Adding upstream version 14.5.upstream/14.5upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1585 insertions, 0 deletions

diff --git a/src/backend/access/transam/parallel.c b/src/backend/access/transam/parallel.c
new file mode 100644
index 0000000..8ce95ab
--- /dev/null
+++ b/src/backend/access/transam/parallel.c
@@ -0,0 +1,1585 @@
+/*-------------------------------------------------------------------------
+ *
+ * parallel.c
+ *	  Infrastructure for launching parallel workers
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/transam/parallel.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/heapam.h"
+#include "access/nbtree.h"
+#include "access/parallel.h"
+#include "access/session.h"
+#include "access/xact.h"
+#include "access/xlog.h"
+#include "catalog/index.h"
+#include "catalog/namespace.h"
+#include "catalog/pg_enum.h"
+#include "catalog/storage.h"
+#include "commands/async.h"
+#include "executor/execParallel.h"
+#include "libpq/libpq.h"
+#include "libpq/pqformat.h"
+#include "libpq/pqmq.h"
+#include "miscadmin.h"
+#include "optimizer/optimizer.h"
+#include "pgstat.h"
+#include "storage/ipc.h"
+#include "storage/predicate.h"
+#include "storage/sinval.h"
+#include "storage/spin.h"
+#include "tcop/tcopprot.h"
+#include "utils/combocid.h"
+#include "utils/guc.h"
+#include "utils/inval.h"
+#include "utils/memutils.h"
+#include "utils/relmapper.h"
+#include "utils/snapmgr.h"
+#include "utils/typcache.h"
+
+/*
+ * We don't want to waste a lot of memory on an error queue which, most of
+ * the time, will process only a handful of small messages.  However, 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 PARALLEL_ERROR_QUEUE_SIZE			16384
+
+/* Magic number for parallel context TOC. */
+#define PARALLEL_MAGIC						0x50477c7c
+
+/*
+ * Magic numbers for per-context parallel state sharing.  Higher-level code
+ * should use smaller values, leaving these very large ones for use by this
+ * module.
+ */
+#define PARALLEL_KEY_FIXED					UINT64CONST(0xFFFFFFFFFFFF0001)
+#define PARALLEL_KEY_ERROR_QUEUE			UINT64CONST(0xFFFFFFFFFFFF0002)
+#define PARALLEL_KEY_LIBRARY				UINT64CONST(0xFFFFFFFFFFFF0003)
+#define PARALLEL_KEY_GUC					UINT64CONST(0xFFFFFFFFFFFF0004)
+#define PARALLEL_KEY_COMBO_CID				UINT64CONST(0xFFFFFFFFFFFF0005)
+#define PARALLEL_KEY_TRANSACTION_SNAPSHOT	UINT64CONST(0xFFFFFFFFFFFF0006)
+#define PARALLEL_KEY_ACTIVE_SNAPSHOT		UINT64CONST(0xFFFFFFFFFFFF0007)
+#define PARALLEL_KEY_TRANSACTION_STATE		UINT64CONST(0xFFFFFFFFFFFF0008)
+#define PARALLEL_KEY_ENTRYPOINT				UINT64CONST(0xFFFFFFFFFFFF0009)
+#define PARALLEL_KEY_SESSION_DSM			UINT64CONST(0xFFFFFFFFFFFF000A)
+#define PARALLEL_KEY_PENDING_SYNCS			UINT64CONST(0xFFFFFFFFFFFF000B)
+#define PARALLEL_KEY_REINDEX_STATE			UINT64CONST(0xFFFFFFFFFFFF000C)
+#define PARALLEL_KEY_RELMAPPER_STATE		UINT64CONST(0xFFFFFFFFFFFF000D)
+#define PARALLEL_KEY_UNCOMMITTEDENUMS		UINT64CONST(0xFFFFFFFFFFFF000E)
+
+/* Fixed-size parallel state. */
+typedef struct FixedParallelState
+{
+	/* Fixed-size state that workers must restore. */
+	Oid			database_id;
+	Oid			authenticated_user_id;
+	Oid			current_user_id;
+	Oid			outer_user_id;
+	Oid			temp_namespace_id;
+	Oid			temp_toast_namespace_id;
+	int			sec_context;
+	bool		is_superuser;
+	PGPROC	   *parallel_leader_pgproc;
+	pid_t		parallel_leader_pid;
+	BackendId	parallel_leader_backend_id;
+	TimestampTz xact_ts;
+	TimestampTz stmt_ts;
+	SerializableXactHandle serializable_xact_handle;
+
+	/* Mutex protects remaining fields. */
+	slock_t		mutex;
+
+	/* Maximum XactLastRecEnd of any worker. */
+	XLogRecPtr	last_xlog_end;
+} FixedParallelState;
+
+/*
+ * Our parallel worker number.  We initialize this to -1, meaning that we are
+ * not a parallel worker.  In parallel workers, it will be set to a value >= 0
+ * and < the number of workers before any user code is invoked; each parallel
+ * worker will get a different parallel worker number.
+ */
+int			ParallelWorkerNumber = -1;
+
+/* Is there a parallel message pending which we need to receive? */
+volatile bool ParallelMessagePending = false;
+
+/* Are we initializing a parallel worker? */
+bool		InitializingParallelWorker = false;
+
+/* Pointer to our fixed parallel state. */
+static FixedParallelState *MyFixedParallelState;
+
+/* List of active parallel contexts. */
+static dlist_head pcxt_list = DLIST_STATIC_INIT(pcxt_list);
+
+/* Backend-local copy of data from FixedParallelState. */
+static pid_t ParallelLeaderPid;
+
+/*
+ * List of internal parallel worker entry points.  We need this for
+ * reasons explained in LookupParallelWorkerFunction(), below.
+ */
+static const struct
+{
+	const char *fn_name;
+	parallel_worker_main_type fn_addr;
+}			InternalParallelWorkers[] =
+
+{
+	{
+		"ParallelQueryMain", ParallelQueryMain
+	},
+	{
+		"_bt_parallel_build_main", _bt_parallel_build_main
+	},
+	{
+		"parallel_vacuum_main", parallel_vacuum_main
+	}
+};
+
+/* Private functions. */
+static void HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg);
+static void WaitForParallelWorkersToExit(ParallelContext *pcxt);
+static parallel_worker_main_type LookupParallelWorkerFunction(const char *libraryname, const char *funcname);
+static void ParallelWorkerShutdown(int code, Datum arg);
+
+
+/*
+ * Establish a new parallel context.  This should be done after entering
+ * parallel mode, and (unless there is an error) the context should be
+ * destroyed before exiting the current subtransaction.
+ */
+ParallelContext *
+CreateParallelContext(const char *library_name, const char *function_name,
+					  int nworkers)
+{
+	MemoryContext oldcontext;
+	ParallelContext *pcxt;
+
+	/* It is unsafe to create a parallel context if not in parallel mode. */
+	Assert(IsInParallelMode());
+
+	/* Number of workers should be non-negative. */
+	Assert(nworkers >= 0);
+
+	/* We might be running in a short-lived memory context. */
+	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+
+	/* Initialize a new ParallelContext. */
+	pcxt = palloc0(sizeof(ParallelContext));
+	pcxt->subid = GetCurrentSubTransactionId();
+	pcxt->nworkers = nworkers;
+	pcxt->nworkers_to_launch = nworkers;
+	pcxt->library_name = pstrdup(library_name);
+	pcxt->function_name = pstrdup(function_name);
+	pcxt->error_context_stack = error_context_stack;
+	shm_toc_initialize_estimator(&pcxt->estimator);
+	dlist_push_head(&pcxt_list, &pcxt->node);
+
+	/* Restore previous memory context. */
+	MemoryContextSwitchTo(oldcontext);
+
+	return pcxt;
+}
+
+/*
+ * Establish the dynamic shared memory segment for a parallel context and
+ * copy state and other bookkeeping information that will be needed by
+ * parallel workers into it.
+ */
+void
+InitializeParallelDSM(ParallelContext *pcxt)
+{
+	MemoryContext oldcontext;
+	Size		library_len = 0;
+	Size		guc_len = 0;
+	Size		combocidlen = 0;
+	Size		tsnaplen = 0;
+	Size		asnaplen = 0;
+	Size		tstatelen = 0;
+	Size		pendingsyncslen = 0;
+	Size		reindexlen = 0;
+	Size		relmapperlen = 0;
+	Size		uncommittedenumslen = 0;
+	Size		segsize = 0;
+	int			i;
+	FixedParallelState *fps;
+	dsm_handle	session_dsm_handle = DSM_HANDLE_INVALID;
+	Snapshot	transaction_snapshot = GetTransactionSnapshot();
+	Snapshot	active_snapshot = GetActiveSnapshot();
+
+	/* We might be running in a very short-lived memory context. */
+	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+
+	/* Allow space to store the fixed-size parallel state. */
+	shm_toc_estimate_chunk(&pcxt->estimator, sizeof(FixedParallelState));
+	shm_toc_estimate_keys(&pcxt->estimator, 1);
+
+	/*
+	 * Normally, the user will have requested at least one worker process, but
+	 * if by chance they have not, we can skip a bunch of things here.
+	 */
+	if (pcxt->nworkers > 0)
+	{
+		/* Get (or create) the per-session DSM segment's handle. */
+		session_dsm_handle = GetSessionDsmHandle();
+
+		/*
+		 * If we weren't able to create a per-session DSM segment, then we can
+		 * continue but we can't safely launch any workers because their
+		 * record typmods would be incompatible so they couldn't exchange
+		 * tuples.
+		 */
+		if (session_dsm_handle == DSM_HANDLE_INVALID)
+			pcxt->nworkers = 0;
+	}
+
+	if (pcxt->nworkers > 0)
+	{
+		/* Estimate space for various kinds of state sharing. */
+		library_len = EstimateLibraryStateSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, library_len);
+		guc_len = EstimateGUCStateSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, guc_len);
+		combocidlen = EstimateComboCIDStateSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, combocidlen);
+		if (IsolationUsesXactSnapshot())
+		{
+			tsnaplen = EstimateSnapshotSpace(transaction_snapshot);
+			shm_toc_estimate_chunk(&pcxt->estimator, tsnaplen);
+		}
+		asnaplen = EstimateSnapshotSpace(active_snapshot);
+		shm_toc_estimate_chunk(&pcxt->estimator, asnaplen);
+		tstatelen = EstimateTransactionStateSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, tstatelen);
+		shm_toc_estimate_chunk(&pcxt->estimator, sizeof(dsm_handle));
+		pendingsyncslen = EstimatePendingSyncsSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, pendingsyncslen);
+		reindexlen = EstimateReindexStateSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, reindexlen);
+		relmapperlen = EstimateRelationMapSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, relmapperlen);
+		uncommittedenumslen = EstimateUncommittedEnumsSpace();
+		shm_toc_estimate_chunk(&pcxt->estimator, uncommittedenumslen);
+		/* If you add more chunks here, you probably need to add keys. */
+		shm_toc_estimate_keys(&pcxt->estimator, 11);
+
+		/* Estimate space need for error queues. */
+		StaticAssertStmt(BUFFERALIGN(PARALLEL_ERROR_QUEUE_SIZE) ==
+						 PARALLEL_ERROR_QUEUE_SIZE,
+						 "parallel error queue size not buffer-aligned");
+		shm_toc_estimate_chunk(&pcxt->estimator,
+							   mul_size(PARALLEL_ERROR_QUEUE_SIZE,
+										pcxt->nworkers));
+		shm_toc_estimate_keys(&pcxt->estimator, 1);
+
+		/* Estimate how much we'll need for the entrypoint info. */
+		shm_toc_estimate_chunk(&pcxt->estimator, strlen(pcxt->library_name) +
+							   strlen(pcxt->function_name) + 2);
+		shm_toc_estimate_keys(&pcxt->estimator, 1);
+	}
+
+	/*
+	 * Create DSM and initialize with new table of contents.  But if the user
+	 * didn't request any workers, then don't bother creating a dynamic shared
+	 * memory segment; instead, just use backend-private memory.
+	 *
+	 * Also, if we can't create a dynamic shared memory segment because the
+	 * maximum number of segments have already been created, then fall back to
+	 * backend-private memory, and plan not to use any workers.  We hope this
+	 * won't happen very often, but it's better to abandon the use of
+	 * parallelism than to fail outright.
+	 */
+	segsize = shm_toc_estimate(&pcxt->estimator);
+	if (pcxt->nworkers > 0)
+		pcxt->seg = dsm_create(segsize, DSM_CREATE_NULL_IF_MAXSEGMENTS);
+	if (pcxt->seg != NULL)
+		pcxt->toc = shm_toc_create(PARALLEL_MAGIC,
+								   dsm_segment_address(pcxt->seg),
+								   segsize);
+	else
+	{
+		pcxt->nworkers = 0;
+		pcxt->private_memory = MemoryContextAlloc(TopMemoryContext, segsize);
+		pcxt->toc = shm_toc_create(PARALLEL_MAGIC, pcxt->private_memory,
+								   segsize);
+	}
+
+	/* Initialize fixed-size state in shared memory. */
+	fps = (FixedParallelState *)
+		shm_toc_allocate(pcxt->toc, sizeof(FixedParallelState));
+	fps->database_id = MyDatabaseId;
+	fps->authenticated_user_id = GetAuthenticatedUserId();
+	fps->outer_user_id = GetCurrentRoleId();
+	fps->is_superuser = session_auth_is_superuser;
+	GetUserIdAndSecContext(&fps->current_user_id, &fps->sec_context);
+	GetTempNamespaceState(&fps->temp_namespace_id,
+						  &fps->temp_toast_namespace_id);
+	fps->parallel_leader_pgproc = MyProc;
+	fps->parallel_leader_pid = MyProcPid;
+	fps->parallel_leader_backend_id = MyBackendId;
+	fps->xact_ts = GetCurrentTransactionStartTimestamp();
+	fps->stmt_ts = GetCurrentStatementStartTimestamp();
+	fps->serializable_xact_handle = ShareSerializableXact();
+	SpinLockInit(&fps->mutex);
+	fps->last_xlog_end = 0;
+	shm_toc_insert(pcxt->toc, PARALLEL_KEY_FIXED, fps);
+
+	/* We can skip the rest of this if we're not budgeting for any workers. */
+	if (pcxt->nworkers > 0)
+	{
+		char	   *libraryspace;
+		char	   *gucspace;
+		char	   *combocidspace;
+		char	   *tsnapspace;
+		char	   *asnapspace;
+		char	   *tstatespace;
+		char	   *pendingsyncsspace;
+		char	   *reindexspace;
+		char	   *relmapperspace;
+		char	   *error_queue_space;
+		char	   *session_dsm_handle_space;
+		char	   *entrypointstate;
+		char	   *uncommittedenumsspace;
+		Size		lnamelen;
+
+		/* Serialize shared libraries we have loaded. */
+		libraryspace = shm_toc_allocate(pcxt->toc, library_len);
+		SerializeLibraryState(library_len, libraryspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_LIBRARY, libraryspace);
+
+		/* Serialize GUC settings. */
+		gucspace = shm_toc_allocate(pcxt->toc, guc_len);
+		SerializeGUCState(guc_len, gucspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_GUC, gucspace);
+
+		/* Serialize combo CID state. */
+		combocidspace = shm_toc_allocate(pcxt->toc, combocidlen);
+		SerializeComboCIDState(combocidlen, combocidspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_COMBO_CID, combocidspace);
+
+		/*
+		 * Serialize the transaction snapshot if the transaction
+		 * isolation-level uses a transaction snapshot.
+		 */
+		if (IsolationUsesXactSnapshot())
+		{
+			tsnapspace = shm_toc_allocate(pcxt->toc, tsnaplen);
+			SerializeSnapshot(transaction_snapshot, tsnapspace);
+			shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
+						   tsnapspace);
+		}
+
+		/* Serialize the active snapshot. */
+		asnapspace = shm_toc_allocate(pcxt->toc, asnaplen);
+		SerializeSnapshot(active_snapshot, asnapspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, asnapspace);
+
+		/* Provide the handle for per-session segment. */
+		session_dsm_handle_space = shm_toc_allocate(pcxt->toc,
+													sizeof(dsm_handle));
+		*(dsm_handle *) session_dsm_handle_space = session_dsm_handle;
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_SESSION_DSM,
+					   session_dsm_handle_space);
+
+		/* Serialize transaction state. */
+		tstatespace = shm_toc_allocate(pcxt->toc, tstatelen);
+		SerializeTransactionState(tstatelen, tstatespace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_STATE, tstatespace);
+
+		/* Serialize pending syncs. */
+		pendingsyncsspace = shm_toc_allocate(pcxt->toc, pendingsyncslen);
+		SerializePendingSyncs(pendingsyncslen, pendingsyncsspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_PENDING_SYNCS,
+					   pendingsyncsspace);
+
+		/* Serialize reindex state. */
+		reindexspace = shm_toc_allocate(pcxt->toc, reindexlen);
+		SerializeReindexState(reindexlen, reindexspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_REINDEX_STATE, reindexspace);
+
+		/* Serialize relmapper state. */
+		relmapperspace = shm_toc_allocate(pcxt->toc, relmapperlen);
+		SerializeRelationMap(relmapperlen, relmapperspace);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_RELMAPPER_STATE,
+					   relmapperspace);
+
+		/* Serialize uncommitted enum state. */
+		uncommittedenumsspace = shm_toc_allocate(pcxt->toc,
+												 uncommittedenumslen);
+		SerializeUncommittedEnums(uncommittedenumsspace, uncommittedenumslen);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_UNCOMMITTEDENUMS,
+					   uncommittedenumsspace);
+
+		/* Allocate space for worker information. */
+		pcxt->worker = palloc0(sizeof(ParallelWorkerInfo) * pcxt->nworkers);
+
+		/*
+		 * Establish error queues in dynamic shared memory.
+		 *
+		 * These queues should be used only for transmitting ErrorResponse,
+		 * NoticeResponse, and NotifyResponse protocol messages.  Tuple data
+		 * should be transmitted via separate (possibly larger?) queues.
+		 */
+		error_queue_space =
+			shm_toc_allocate(pcxt->toc,
+							 mul_size(PARALLEL_ERROR_QUEUE_SIZE,
+									  pcxt->nworkers));
+		for (i = 0; i < pcxt->nworkers; ++i)
+		{
+			char	   *start;
+			shm_mq	   *mq;
+
+			start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
+			mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
+			shm_mq_set_receiver(mq, MyProc);
+			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		}
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, error_queue_space);
+
+		/*
+		 * Serialize entrypoint information.  It's unsafe to pass function
+		 * pointers across processes, as the function pointer may be different
+		 * in each process in EXEC_BACKEND builds, so we always pass library
+		 * and function name.  (We use library name "postgres" for functions
+		 * in the core backend.)
+		 */
+		lnamelen = strlen(pcxt->library_name);
+		entrypointstate = shm_toc_allocate(pcxt->toc, lnamelen +
+										   strlen(pcxt->function_name) + 2);
+		strcpy(entrypointstate, pcxt->library_name);
+		strcpy(entrypointstate + lnamelen + 1, pcxt->function_name);
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_ENTRYPOINT, entrypointstate);
+	}
+
+	/* Restore previous memory context. */
+	MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Reinitialize the dynamic shared memory segment for a parallel context such
+ * that we could launch workers for it again.
+ */
+void
+ReinitializeParallelDSM(ParallelContext *pcxt)
+{
+	FixedParallelState *fps;
+
+	/* Wait for any old workers to exit. */
+	if (pcxt->nworkers_launched > 0)
+	{
+		WaitForParallelWorkersToFinish(pcxt);
+		WaitForParallelWorkersToExit(pcxt);
+		pcxt->nworkers_launched = 0;
+		if (pcxt->known_attached_workers)
+		{
+			pfree(pcxt->known_attached_workers);
+			pcxt->known_attached_workers = NULL;
+			pcxt->nknown_attached_workers = 0;
+		}
+	}
+
+	/* Reset a few bits of fixed parallel state to a clean state. */
+	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
+	fps->last_xlog_end = 0;
+
+	/* Recreate error queues (if they exist). */
+	if (pcxt->nworkers > 0)
+	{
+		char	   *error_queue_space;
+		int			i;
+
+		error_queue_space =
+			shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+		for (i = 0; i < pcxt->nworkers; ++i)
+		{
+			char	   *start;
+			shm_mq	   *mq;
+
+			start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
+			mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
+			shm_mq_set_receiver(mq, MyProc);
+			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		}
+	}
+}
+
+/*
+ * Reinitialize parallel workers for a parallel context such that we could
+ * launch a different number of workers.  This is required for cases where
+ * we need to reuse the same DSM segment, but the number of workers can
+ * vary from run-to-run.
+ */
+void
+ReinitializeParallelWorkers(ParallelContext *pcxt, int nworkers_to_launch)
+{
+	/*
+	 * The number of workers that need to be launched must be less than the
+	 * number of workers with which the parallel context is initialized.
+	 */
+	Assert(pcxt->nworkers >= nworkers_to_launch);
+	pcxt->nworkers_to_launch = nworkers_to_launch;
+}
+
+/*
+ * Launch parallel workers.
+ */
+void
+LaunchParallelWorkers(ParallelContext *pcxt)
+{
+	MemoryContext oldcontext;
+	BackgroundWorker worker;
+	int			i;
+	bool		any_registrations_failed = false;
+
+	/* Skip this if we have no workers. */
+	if (pcxt->nworkers == 0 || pcxt->nworkers_to_launch == 0)
+		return;
+
+	/* We need to be a lock group leader. */
+	BecomeLockGroupLeader();
+
+	/* If we do have workers, we'd better have a DSM segment. */
+	Assert(pcxt->seg != NULL);
+
+	/* We might be running in a short-lived memory context. */
+	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+
+	/* Configure a worker. */
+	memset(&worker, 0, sizeof(worker));
+	snprintf(worker.bgw_name, BGW_MAXLEN, "parallel worker for PID %d",
+			 MyProcPid);
+	snprintf(worker.bgw_type, BGW_MAXLEN, "parallel worker");
+	worker.bgw_flags =
+		BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION
+		| BGWORKER_CLASS_PARALLEL;
+	worker.bgw_start_time = BgWorkerStart_ConsistentState;
+	worker.bgw_restart_time = BGW_NEVER_RESTART;
+	sprintf(worker.bgw_library_name, "postgres");
+	sprintf(worker.bgw_function_name, "ParallelWorkerMain");
+	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
+	worker.bgw_notify_pid = MyProcPid;
+
+	/*
+	 * Start workers.
+	 *
+	 * The caller must be able to tolerate ending up with fewer workers than
+	 * expected, so there is no need to throw an error here if registration
+	 * fails.  It wouldn't help much anyway, because registering the worker in
+	 * no way guarantees that it will start up and initialize successfully.
+	 */
+	for (i = 0; i < pcxt->nworkers_to_launch; ++i)
+	{
+		memcpy(worker.bgw_extra, &i, sizeof(int));
+		if (!any_registrations_failed &&
+			RegisterDynamicBackgroundWorker(&worker,
+											&pcxt->worker[i].bgwhandle))
+		{
+			shm_mq_set_handle(pcxt->worker[i].error_mqh,
+							  pcxt->worker[i].bgwhandle);
+			pcxt->nworkers_launched++;
+		}
+		else
+		{
+			/*
+			 * If we weren't able to register the worker, then we've bumped up
+			 * against the max_worker_processes limit, and future
+			 * registrations will probably fail too, so arrange to skip them.
+			 * But we still have to execute this code for the remaining slots
+			 * to make sure that we forget about the error queues we budgeted
+			 * for those workers.  Otherwise, we'll wait for them to start,
+			 * but they never will.
+			 */
+			any_registrations_failed = true;
+			pcxt->worker[i].bgwhandle = NULL;
+			shm_mq_detach(pcxt->worker[i].error_mqh);
+			pcxt->worker[i].error_mqh = NULL;
+		}
+	}
+
+	/*
+	 * Now that nworkers_launched has taken its final value, we can initialize
+	 * known_attached_workers.
+	 */
+	if (pcxt->nworkers_launched > 0)
+	{
+		pcxt->known_attached_workers =
+			palloc0(sizeof(bool) * pcxt->nworkers_launched);
+		pcxt->nknown_attached_workers = 0;
+	}
+
+	/* Restore previous memory context. */
+	MemoryContextSwitchTo(oldcontext);
+}
+
+/*
+ * Wait for all workers to attach to their error queues, and throw an error if
+ * any worker fails to do this.
+ *
+ * Callers can assume that if this function returns successfully, then the
+ * number of workers given by pcxt->nworkers_launched have initialized and
+ * attached to their error queues.  Whether or not these workers are guaranteed
+ * to still be running depends on what code the caller asked them to run;
+ * this function does not guarantee that they have not exited.  However, it
+ * does guarantee that any workers which exited must have done so cleanly and
+ * after successfully performing the work with which they were tasked.
+ *
+ * If this function is not called, then some of the workers that were launched
+ * may not have been started due to a fork() failure, or may have exited during
+ * early startup prior to attaching to the error queue, so nworkers_launched
+ * cannot be viewed as completely reliable.  It will never be less than the
+ * number of workers which actually started, but it might be more.  Any workers
+ * that failed to start will still be discovered by
+ * WaitForParallelWorkersToFinish and an error will be thrown at that time,
+ * provided that function is eventually reached.
+ *
+ * In general, the leader process should do as much work as possible before
+ * calling this function.  fork() failures and other early-startup failures
+ * are very uncommon, and having the leader sit idle when it could be doing
+ * useful work is undesirable.  However, if the leader needs to wait for
+ * all of its workers or for a specific worker, it may want to call this
+ * function before doing so.  If not, it must make some other provision for
+ * the failure-to-start case, lest it wait forever.  On the other hand, a
+ * leader which never waits for a worker that might not be started yet, or
+ * at least never does so prior to WaitForParallelWorkersToFinish(), need not
+ * call this function at all.
+ */
+void
+WaitForParallelWorkersToAttach(ParallelContext *pcxt)
+{
+	int			i;
+
+	/* Skip this if we have no launched workers. */
+	if (pcxt->nworkers_launched == 0)
+		return;
+
+	for (;;)
+	{
+		/*
+		 * This will process any parallel messages that are pending and it may
+		 * also throw an error propagated from a worker.
+		 */
+		CHECK_FOR_INTERRUPTS();
+
+		for (i = 0; i < pcxt->nworkers_launched; ++i)
+		{
+			BgwHandleStatus status;
+			shm_mq	   *mq;
+			int			rc;
+			pid_t		pid;
+
+			if (pcxt->known_attached_workers[i])
+				continue;
+
+			/*
+			 * If error_mqh is NULL, then the worker has already exited
+			 * cleanly.
+			 */
+			if (pcxt->worker[i].error_mqh == NULL)
+			{
+				pcxt->known_attached_workers[i] = true;
+				++pcxt->nknown_attached_workers;
+				continue;
+			}
+
+			status = GetBackgroundWorkerPid(pcxt->worker[i].bgwhandle, &pid);
+			if (status == BGWH_STARTED)
+			{
+				/* Has the worker attached to the error queue? */
+				mq = shm_mq_get_queue(pcxt->worker[i].error_mqh);
+				if (shm_mq_get_sender(mq) != NULL)
+				{
+					/* Yes, so it is known to be attached. */
+					pcxt->known_attached_workers[i] = true;
+					++pcxt->nknown_attached_workers;
+				}
+			}
+			else if (status == BGWH_STOPPED)
+			{
+				/*
+				 * If the worker stopped without attaching to the error queue,
+				 * throw an error.
+				 */
+				mq = shm_mq_get_queue(pcxt->worker[i].error_mqh);
+				if (shm_mq_get_sender(mq) == NULL)
+					ereport(ERROR,
+							(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+							 errmsg("parallel worker failed to initialize"),
+							 errhint("More details may be available in the server log.")));
+
+				pcxt->known_attached_workers[i] = true;
+				++pcxt->nknown_attached_workers;
+			}
+			else
+			{
+				/*
+				 * Worker not yet started, so we must wait.  The postmaster
+				 * will notify us if the worker's state changes.  Our latch
+				 * might also get set for some other reason, but if so we'll
+				 * just end up waiting for the same worker again.
+				 */
+				rc = WaitLatch(MyLatch,
+							   WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
+							   -1, WAIT_EVENT_BGWORKER_STARTUP);
+
+				if (rc & WL_LATCH_SET)
+					ResetLatch(MyLatch);
+			}
+		}
+
+		/* If all workers are known to have started, we're done. */
+		if (pcxt->nknown_attached_workers >= pcxt->nworkers_launched)
+		{
+			Assert(pcxt->nknown_attached_workers == pcxt->nworkers_launched);
+			break;
+		}
+	}
+}
+
+/*
+ * Wait for all workers to finish computing.
+ *
+ * Even if the parallel operation seems to have completed successfully, it's
+ * important to call this function afterwards.  We must not miss any errors
+ * the workers may have thrown during the parallel operation, or any that they
+ * may yet throw while shutting down.
+ *
+ * Also, we want to update our notion of XactLastRecEnd based on worker
+ * feedback.
+ */
+void
+WaitForParallelWorkersToFinish(ParallelContext *pcxt)
+{
+	for (;;)
+	{
+		bool		anyone_alive = false;
+		int			nfinished = 0;
+		int			i;
+
+		/*
+		 * This will process any parallel messages that are pending, which may
+		 * change the outcome of the loop that follows.  It may also throw an
+		 * error propagated from a worker.
+		 */
+		CHECK_FOR_INTERRUPTS();
+
+		for (i = 0; i < pcxt->nworkers_launched; ++i)
+		{
+			/*
+			 * If error_mqh is NULL, then the worker has already exited
+			 * cleanly.  If we have received a message through error_mqh from
+			 * the worker, we know it started up cleanly, and therefore we're
+			 * certain to be notified when it exits.
+			 */
+			if (pcxt->worker[i].error_mqh == NULL)
+				++nfinished;
+			else if (pcxt->known_attached_workers[i])
+			{
+				anyone_alive = true;
+				break;
+			}
+		}
+
+		if (!anyone_alive)
+		{
+			/* If all workers are known to have finished, we're done. */
+			if (nfinished >= pcxt->nworkers_launched)
+			{
+				Assert(nfinished == pcxt->nworkers_launched);
+				break;
+			}
+
+			/*
+			 * We didn't detect any living workers, but not all workers are
+			 * known to have exited cleanly.  Either not all workers have
+			 * launched yet, or maybe some of them failed to start or
+			 * terminated abnormally.
+			 */
+			for (i = 0; i < pcxt->nworkers_launched; ++i)
+			{
+				pid_t		pid;
+				shm_mq	   *mq;
+
+				/*
+				 * If the worker is BGWH_NOT_YET_STARTED or BGWH_STARTED, we
+				 * should just keep waiting.  If it is BGWH_STOPPED, then
+				 * further investigation is needed.
+				 */
+				if (pcxt->worker[i].error_mqh == NULL ||
+					pcxt->worker[i].bgwhandle == NULL ||
+					GetBackgroundWorkerPid(pcxt->worker[i].bgwhandle,
+										   &pid) != BGWH_STOPPED)
+					continue;
+
+				/*
+				 * Check whether the worker ended up stopped without ever
+				 * attaching to the error queue.  If so, the postmaster was
+				 * unable to fork the worker or it exited without initializing
+				 * properly.  We must throw an error, since the caller may
+				 * have been expecting the worker to do some work before
+				 * exiting.
+				 */
+				mq = shm_mq_get_queue(pcxt->worker[i].error_mqh);
+				if (shm_mq_get_sender(mq) == NULL)
+					ereport(ERROR,
+							(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+							 errmsg("parallel worker failed to initialize"),
+							 errhint("More details may be available in the server log.")));
+
+				/*
+				 * The worker is stopped, but is attached to the error queue.
+				 * Unless there's a bug somewhere, this will only happen when
+				 * the worker writes messages and terminates after the
+				 * CHECK_FOR_INTERRUPTS() near the top of this function and
+				 * before the call to GetBackgroundWorkerPid().  In that case,
+				 * or latch should have been set as well and the right things
+				 * will happen on the next pass through the loop.
+				 */
+			}
+		}
+
+		(void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, -1,
+						 WAIT_EVENT_PARALLEL_FINISH);
+		ResetLatch(MyLatch);
+	}
+
+	if (pcxt->toc != NULL)
+	{
+		FixedParallelState *fps;
+
+		fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
+		if (fps->last_xlog_end > XactLastRecEnd)
+			XactLastRecEnd = fps->last_xlog_end;
+	}
+}
+
+/*
+ * Wait for all workers to exit.
+ *
+ * This function ensures that workers have been completely shutdown.  The
+ * difference between WaitForParallelWorkersToFinish and this function is
+ * that the former just ensures that last message sent by a worker backend is
+ * received by the leader backend whereas this ensures the complete shutdown.
+ */
+static void
+WaitForParallelWorkersToExit(ParallelContext *pcxt)
+{
+	int			i;
+
+	/* Wait until the workers actually die. */
+	for (i = 0; i < pcxt->nworkers_launched; ++i)
+	{
+		BgwHandleStatus status;
+
+		if (pcxt->worker == NULL || pcxt->worker[i].bgwhandle == NULL)
+			continue;
+
+		status = WaitForBackgroundWorkerShutdown(pcxt->worker[i].bgwhandle);
+
+		/*
+		 * If the postmaster kicked the bucket, we have no chance of cleaning
+		 * up safely -- we won't be able to tell when our workers are actually
+		 * dead.  This doesn't necessitate a PANIC since they will all abort
+		 * eventually, but we can't safely continue this session.
+		 */
+		if (status == BGWH_POSTMASTER_DIED)
+			ereport(FATAL,
+					(errcode(ERRCODE_ADMIN_SHUTDOWN),
+					 errmsg("postmaster exited during a parallel transaction")));
+
+		/* Release memory. */
+		pfree(pcxt->worker[i].bgwhandle);
+		pcxt->worker[i].bgwhandle = NULL;
+	}
+}
+
+/*
+ * Destroy a parallel context.
+ *
+ * If expecting a clean exit, you should use WaitForParallelWorkersToFinish()
+ * first, before calling this function.  When this function is invoked, any
+ * remaining workers are forcibly killed; the dynamic shared memory segment
+ * is unmapped; and we then wait (uninterruptibly) for the workers to exit.
+ */
+void
+DestroyParallelContext(ParallelContext *pcxt)
+{
+	int			i;
+
+	/*
+	 * Be careful about order of operations here!  We remove the parallel
+	 * context from the list before we do anything else; otherwise, if an
+	 * error occurs during a subsequent step, we might try to nuke it again
+	 * from AtEOXact_Parallel or AtEOSubXact_Parallel.
+	 */
+	dlist_delete(&pcxt->node);
+
+	/* Kill each worker in turn, and forget their error queues. */
+	if (pcxt->worker != NULL)
+	{
+		for (i = 0; i < pcxt->nworkers_launched; ++i)
+		{
+			if (pcxt->worker[i].error_mqh != NULL)
+			{
+				TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);
+
+				shm_mq_detach(pcxt->worker[i].error_mqh);
+				pcxt->worker[i].error_mqh = NULL;
+			}
+		}
+	}
+
+	/*
+	 * If we have allocated a shared memory segment, detach it.  This will
+	 * implicitly detach the error queues, and any other shared memory queues,
+	 * stored there.
+	 */
+	if (pcxt->seg != NULL)
+	{
+		dsm_detach(pcxt->seg);
+		pcxt->seg = NULL;
+	}
+
+	/*
+	 * If this parallel context is actually in backend-private memory rather
+	 * than shared memory, free that memory instead.
+	 */
+	if (pcxt->private_memory != NULL)
+	{
+		pfree(pcxt->private_memory);
+		pcxt->private_memory = NULL;
+	}
+
+	/*
+	 * We can't finish transaction commit or abort until all of the workers
+	 * have exited.  This means, in particular, that we can't respond to
+	 * interrupts at this stage.
+	 */
+	HOLD_INTERRUPTS();
+	WaitForParallelWorkersToExit(pcxt);
+	RESUME_INTERRUPTS();
+
+	/* Free the worker array itself. */
+	if (pcxt->worker != NULL)
+	{
+		pfree(pcxt->worker);
+		pcxt->worker = NULL;
+	}
+
+	/* Free memory. */
+	pfree(pcxt->library_name);
+	pfree(pcxt->function_name);
+	pfree(pcxt);
+}
+
+/*
+ * Are there any parallel contexts currently active?
+ */
+bool
+ParallelContextActive(void)
+{
+	return !dlist_is_empty(&pcxt_list);
+}
+
+/*
+ * Handle receipt of an interrupt indicating a parallel 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
+ * HandleParallelMessages().
+ */
+void
+HandleParallelMessageInterrupt(void)
+{
+	InterruptPending = true;
+	ParallelMessagePending = true;
+	SetLatch(MyLatch);
+}
+
+/*
+ * Handle any queued protocol messages received from parallel workers.
+ */
+void
+HandleParallelMessages(void)
+{
+	dlist_iter	iter;
+	MemoryContext oldcontext;
+
+	static MemoryContext hpm_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 (hpm_context == NULL)	/* first time through? */
+		hpm_context = AllocSetContextCreate(TopMemoryContext,
+											"HandleParallelMessages",
+											ALLOCSET_DEFAULT_SIZES);
+	else
+		MemoryContextReset(hpm_context);
+
+	oldcontext = MemoryContextSwitchTo(hpm_context);
+
+	/* OK to process messages.  Reset the flag saying there are more to do. */
+	ParallelMessagePending = false;
+
+	dlist_foreach(iter, &pcxt_list)
+	{
+		ParallelContext *pcxt;
+		int			i;
+
+		pcxt = dlist_container(ParallelContext, node, iter.cur);
+		if (pcxt->worker == NULL)
+			continue;
+
+		for (i = 0; i < pcxt->nworkers_launched; ++i)
+		{
+			/*
+			 * Read as many messages as we can from each worker, but stop when
+			 * either (1) the worker's error queue goes away, which can happen
+			 * if we receive a Terminate message from the worker; or (2) no
+			 * more messages can be read from the worker without blocking.
+			 */
+			while (pcxt->worker[i].error_mqh != NULL)
+			{
+				shm_mq_result res;
+				Size		nbytes;
+				void	   *data;
+
+				res = shm_mq_receive(pcxt->worker[i].error_mqh, &nbytes,
+									 &data, true);
+				if (res == SHM_MQ_WOULD_BLOCK)
+					break;
+				else if (res == SHM_MQ_SUCCESS)
+				{
+					StringInfoData msg;
+
+					initStringInfo(&msg);
+					appendBinaryStringInfo(&msg, data, nbytes);
+					HandleParallelMessage(pcxt, i, &msg);
+					pfree(msg.data);
+				}
+				else
+					ereport(ERROR,
+							(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+							 errmsg("lost connection to parallel worker")));
+			}
+		}
+	}
+
+	MemoryContextSwitchTo(oldcontext);
+
+	/* Might as well clear the context on our way out */
+	MemoryContextReset(hpm_context);
+
+	RESUME_INTERRUPTS();
+}
+
+/*
+ * Handle a single protocol message received from a single parallel worker.
+ */
+static void
+HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
+{
+	char		msgtype;
+
+	if (pcxt->known_attached_workers != NULL &&
+		!pcxt->known_attached_workers[i])
+	{
+		pcxt->known_attached_workers[i] = true;
+		pcxt->nknown_attached_workers++;
+	}
+
+	msgtype = pq_getmsgbyte(msg);
+
+	switch (msgtype)
+	{
+		case 'K':				/* BackendKeyData */
+			{
+				int32		pid = pq_getmsgint(msg, 4);
+
+				(void) pq_getmsgint(msg, 4);	/* discard cancel key */
+				(void) pq_getmsgend(msg);
+				pcxt->worker[i].pid = pid;
+				break;
+			}
+
+		case 'E':				/* ErrorResponse */
+		case 'N':				/* NoticeResponse */
+			{
+				ErrorData	edata;
+				ErrorContextCallback *save_error_context_stack;
+
+				/* Parse ErrorResponse or NoticeResponse. */
+				pq_parse_errornotice(msg, &edata);
+
+				/* Death of a worker isn't enough justification for suicide. */
+				edata.elevel = Min(edata.elevel, ERROR);
+
+				/*
+				 * If desired, add a context line to show that this is a
+				 * message propagated from a parallel worker.  Otherwise, it
+				 * can sometimes be confusing to understand what actually
+				 * happened.  (We don't do this in FORCE_PARALLEL_REGRESS mode
+				 * because it causes test-result instability depending on
+				 * whether a parallel worker is actually used or not.)
+				 */
+				if (force_parallel_mode != FORCE_PARALLEL_REGRESS)
+				{
+					if (edata.context)
+						edata.context = psprintf("%s\n%s", edata.context,
+												 _("parallel worker"));
+					else
+						edata.context = pstrdup(_("parallel worker"));
+				}
+
+				/*
+				 * Context beyond that should use the error context callbacks
+				 * that were in effect when the ParallelContext was created,
+				 * not the current ones.
+				 */
+				save_error_context_stack = error_context_stack;
+				error_context_stack = pcxt->error_context_stack;
+
+				/* Rethrow error or print notice. */
+				ThrowErrorData(&edata);
+
+				/* Not an error, so restore previous context stack. */
+				error_context_stack = save_error_context_stack;
+
+				break;
+			}
+
+		case 'A':				/* NotifyResponse */
+			{
+				/* Propagate NotifyResponse. */
+				int32		pid;
+				const char *channel;
+				const char *payload;
+
+				pid = pq_getmsgint(msg, 4);
+				channel = pq_getmsgrawstring(msg);
+				payload = pq_getmsgrawstring(msg);
+				pq_endmessage(msg);
+
+				NotifyMyFrontEnd(channel, payload, pid);
+
+				break;
+			}
+
+		case 'X':				/* Terminate, indicating clean exit */
+			{
+				shm_mq_detach(pcxt->worker[i].error_mqh);
+				pcxt->worker[i].error_mqh = NULL;
+				break;
+			}
+
+		default:
+			{
+				elog(ERROR, "unrecognized message type received from parallel worker: %c (message length %d bytes)",
+					 msgtype, msg->len);
+			}
+	}
+}
+
+/*
+ * End-of-subtransaction cleanup for parallel contexts.
+ *
+ * Currently, it's forbidden to enter or leave a subtransaction while
+ * parallel mode is in effect, so we could just blow away everything.  But
+ * we may want to relax that restriction in the future, so this code
+ * contemplates that there may be multiple subtransaction IDs in pcxt_list.
+ */
+void
+AtEOSubXact_Parallel(bool isCommit, SubTransactionId mySubId)
+{
+	while (!dlist_is_empty(&pcxt_list))
+	{
+		ParallelContext *pcxt;
+
+		pcxt = dlist_head_element(ParallelContext, node, &pcxt_list);
+		if (pcxt->subid != mySubId)
+			break;
+		if (isCommit)
+			elog(WARNING, "leaked parallel context");
+		DestroyParallelContext(pcxt);
+	}
+}
+
+/*
+ * End-of-transaction cleanup for parallel contexts.
+ */
+void
+AtEOXact_Parallel(bool isCommit)
+{
+	while (!dlist_is_empty(&pcxt_list))
+	{
+		ParallelContext *pcxt;
+
+		pcxt = dlist_head_element(ParallelContext, node, &pcxt_list);
+		if (isCommit)
+			elog(WARNING, "leaked parallel context");
+		DestroyParallelContext(pcxt);
+	}
+}
+
+/*
+ * Main entrypoint for parallel workers.
+ */
+void
+ParallelWorkerMain(Datum main_arg)
+{
+	dsm_segment *seg;
+	shm_toc    *toc;
+	FixedParallelState *fps;
+	char	   *error_queue_space;
+	shm_mq	   *mq;
+	shm_mq_handle *mqh;
+	char	   *libraryspace;
+	char	   *entrypointstate;
+	char	   *library_name;
+	char	   *function_name;
+	parallel_worker_main_type entrypt;
+	char	   *gucspace;
+	char	   *combocidspace;
+	char	   *tsnapspace;
+	char	   *asnapspace;
+	char	   *tstatespace;
+	char	   *pendingsyncsspace;
+	char	   *reindexspace;
+	char	   *relmapperspace;
+	char	   *uncommittedenumsspace;
+	StringInfoData msgbuf;
+	char	   *session_dsm_handle_space;
+	Snapshot	tsnapshot;
+	Snapshot	asnapshot;
+
+	/* Set flag to indicate that we're initializing a parallel worker. */
+	InitializingParallelWorker = true;
+
+	/* Establish signal handlers. */
+	pqsignal(SIGTERM, die);
+	BackgroundWorkerUnblockSignals();
+
+	/* Determine and set our parallel worker number. */
+	Assert(ParallelWorkerNumber == -1);
+	memcpy(&ParallelWorkerNumber, MyBgworkerEntry->bgw_extra, sizeof(int));
+
+	/* Set up a memory context to work in, just for cleanliness. */
+	CurrentMemoryContext = AllocSetContextCreate(TopMemoryContext,
+												 "Parallel worker",
+												 ALLOCSET_DEFAULT_SIZES);
+
+	/*
+	 * Attach to the dynamic shared memory segment for the parallel query, and
+	 * find its table of contents.
+	 *
+	 * Note: at this point, we have not created any ResourceOwner in this
+	 * process.  This will result in our DSM mapping surviving until process
+	 * exit, which is fine.  If there were a ResourceOwner, it would acquire
+	 * ownership of the mapping, but we have no need for that.
+	 */
+	seg = dsm_attach(DatumGetUInt32(main_arg));
+	if (seg == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("could not map dynamic shared memory segment")));
+	toc = shm_toc_attach(PARALLEL_MAGIC, dsm_segment_address(seg));
+	if (toc == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("invalid magic number in dynamic shared memory segment")));
+
+	/* Look up fixed parallel state. */
+	fps = shm_toc_lookup(toc, PARALLEL_KEY_FIXED, false);
+	MyFixedParallelState = fps;
+
+	/* Arrange to signal the leader if we exit. */
+	ParallelLeaderPid = fps->parallel_leader_pid;
+	ParallelLeaderBackendId = fps->parallel_leader_backend_id;
+	on_shmem_exit(ParallelWorkerShutdown, (Datum) 0);
+
+	/*
+	 * Now we can find and attach to the error queue provided for us.  That's
+	 * good, because until we do that, any errors that happen here will not be
+	 * reported back to the process that requested that this worker be
+	 * launched.
+	 */
+	error_queue_space = shm_toc_lookup(toc, PARALLEL_KEY_ERROR_QUEUE, false);
+	mq = (shm_mq *) (error_queue_space +
+					 ParallelWorkerNumber * PARALLEL_ERROR_QUEUE_SIZE);
+	shm_mq_set_sender(mq, MyProc);
+	mqh = shm_mq_attach(mq, seg, NULL);
+	pq_redirect_to_shm_mq(seg, mqh);
+	pq_set_parallel_leader(fps->parallel_leader_pid,
+						   fps->parallel_leader_backend_id);
+
+	/*
+	 * Send a BackendKeyData message to the process that initiated parallelism
+	 * so that it has access to our PID before it receives any other messages
+	 * from us.  Our cancel key is sent, too, since that's the way the
+	 * protocol message is defined, but it won't actually be used for anything
+	 * in this case.
+	 */
+	pq_beginmessage(&msgbuf, 'K');
+	pq_sendint32(&msgbuf, (int32) MyProcPid);
+	pq_sendint32(&msgbuf, (int32) MyCancelKey);
+	pq_endmessage(&msgbuf);
+
+	/*
+	 * Hooray! Primary initialization is complete.  Now, we need to set up our
+	 * backend-local state to match the original backend.
+	 */
+
+	/*
+	 * Join locking group.  We must do this before anything that could try to
+	 * acquire a heavyweight lock, because any heavyweight locks acquired to
+	 * this point could block either directly against the parallel group
+	 * leader or against some process which in turn waits for a lock that
+	 * conflicts with the parallel group leader, causing an undetected
+	 * deadlock.  (If we can't join the lock group, the leader has gone away,
+	 * so just exit quietly.)
+	 */
+	if (!BecomeLockGroupMember(fps->parallel_leader_pgproc,
+							   fps->parallel_leader_pid))
+		return;
+
+	/*
+	 * Restore transaction and statement start-time timestamps.  This must
+	 * happen before anything that would start a transaction, else asserts in
+	 * xact.c will fire.
+	 */
+	SetParallelStartTimestamps(fps->xact_ts, fps->stmt_ts);
+
+	/*
+	 * Identify the entry point to be called.  In theory this could result in
+	 * loading an additional library, though most likely the entry point is in
+	 * the core backend or in a library we just loaded.
+	 */
+	entrypointstate = shm_toc_lookup(toc, PARALLEL_KEY_ENTRYPOINT, false);
+	library_name = entrypointstate;
+	function_name = entrypointstate + strlen(library_name) + 1;
+
+	entrypt = LookupParallelWorkerFunction(library_name, function_name);
+
+	/* Restore database connection. */
+	BackgroundWorkerInitializeConnectionByOid(fps->database_id,
+											  fps->authenticated_user_id,
+											  0);
+
+	/*
+	 * Set the client encoding to the database encoding, since that is what
+	 * the leader will expect.
+	 */
+	SetClientEncoding(GetDatabaseEncoding());
+
+	/*
+	 * Load libraries that were loaded by original backend.  We want to do
+	 * this before restoring GUCs, because the libraries might define custom
+	 * variables.
+	 */
+	libraryspace = shm_toc_lookup(toc, PARALLEL_KEY_LIBRARY, false);
+	StartTransactionCommand();
+	RestoreLibraryState(libraryspace);
+
+	/* Restore GUC values from launching backend. */
+	gucspace = shm_toc_lookup(toc, PARALLEL_KEY_GUC, false);
+	RestoreGUCState(gucspace);
+	CommitTransactionCommand();
+
+	/* Crank up a transaction state appropriate to a parallel worker. */
+	tstatespace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_STATE, false);
+	StartParallelWorkerTransaction(tstatespace);
+
+	/* Restore combo CID state. */
+	combocidspace = shm_toc_lookup(toc, PARALLEL_KEY_COMBO_CID, false);
+	RestoreComboCIDState(combocidspace);
+
+	/* Attach to the per-session DSM segment and contained objects. */
+	session_dsm_handle_space =
+		shm_toc_lookup(toc, PARALLEL_KEY_SESSION_DSM, false);
+	AttachSession(*(dsm_handle *) session_dsm_handle_space);
+
+	/*
+	 * If the transaction isolation level is REPEATABLE READ or SERIALIZABLE,
+	 * the leader has serialized the transaction snapshot and we must restore
+	 * it. At lower isolation levels, there is no transaction-lifetime
+	 * snapshot, but we need TransactionXmin to get set to a value which is
+	 * less than or equal to the xmin of every snapshot that will be used by
+	 * this worker. The easiest way to accomplish that is to install the
+	 * active snapshot as the transaction snapshot. Code running in this
+	 * parallel worker might take new snapshots via GetTransactionSnapshot()
+	 * or GetLatestSnapshot(), but it shouldn't have any way of acquiring a
+	 * snapshot older than the active snapshot.
+	 */
+	asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, false);
+	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, true);
+	asnapshot = RestoreSnapshot(asnapspace);
+	tsnapshot = tsnapspace ? RestoreSnapshot(tsnapspace) : asnapshot;
+	RestoreTransactionSnapshot(tsnapshot,
+							   fps->parallel_leader_pgproc);
+	PushActiveSnapshot(asnapshot);
+
+	/*
+	 * We've changed which tuples we can see, and must therefore invalidate
+	 * system caches.
+	 */
+	InvalidateSystemCaches();
+
+	/*
+	 * Restore current role id.  Skip verifying whether session user is
+	 * allowed to become this role and blindly restore the leader's state for
+	 * current role.
+	 */
+	SetCurrentRoleId(fps->outer_user_id, fps->is_superuser);
+
+	/* Restore user ID and security context. */
+	SetUserIdAndSecContext(fps->current_user_id, fps->sec_context);
+
+	/* Restore temp-namespace state to ensure search path matches leader's. */
+	SetTempNamespaceState(fps->temp_namespace_id,
+						  fps->temp_toast_namespace_id);
+
+	/* Restore pending syncs. */
+	pendingsyncsspace = shm_toc_lookup(toc, PARALLEL_KEY_PENDING_SYNCS,
+									   false);
+	RestorePendingSyncs(pendingsyncsspace);
+
+	/* Restore reindex state. */
+	reindexspace = shm_toc_lookup(toc, PARALLEL_KEY_REINDEX_STATE, false);
+	RestoreReindexState(reindexspace);
+
+	/* Restore relmapper state. */
+	relmapperspace = shm_toc_lookup(toc, PARALLEL_KEY_RELMAPPER_STATE, false);
+	RestoreRelationMap(relmapperspace);
+
+	/* Restore uncommitted enums. */
+	uncommittedenumsspace = shm_toc_lookup(toc, PARALLEL_KEY_UNCOMMITTEDENUMS,
+										   false);
+	RestoreUncommittedEnums(uncommittedenumsspace);
+
+	/* Attach to the leader's serializable transaction, if SERIALIZABLE. */
+	AttachSerializableXact(fps->serializable_xact_handle);
+
+	/*
+	 * We've initialized all of our state now; nothing should change
+	 * hereafter.
+	 */
+	InitializingParallelWorker = false;
+	EnterParallelMode();
+
+	/*
+	 * Time to do the real work: invoke the caller-supplied code.
+	 */
+	entrypt(seg, toc);
+
+	/* Must exit parallel mode to pop active snapshot. */
+	ExitParallelMode();
+
+	/* Must pop active snapshot so snapmgr.c doesn't complain. */
+	PopActiveSnapshot();
+
+	/* Shut down the parallel-worker transaction. */
+	EndParallelWorkerTransaction();
+
+	/* Detach from the per-session DSM segment. */
+	DetachSession();
+
+	/* Report success. */
+	pq_putmessage('X', NULL, 0);
+}
+
+/*
+ * Update shared memory with the ending location of the last WAL record we
+ * wrote, if it's greater than the value already stored there.
+ */
+void
+ParallelWorkerReportLastRecEnd(XLogRecPtr last_xlog_end)
+{
+	FixedParallelState *fps = MyFixedParallelState;
+
+	Assert(fps != NULL);
+	SpinLockAcquire(&fps->mutex);
+	if (fps->last_xlog_end < last_xlog_end)
+		fps->last_xlog_end = last_xlog_end;
+	SpinLockRelease(&fps->mutex);
+}
+
+/*
+ * Make sure the leader tries to read from our error queue one more time.
+ * This guards against the case where we exit uncleanly without sending an
+ * ErrorResponse to the leader, for example because some code calls proc_exit
+ * directly.
+ */
+static void
+ParallelWorkerShutdown(int code, Datum arg)
+{
+	SendProcSignal(ParallelLeaderPid,
+				   PROCSIG_PARALLEL_MESSAGE,
+				   ParallelLeaderBackendId);
+}
+
+/*
+ * Look up (and possibly load) a parallel worker entry point function.
+ *
+ * For functions contained in the core code, we use library name "postgres"
+ * and consult the InternalParallelWorkers array.  External functions are
+ * looked up, and loaded if necessary, using load_external_function().
+ *
+ * The point of this is to pass function names as strings across process
+ * boundaries.  We can't pass actual function addresses because of the
+ * possibility that the function has been loaded at a different address
+ * in a different process.  This is obviously a hazard for functions in
+ * loadable libraries, but it can happen even for functions in the core code
+ * on platforms using EXEC_BACKEND (e.g., Windows).
+ *
+ * At some point it might be worthwhile to get rid of InternalParallelWorkers[]
+ * in favor of applying load_external_function() for core functions too;
+ * but that raises portability issues that are not worth addressing now.
+ */
+static parallel_worker_main_type
+LookupParallelWorkerFunction(const char *libraryname, const char *funcname)
+{
+	/*
+	 * If the function is to be loaded from postgres itself, search the
+	 * InternalParallelWorkers array.
+	 */
+	if (strcmp(libraryname, "postgres") == 0)
+	{
+		int			i;
+
+		for (i = 0; i < lengthof(InternalParallelWorkers); i++)
+		{
+			if (strcmp(InternalParallelWorkers[i].fn_name, funcname) == 0)
+				return InternalParallelWorkers[i].fn_addr;
+		}
+
+		/* We can only reach this by programming error. */
+		elog(ERROR, "internal function \"%s\" not found", funcname);
+	}
+
+	/* Otherwise load from external library. */
+	return (parallel_worker_main_type)
+		load_external_function(libraryname, funcname, true, NULL);
+}