text: /*-------------------------------------------------------------------------
   *
   * parallel.c
   *	  Infrastructure for launching parallel workers
   *
   * Portions Copyright (c) 1996-2018, 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/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 "commands/async.h"
  #include "executor/execParallel.h"
  #include "libpq/libpq.h"
  #include "libpq/pqformat.h"
  #include "libpq/pqmq.h"
  #include "miscadmin.h"
  #include "optimizer/planmain.h"
  #include "pgstat.h"
  #include "storage/ipc.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/resowner.h"
  #include "utils/snapmgr.h"
  #include "utils/typcache.h"
  
  
  /* POLAR px */
  #include "access/px_btbuild.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_REINDEX_STATE			UINT64CONST(0xFFFFFFFFFFFF000B)
  
  /* 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_master_pgproc;
  	pid_t		parallel_master_pid;
  	BackendId	parallel_master_backend_id;
  	TimestampTz xact_ts;
  	TimestampTz stmt_ts;
  
  	/* 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 ParallelMasterPid;
  
  /*
   * 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
  	},
  	/* POLAR px */
  	{
  		"polar_px_bt_build_main", polar_px_bt_build_main
  	},
  	/* POLAR End*/
  };
  
  /* 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, bool serializable_okay)
  {
  	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);
  
  	/*
  	 * If dynamic shared memory is not available, we won't be able to use
  	 * background workers.
  	 */
  	if (dynamic_shared_memory_type == DSM_IMPL_NONE)
  		nworkers = 0;
  
  	/*
  	 * If we are running under serializable isolation, we can't use parallel
  	 * workers, at least not until somebody enhances that mechanism to be
  	 * parallel-aware.  Utility statement callers may ask us to ignore this
  	 * restriction because they're always able to safely ignore the fact that
  	 * SIREAD locks do not work with parallelism.
  	 */
  	if (IsolationIsSerializable() && !serializable_okay)
  		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->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		reindexlen = 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);
  		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));
  		reindexlen = EstimateReindexStateSpace();
  		shm_toc_estimate_chunk(&pcxt->estimator, reindexlen);
  		/* If you add more chunks here, you probably need to add keys. */
  		shm_toc_estimate_keys(&pcxt->estimator, 8);
  
  		/* 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_master_pgproc = MyProc;
  	fps->parallel_master_pid = MyProcPid;
  	fps->parallel_master_backend_id = MyBackendId;
  	fps->xact_ts = GetCurrentTransactionStartTimestamp();
  	fps->stmt_ts = GetCurrentStatementStartTimestamp();
  	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	   *reindexspace;
  		char	   *error_queue_space;
  		char	   *session_dsm_handle_space;
  		char	   *entrypointstate;
  		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 transaction snapshot and active snapshot. */
  		tsnapspace = shm_toc_allocate(pcxt->toc, tsnaplen);
  		SerializeSnapshot(transaction_snapshot, tsnapspace);
  		shm_toc_insert(pcxt->toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT,
  					   tsnapspace);
  		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 reindex state. */
  		reindexspace = shm_toc_allocate(pcxt->toc, reindexlen);
  		SerializeReindexState(reindexlen, reindexspace);
  		shm_toc_insert(pcxt->toc, PARALLEL_KEY_REINDEX_STATE, reindexspace);
  
  		/* 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);
  		}
  	}
  }
  
  /*
   * 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)
  		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; ++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_POSTMASTER_DEATH,
  							   -1, WAIT_EVENT_BGWORKER_STARTUP);
  
  				/* emergency bailout if postmaster has died */
  				if (rc & WL_POSTMASTER_DEATH)
  					proc_exit(1);
  
  				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.
  				 */
  			}
  		}
  
  		WaitLatch(MyLatch, WL_LATCH_SET, -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 former just ensures that last message sent by worker backend is
   * received by master 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	   *reindexspace;
  	StringInfoData msgbuf;
  	char	   *session_dsm_handle_space;
  
  	/* 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 and resource owner. */
  	Assert(CurrentResourceOwner == NULL);
  	CurrentResourceOwner = ResourceOwnerCreate(NULL, "parallel toplevel");
  	CurrentMemoryContext = AllocSetContextCreate(TopMemoryContext,
  												 "Parallel worker",
  												 ALLOCSET_DEFAULT_SIZES);
  
  	/*
  	 * Now that we have a resource owner, we can attach to the dynamic shared
  	 * memory segment and read the table of contents.
  	 */
  	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. */
  	ParallelMasterPid = fps->parallel_master_pid;
  	ParallelMasterBackendId = fps->parallel_master_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_master(fps->parallel_master_pid,
  						   fps->parallel_master_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_master_pgproc,
  							   fps->parallel_master_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);
  
  	/* Restore transaction snapshot. */
  	tsnapspace = shm_toc_lookup(toc, PARALLEL_KEY_TRANSACTION_SNAPSHOT, false);
  	RestoreTransactionSnapshot(RestoreSnapshot(tsnapspace),
  							   fps->parallel_master_pgproc);
  
  	/* Restore active snapshot. */
  	asnapspace = shm_toc_lookup(toc, PARALLEL_KEY_ACTIVE_SNAPSHOT, false);
  	PushActiveSnapshot(RestoreSnapshot(asnapspace));
  
  	/*
  	 * 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 reindex state. */
  	reindexspace = shm_toc_lookup(toc, PARALLEL_KEY_REINDEX_STATE, false);
  	RestoreReindexState(reindexspace);
  
  	/*
  	 * 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 resowner.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(ParallelMasterPid,
  				   PROCSIG_PARALLEL_MESSAGE,
  				   ParallelMasterBackendId);
  }
  
  /*
   * 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);
  }
  