContextCreate(TopMemoryContext,
                       "ProcessParallelApplyMessages",
                       ALLOCSET_DEFAULT_SIZES);
  else
    MemoryContextReset(hpam_context);

  oldcontext = MemoryContextSwitchTo(hpam_context);

  ParallelApplyMessagePending = false;

  foreach(lc, ParallelApplyWorkerPool)
  {
    shm_mq_result res;
    Size    nbytes;
    void     *data;
    ParallelApplyWorkerInfo *winfo = (ParallelApplyWorkerInfo *) lfirst(lc);

    /*
     * The leader will detach from the error queue and set it to NULL
     * before preparing to stop all parallel apply workers, so we don't
     * need to handle error messages anymore. See
     * logicalrep_worker_detach.
     */
    if (!winfo->error_mq_handle)
      continue;

    res = shm_mq_receive(winfo->error_mq_handle, &nbytes, &data, true);

    if (res == SHM_MQ_WOULD_BLOCK)
      continue;
    else if (res == SHM_MQ_SUCCESS)
    {
      StringInfoData msg;

      initStringInfo(&msg);
      appendBinaryStringInfo(&msg, data, nbytes);
      ProcessParallelApplyMessage(&msg);
      pfree(msg.data);
    }
    else
      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("lost connection to the logical replication parallel apply worker")));
  }

  MemoryContextSwitchTo(oldcontext);

  /* Might as well clear the context on our way out */
  MemoryContextReset(hpam_context);

  RESUME_INTERRUPTS();
}

/*
 * Send the data to the specified parallel apply worker via shared-memory
 * queue.
 *
 * Returns false if the attempt to send data via shared memory times out, true
 * otherwise.
 */
bool
pa_send_data(ParallelApplyWorkerInfo *winfo, Size nbytes, const void *data)
{
  int     rc;
  shm_mq_result result;
  TimestampTz startTime = 0;

  Assert(!IsTransactionState());
  Assert(!winfo->serialize_changes);

  /*
   * We don't try to send data to parallel worker for 'immediate' mode. This
   * is primarily used for testing purposes.
   */
  if (unlikely(debug_logical_replication_streaming == DEBUG_LOGICAL_REP_STREAMING_IMMEDIATE))
    return false;

/*
 * This timeout is a bit arbitrary but testing revealed that it is sufficient
 * to send the message unless the parallel apply worker is waiting on some
 * lock or there is a serious resource crunch. See the comments atop this file
 * to know why we are using a non-blocking way to send the message.
 */
#define SHM_SEND_RETRY_INTERVAL_MS 1000
#define SHM_SEND_TIMEOUT_MS   (10000 - SHM_SEND_RETRY_INTERVAL_MS)

  for (;;)
  {
    result = shm_mq_send(winfo->mq_handle, nbytes, data, true, true);

    if (result == SHM_MQ_SUCCESS)
      return true;
    else if (result == SHM_MQ_DETACHED)
      ereport(ERROR,
          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
           errmsg("could not send data to shared-memory queue")));

    Assert(result == SHM_MQ_WOULD_BLOCK);

    /* Wait before retrying. */
    rc = WaitLatch(MyLatch,
             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
             SHM_SEND_RETRY_INTERVAL_MS,
             WAIT_EVENT_LOGICAL_APPLY_SEND_DATA);

    if (rc & WL_LATCH_SET)
    {
      ResetLatch(MyLatch);
      CHECK_FOR_INTERRUPTS();
    }

    if (startTime == 0)
      startTime = GetCurrentTimestamp();
    else if (TimestampDifferenceExceeds(startTime, GetCurrentTimestamp(),
                      SHM_SEND_TIMEOUT_MS))
      return false;
  }
}

/*
 * Switch to PARTIAL_SERIALIZE mode for the current transaction -- this means
 * that the current data and any subsequent data for this transaction will be
 * serialized to a file. This is done to prevent possible deadlocks with
 * another parallel apply worker (refer to the comments atop this file).
 */
void
pa_switch_to_partial_serialize(ParallelApplyWorkerInfo *winfo,
                 bool stream_locked)
{
  ereport(LOG,
      (errmsg("logical replication apply worker will serialize the remaining changes of remote transaction %u to a file",
          winfo->shared->xid)));

  /*
   * The parallel apply worker could be stuck for some reason (say waiting
   * on some lock by other backend), so stop trying to send data directly to
   * it and start serializing data to the file instead.
   */
  winfo->serialize_changes = true;

  /* Initialize the stream fileset. */
  stream_start_internal(winfo->shared->xid, true);

  /*
   * Acquires the stream lock if not already to make sure that the parallel
   * apply worker will wait for the leader to release the stream lock until
   * the end of the transaction.
   */
  if (!stream_locked)
    pa_lock_stream(winfo->shared->xid, AccessExclusiveLock);

  pa_set_fileset_state(winfo->shared, FS_SERIALIZE_IN_PROGRESS);
}

/*
 * Wait until the parallel apply worker's transaction state has reached or
 * exceeded the given xact_state.
 */
static void
pa_wait_for_xact_state(ParallelApplyWorkerInfo *winfo,
             ParallelTransState xact_state)
{
  for (;;)
  {
    /*
     * Stop if the transaction state has reached or exceeded the given
     * xact_state.
     */
    if (pa_get_xact_state(winfo->shared) >= xact_state)
      break;

    /* Wait to be signalled. */
    (void) WaitLatch(MyLatch,
             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
             10L,
             WAIT_EVENT_LOGICAL_PARALLEL_APPLY_STATE_CHANGE);

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

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

/*
 * Wait until the parallel apply worker's transaction finishes.
 */
static void
pa_wait_for_xact_finish(ParallelApplyWorkerInfo *winfo)
{
  /*
   * Wait until the parallel apply worker set the state to
   * PARALLEL_TRANS_STARTED which means it has acquired the transaction
   * lock. This is to prevent leader apply worker from acquiring the
   * transaction lock earlier than the parallel apply worker.
   */
  pa_wait_for_xact_state(winfo, PARALLEL_TRANS_STARTED);

  /*
   * Wait for the transaction lock to be released. This is required to
   * detect deadlock among leader and parallel apply workers. Refer to the
   * comments atop this file.
   */
  pa_lock_transaction(winfo->shared->xid, AccessShareLock);
  pa_unlock_transaction(winfo->shared->xid, AccessShareLock);

  /*
   * Check if the state becomes PARALLEL_TRANS_FINISHED in case the parallel
   * apply worker failed while applying changes causing the lock to be
   * released.
   */
  if (pa_get_xact_state(winfo->shared) != PARALLEL_TRANS_FINISHED)
    ereport(ERROR,
        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
         errmsg("lost connection to the logical replication parallel apply worker")));
}

/*
 * Set the transaction state for a given parallel apply worker.
 */
void
pa_set_xact_state(ParallelApplyWorkerShared *wshared,
          ParallelTransState xact_state)
{
  SpinLockAcquire(&wshared->mutex);
  wshared->xact_state = xact_state;
  SpinLockRelease(&wshared->mutex);
}

/*
 * Get the transaction state for a given parallel apply worker.
 */
static ParallelTransState
pa_get_xact_state(ParallelApplyWorkerShared *wshared)
{
  ParallelTransState xact_state;

  SpinLockAcquire(&wshared->mutex);
  xact_state = wshared->xact_state;
  SpinLockRelease(&wshared->mutex);

  return xact_state;
}

/*
 * Cache the parallel apply worker information.
 */
void
pa_set_stream_apply_worker(ParallelApplyWorkerInfo *winfo)
{
  stream_apply_worker = winfo;
}

/*
 * Form a unique savepoint name for the streaming transaction.
 *
 * Note that different subscriptions for publications on different nodes can
 * receive same remote xid, so we need to use subscription id along with it.
 *
 * Returns the name in the supplied buffer.
 */
static void
pa_savepoint_name(Oid suboid, TransactionId xid, char *spname, Size szsp)
{
  snprintf(spname, szsp, "pg_sp_%u_%u", suboid, xid);
}

/*
 * Define a savepoint for a subxact in parallel apply worker if needed.
 *
 * The parallel apply worker can figure out if a new subtransaction was
 * started by checking if the new change arrived with a different xid. In that
 * case define a named savepoint, so that we are able to rollback to it
 * if required.
 */
void
pa_start_subtrans(TransactionId current_xid, TransactionId top_xid)
{
  if (current_xid != top_xid &&
    !list_member_xid(subxactlist, current_xid))
  {
    MemoryContext oldctx;
    char    spname[NAMEDATALEN];

    pa_savepoint_name(MySubscription->oid, current_xid,
              spname, sizeof(spname));

    elog(DEBUG1, "defining savepoint %s in logical replication parallel apply worker", spname);

    /* We must be in transaction block to define the SAVEPOINT. */
    if (!IsTransactionBlock())
    {
      if (!IsTransactionState())
        StartTransactionCommand();

      BeginTransactionBlock();
      CommitTransactionCommand();
    }

    DefineSavepoint(spname);

    /*
     * CommitTransactionCommand is needed to start a subtransaction after
     * issuing a SAVEPOINT inside a transaction block (see
     * StartSubTransaction()).
     */
    CommitTransactionCommand();

    oldctx = MemoryContextSwitchTo(TopTransactionContext);
    subxactlist = lappend_xid(subxactlist, current_xid);
    MemoryContextSwitchTo(oldctx);
  }
}

/* Reset the list that maintains subtransactions. */
void
pa_reset_subtrans(void)
{
  /*
   * We don't need to free this explicitly as the allocated memory will be
   * freed at the transaction end.
   */
  subxactlist = NIL;
}

/*
 * Handle STREAM ABORT message when the transaction was applied in a parallel
 * apply worker.
 */
void
pa_stream_abort(LogicalRepStreamAbortData *abort_data)
{
  TransactionId xid = abort_data->xid;
  TransactionId subxid = abort_data->subxid;

  /*
   * Update origin state so we can restart streaming from correct position
   * in case of crash.
   */
  replorigin_session_origin_lsn = abort_data->abort_lsn;
  replorigin_session_origin_timestamp = abort_data->abort_time;

  /*
   * If the two XIDs are the same, it's in fact abort of toplevel xact, so
   * just free the subxactlist.
   */
  if (subxid == xid)
  {
    pa_set_xact_state(MyParallelShared, PARALLEL_TRANS_FINISHED);

    /*
     * Release the lock as we might be processing an empty streaming
     * transaction in which case the lock won't be released during
     * transaction rollback.
     *
     * Note that it's ok to release the transaction lock before aborting
     * the transaction because even if the parallel apply worker dies due
     * to crash or some other reason, such a transaction would still be
     * considered aborted.
     */
    pa_unlock_transaction(xid, AccessExclusiveLock);

    AbortCurrentTransaction();

    if (IsTransactionBlock())
    {
      EndTransactionBlock(false);
      CommitTransactionCommand();
    }

    pa_reset_subtrans();

    pgstat_report_activity(STATE_IDLE, NULL);
  }
  else
  {
    /* OK, so it's a subxact. Rollback to the savepoint. */
    int     i;
    char    spname[NAMEDATALEN];

    pa_savepoint_name(MySubscription->oid, subxid, spname, sizeof(spname));

    elog(DEBUG1, "rolling back to savepoint %s in logical replication parallel apply worker", spname);

    /*
     * Search the subxactlist, determine the offset tracked for the
     * subxact, and truncate the list.
     *
     * Note that for an empty sub-transaction we won't find the subxid
     * here.
     */
    for (i = list_length(subxactlist) - 1; i >= 0; i--)
    {
      TransactionId xid_tmp = lfirst_xid(list_nth_cell(subxactlist, i));

      if (xid_tmp == subxid)
      {
        RollbackToSavepoint(spname);
        CommitTransactionCommand();
        subxactlist = list_truncate(subxactlist, i);
        break;
      }
    }
  }
}

/*
 * Set the fileset state for a particular parallel apply worker. The fileset
 * will be set once the leader worker serialized all changes to the file
 * so that it can be used by parallel apply worker.
 */
void
pa_set_fileset_state(ParallelApplyWorkerShared *wshared,
           PartialFileSetState fileset_state)
{
  SpinLockAcquire(&wshared->mutex);
  wshared->fileset_state = fileset_state;

  if (fileset_state == FS_SERIALIZE_DONE)
  {
    Assert(am_leader_apply_worker());
    Assert(MyLogicalRepWorker->stream_fileset);
    wshared->fileset = *MyLogicalRepWorker->stream_fileset;
  }

  SpinLockRelease(&wshared->mutex);
}

/*
 * Get the fileset state for the current parallel apply worker.
 */
static PartialFileSetState
pa_get_fileset_state(void)
{
  PartialFileSetState fileset_state;

  Assert(am_parallel_apply_worker());

  SpinLockAcquire(&MyParallelShared->mutex);
  fileset_state = MyParallelShared->fileset_state;
  SpinLockRelease(&MyParallelShared->mutex);

  return fileset_state;
}

/*
 * Helper functions to acquire and release a lock for each stream block.
 *
 * Set locktag_field4 to PARALLEL_APPLY_LOCK_STREAM to indicate that it's a
 * stream lock.
 *
 * Refer to the comments atop this file to see how the stream lock is used.
 */
void
pa_lock_stream(TransactionId xid, LOCKMODE lockmode)
{
  LockApplyTransactionForSession(MyLogicalRepWorker->subid, xid,
                   PARALLEL_APPLY_LOCK_STREAM, lockmode);
}

void
pa_unlock_stream(TransactionId xid, LOCKMODE lockmode)
{
  UnlockApplyTransactionForSession(MyLogicalRepWorker->subid, xid,
                   PARALLEL_APPLY_LOCK_STREAM, lockmode);
}

/*
 * Helper functions to acquire and release a lock for each local transaction
 * apply.
 *
 * Set locktag_field4 to PARALLEL_APPLY_LOCK_XACT to indicate that it's a
 * transaction lock.
 *
 * Note that all the callers must pass a remote transaction ID instead of a
 * local transaction ID as xid. This is because the local transaction ID will
 * only be assigned while applying the first change in the parallel apply but
 * it's possible that the first change in the parallel apply worker is blocked
 * by a concurrently executing transaction in another parallel apply worker. We
 * can only communicate the local transaction id to the leader after applying
 * the first change so it won't be able to wait after sending the xact finish
 * command using this lock.
 *
 * Refer to the comments atop this file to see how the transaction lock is
 * used.
 */
void
pa_lock_transaction(TransactionId xid, LOCKMODE lockmode)
{
  LockApplyTransactionForSession(MyLogicalRepWorker->subid, xid,
                   PARALLEL_APPLY_LOCK_XACT, lockmode);
}

void
pa_unlock_transaction(TransactionId xid, LOCKMODE lockmode)
{
  UnlockApplyTransactionForSession(MyLogicalRepWorker->subid, xid,
                   PARALLEL_APPLY_LOCK_XACT, lockmode);
}

/*
 * Decrement the number of pending streaming blocks and wait on the stream lock
 * if there is no pending block available.
 */
void
pa_decr_and_wait_stream_block(void)
{
  Assert(am_parallel_apply_worker());

  /*
   * It is only possible to not have any pending stream chunks when we are
   * applying spooled messages.
   */
  if (pg_atomic_read_u32(&MyParallelShared->pending_stream_count) == 0)
  {
    if (pa_has_spooled_message_pending())
      return;

    elog(ERROR, "invalid pending streaming chunk 0");
  }

  if (pg_atomic_sub_fetch_u32(&MyParallelShared->pending_stream_count, 1) == 0)
  {
    pa_lock_stream(MyParallelShared->xid, AccessShareLock);
    pa_unlock_stream(MyParallelShared->xid, AccessShareLock);
  }
}

/*
 * Finish processing the streaming transaction in the leader apply worker.
 */
void
pa_xact_finish(ParallelApplyWorkerInfo *winfo, XLogRecPtr remote_lsn)
{
  Assert(am_leader_apply_worker());

  /*
   * Unlock the shared object lock so that parallel apply worker can
   * continue to receive and apply changes.
   */
  pa_unlock_stream(winfo->shared->xid, AccessExclusiveLock);

  /*
   * Wait for that worker to finish. This is necessary to maintain commit
   * order which avoids failures due to transaction dependencies and
   * deadlocks.
   */
  pa_wait_for_xact_finish(winfo);

  if (!XLogRecPtrIsInvalid(remote_lsn))
    store_flush_position(remote_lsn, winfo->shared->last_commit_end);

  pa_free_worker(winfo);
}

Coverage Report

Created: 2025-06-15 06:31