Adding upstream version 14.5.upstream/14.5upstream

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

1 files changed, 1325 insertions, 0 deletions

diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
new file mode 100644
index 0000000..c40410d
--- /dev/null
+++ b/src/backend/postmaster/bgworker.c
@@ -0,0 +1,1325 @@
+/*--------------------------------------------------------------------
+ * bgworker.c
+ *		POSTGRES pluggable background workers implementation
+ *
+ * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/backend/postmaster/bgworker.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/parallel.h"
+#include "libpq/pqsignal.h"
+#include "miscadmin.h"
+#include "pgstat.h"
+#include "port/atomics.h"
+#include "postmaster/bgworker_internals.h"
+#include "postmaster/interrupt.h"
+#include "postmaster/postmaster.h"
+#include "replication/logicallauncher.h"
+#include "replication/logicalworker.h"
+#include "storage/dsm.h"
+#include "storage/ipc.h"
+#include "storage/latch.h"
+#include "storage/lwlock.h"
+#include "storage/pg_shmem.h"
+#include "storage/pmsignal.h"
+#include "storage/proc.h"
+#include "storage/procsignal.h"
+#include "storage/shmem.h"
+#include "tcop/tcopprot.h"
+#include "utils/ascii.h"
+#include "utils/ps_status.h"
+#include "utils/timeout.h"
+
+/*
+ * The postmaster's list of registered background workers, in private memory.
+ */
+slist_head	BackgroundWorkerList = SLIST_STATIC_INIT(BackgroundWorkerList);
+
+/*
+ * BackgroundWorkerSlots exist in shared memory and can be accessed (via
+ * the BackgroundWorkerArray) by both the postmaster and by regular backends.
+ * However, the postmaster cannot take locks, even spinlocks, because this
+ * might allow it to crash or become wedged if shared memory gets corrupted.
+ * Such an outcome is intolerable.  Therefore, we need a lockless protocol
+ * for coordinating access to this data.
+ *
+ * The 'in_use' flag is used to hand off responsibility for the slot between
+ * the postmaster and the rest of the system.  When 'in_use' is false,
+ * the postmaster will ignore the slot entirely, except for the 'in_use' flag
+ * itself, which it may read.  In this state, regular backends may modify the
+ * slot.  Once a backend sets 'in_use' to true, the slot becomes the
+ * responsibility of the postmaster.  Regular backends may no longer modify it,
+ * but the postmaster may examine it.  Thus, a backend initializing a slot
+ * must fully initialize the slot - and insert a write memory barrier - before
+ * marking it as in use.
+ *
+ * As an exception, however, even when the slot is in use, regular backends
+ * may set the 'terminate' flag for a slot, telling the postmaster not
+ * to restart it.  Once the background worker is no longer running, the slot
+ * will be released for reuse.
+ *
+ * In addition to coordinating with the postmaster, backends modifying this
+ * data structure must coordinate with each other.  Since they can take locks,
+ * this is straightforward: any backend wishing to manipulate a slot must
+ * take BackgroundWorkerLock in exclusive mode.  Backends wishing to read
+ * data that might get concurrently modified by other backends should take
+ * this lock in shared mode.  No matter what, backends reading this data
+ * structure must be able to tolerate concurrent modifications by the
+ * postmaster.
+ */
+typedef struct BackgroundWorkerSlot
+{
+	bool		in_use;
+	bool		terminate;
+	pid_t		pid;			/* InvalidPid = not started yet; 0 = dead */
+	uint64		generation;		/* incremented when slot is recycled */
+	BackgroundWorker worker;
+} BackgroundWorkerSlot;
+
+/*
+ * In order to limit the total number of parallel workers (according to
+ * max_parallel_workers GUC), we maintain the number of active parallel
+ * workers.  Since the postmaster cannot take locks, two variables are used for
+ * this purpose: the number of registered parallel workers (modified by the
+ * backends, protected by BackgroundWorkerLock) and the number of terminated
+ * parallel workers (modified only by the postmaster, lockless).  The active
+ * number of parallel workers is the number of registered workers minus the
+ * terminated ones.  These counters can of course overflow, but it's not
+ * important here since the subtraction will still give the right number.
+ */
+typedef struct BackgroundWorkerArray
+{
+	int			total_slots;
+	uint32		parallel_register_count;
+	uint32		parallel_terminate_count;
+	BackgroundWorkerSlot slot[FLEXIBLE_ARRAY_MEMBER];
+} BackgroundWorkerArray;
+
+struct BackgroundWorkerHandle
+{
+	int			slot;
+	uint64		generation;
+};
+
+static BackgroundWorkerArray *BackgroundWorkerData;
+
+/*
+ * List of internal background worker entry points.  We need this for
+ * reasons explained in LookupBackgroundWorkerFunction(), below.
+ */
+static const struct
+{
+	const char *fn_name;
+	bgworker_main_type fn_addr;
+}			InternalBGWorkers[] =
+
+{
+	{
+		"ParallelWorkerMain", ParallelWorkerMain
+	},
+	{
+		"ApplyLauncherMain", ApplyLauncherMain
+	},
+	{
+		"ApplyWorkerMain", ApplyWorkerMain
+	}
+};
+
+/* Private functions. */
+static bgworker_main_type LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname);
+
+
+/*
+ * Calculate shared memory needed.
+ */
+Size
+BackgroundWorkerShmemSize(void)
+{
+	Size		size;
+
+	/* Array of workers is variably sized. */
+	size = offsetof(BackgroundWorkerArray, slot);
+	size = add_size(size, mul_size(max_worker_processes,
+								   sizeof(BackgroundWorkerSlot)));
+
+	return size;
+}
+
+/*
+ * Initialize shared memory.
+ */
+void
+BackgroundWorkerShmemInit(void)
+{
+	bool		found;
+
+	BackgroundWorkerData = ShmemInitStruct("Background Worker Data",
+										   BackgroundWorkerShmemSize(),
+										   &found);
+	if (!IsUnderPostmaster)
+	{
+		slist_iter	siter;
+		int			slotno = 0;
+
+		BackgroundWorkerData->total_slots = max_worker_processes;
+		BackgroundWorkerData->parallel_register_count = 0;
+		BackgroundWorkerData->parallel_terminate_count = 0;
+
+		/*
+		 * Copy contents of worker list into shared memory.  Record the shared
+		 * memory slot assigned to each worker.  This ensures a 1-to-1
+		 * correspondence between the postmaster's private list and the array
+		 * in shared memory.
+		 */
+		slist_foreach(siter, &BackgroundWorkerList)
+		{
+			BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
+			RegisteredBgWorker *rw;
+
+			rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
+			Assert(slotno < max_worker_processes);
+			slot->in_use = true;
+			slot->terminate = false;
+			slot->pid = InvalidPid;
+			slot->generation = 0;
+			rw->rw_shmem_slot = slotno;
+			rw->rw_worker.bgw_notify_pid = 0;	/* might be reinit after crash */
+			memcpy(&slot->worker, &rw->rw_worker, sizeof(BackgroundWorker));
+			++slotno;
+		}
+
+		/*
+		 * Mark any remaining slots as not in use.
+		 */
+		while (slotno < max_worker_processes)
+		{
+			BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
+
+			slot->in_use = false;
+			++slotno;
+		}
+	}
+	else
+		Assert(found);
+}
+
+/*
+ * Search the postmaster's backend-private list of RegisteredBgWorker objects
+ * for the one that maps to the given slot number.
+ */
+static RegisteredBgWorker *
+FindRegisteredWorkerBySlotNumber(int slotno)
+{
+	slist_iter	siter;
+
+	slist_foreach(siter, &BackgroundWorkerList)
+	{
+		RegisteredBgWorker *rw;
+
+		rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
+		if (rw->rw_shmem_slot == slotno)
+			return rw;
+	}
+
+	return NULL;
+}
+
+/*
+ * Notice changes to shared memory made by other backends.
+ * Accept new worker requests only if allow_new_workers is true.
+ *
+ * This code runs in the postmaster, so we must be very careful not to assume
+ * that shared memory contents are sane.  Otherwise, a rogue backend could
+ * take out the postmaster.
+ */
+void
+BackgroundWorkerStateChange(bool allow_new_workers)
+{
+	int			slotno;
+
+	/*
+	 * The total number of slots stored in shared memory should match our
+	 * notion of max_worker_processes.  If it does not, something is very
+	 * wrong.  Further down, we always refer to this value as
+	 * max_worker_processes, in case shared memory gets corrupted while we're
+	 * looping.
+	 */
+	if (max_worker_processes != BackgroundWorkerData->total_slots)
+	{
+		ereport(LOG,
+				(errmsg("inconsistent background worker state (max_worker_processes=%d, total_slots=%d)",
+						max_worker_processes,
+						BackgroundWorkerData->total_slots)));
+		return;
+	}
+
+	/*
+	 * Iterate through slots, looking for newly-registered workers or workers
+	 * who must die.
+	 */
+	for (slotno = 0; slotno < max_worker_processes; ++slotno)
+	{
+		BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
+		RegisteredBgWorker *rw;
+
+		if (!slot->in_use)
+			continue;
+
+		/*
+		 * Make sure we don't see the in_use flag before the updated slot
+		 * contents.
+		 */
+		pg_read_barrier();
+
+		/* See whether we already know about this worker. */
+		rw = FindRegisteredWorkerBySlotNumber(slotno);
+		if (rw != NULL)
+		{
+			/*
+			 * In general, the worker data can't change after it's initially
+			 * registered.  However, someone can set the terminate flag.
+			 */
+			if (slot->terminate && !rw->rw_terminate)
+			{
+				rw->rw_terminate = true;
+				if (rw->rw_pid != 0)
+					kill(rw->rw_pid, SIGTERM);
+				else
+				{
+					/* Report never-started, now-terminated worker as dead. */
+					ReportBackgroundWorkerPID(rw);
+				}
+			}
+			continue;
+		}
+
+		/*
+		 * If we aren't allowing new workers, then immediately mark it for
+		 * termination; the next stanza will take care of cleaning it up.
+		 * Doing this ensures that any process waiting for the worker will get
+		 * awoken, even though the worker will never be allowed to run.
+		 */
+		if (!allow_new_workers)
+			slot->terminate = true;
+
+		/*
+		 * If the worker is marked for termination, we don't need to add it to
+		 * the registered workers list; we can just free the slot. However, if
+		 * bgw_notify_pid is set, the process that registered the worker may
+		 * need to know that we've processed the terminate request, so be sure
+		 * to signal it.
+		 */
+		if (slot->terminate)
+		{
+			int			notify_pid;
+
+			/*
+			 * We need a memory barrier here to make sure that the load of
+			 * bgw_notify_pid and the update of parallel_terminate_count
+			 * complete before the store to in_use.
+			 */
+			notify_pid = slot->worker.bgw_notify_pid;
+			if ((slot->worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
+				BackgroundWorkerData->parallel_terminate_count++;
+			slot->pid = 0;
+
+			pg_memory_barrier();
+			slot->in_use = false;
+
+			if (notify_pid != 0)
+				kill(notify_pid, SIGUSR1);
+
+			continue;
+		}
+
+		/*
+		 * Copy the registration data into the registered workers list.
+		 */
+		rw = malloc(sizeof(RegisteredBgWorker));
+		if (rw == NULL)
+		{
+			ereport(LOG,
+					(errcode(ERRCODE_OUT_OF_MEMORY),
+					 errmsg("out of memory")));
+			return;
+		}
+
+		/*
+		 * Copy strings in a paranoid way.  If shared memory is corrupted, the
+		 * source data might not even be NUL-terminated.
+		 */
+		ascii_safe_strlcpy(rw->rw_worker.bgw_name,
+						   slot->worker.bgw_name, BGW_MAXLEN);
+		ascii_safe_strlcpy(rw->rw_worker.bgw_type,
+						   slot->worker.bgw_type, BGW_MAXLEN);
+		ascii_safe_strlcpy(rw->rw_worker.bgw_library_name,
+						   slot->worker.bgw_library_name, BGW_MAXLEN);
+		ascii_safe_strlcpy(rw->rw_worker.bgw_function_name,
+						   slot->worker.bgw_function_name, BGW_MAXLEN);
+
+		/*
+		 * Copy various fixed-size fields.
+		 *
+		 * flags, start_time, and restart_time are examined by the postmaster,
+		 * but nothing too bad will happen if they are corrupted.  The
+		 * remaining fields will only be examined by the child process.  It
+		 * might crash, but we won't.
+		 */
+		rw->rw_worker.bgw_flags = slot->worker.bgw_flags;
+		rw->rw_worker.bgw_start_time = slot->worker.bgw_start_time;
+		rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
+		rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
+		memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
+
+		/*
+		 * Copy the PID to be notified about state changes, but only if the
+		 * postmaster knows about a backend with that PID.  It isn't an error
+		 * if the postmaster doesn't know about the PID, because the backend
+		 * that requested the worker could have died (or been killed) just
+		 * after doing so.  Nonetheless, at least until we get some experience
+		 * with how this plays out in the wild, log a message at a relative
+		 * high debug level.
+		 */
+		rw->rw_worker.bgw_notify_pid = slot->worker.bgw_notify_pid;
+		if (!PostmasterMarkPIDForWorkerNotify(rw->rw_worker.bgw_notify_pid))
+		{
+			elog(DEBUG1, "worker notification PID %ld is not valid",
+				 (long) rw->rw_worker.bgw_notify_pid);
+			rw->rw_worker.bgw_notify_pid = 0;
+		}
+
+		/* Initialize postmaster bookkeeping. */
+		rw->rw_backend = NULL;
+		rw->rw_pid = 0;
+		rw->rw_child_slot = 0;
+		rw->rw_crashed_at = 0;
+		rw->rw_shmem_slot = slotno;
+		rw->rw_terminate = false;
+
+		/* Log it! */
+		ereport(DEBUG1,
+				(errmsg_internal("registering background worker \"%s\"",
+								 rw->rw_worker.bgw_name)));
+
+		slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
+	}
+}
+
+/*
+ * Forget about a background worker that's no longer needed.
+ *
+ * The worker must be identified by passing an slist_mutable_iter that
+ * points to it.  This convention allows deletion of workers during
+ * searches of the worker list, and saves having to search the list again.
+ *
+ * Caller is responsible for notifying bgw_notify_pid, if appropriate.
+ *
+ * This function must be invoked only in the postmaster.
+ */
+void
+ForgetBackgroundWorker(slist_mutable_iter *cur)
+{
+	RegisteredBgWorker *rw;
+	BackgroundWorkerSlot *slot;
+
+	rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
+
+	Assert(rw->rw_shmem_slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+	Assert(slot->in_use);
+
+	/*
+	 * We need a memory barrier here to make sure that the update of
+	 * parallel_terminate_count completes before the store to in_use.
+	 */
+	if ((rw->rw_worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
+		BackgroundWorkerData->parallel_terminate_count++;
+
+	pg_memory_barrier();
+	slot->in_use = false;
+
+	ereport(DEBUG1,
+			(errmsg_internal("unregistering background worker \"%s\"",
+							 rw->rw_worker.bgw_name)));
+
+	slist_delete_current(cur);
+	free(rw);
+}
+
+/*
+ * Report the PID of a newly-launched background worker in shared memory.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+ReportBackgroundWorkerPID(RegisteredBgWorker *rw)
+{
+	BackgroundWorkerSlot *slot;
+
+	Assert(rw->rw_shmem_slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+	slot->pid = rw->rw_pid;
+
+	if (rw->rw_worker.bgw_notify_pid != 0)
+		kill(rw->rw_worker.bgw_notify_pid, SIGUSR1);
+}
+
+/*
+ * Report that the PID of a background worker is now zero because a
+ * previously-running background worker has exited.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+ReportBackgroundWorkerExit(slist_mutable_iter *cur)
+{
+	RegisteredBgWorker *rw;
+	BackgroundWorkerSlot *slot;
+	int			notify_pid;
+
+	rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
+
+	Assert(rw->rw_shmem_slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+	slot->pid = rw->rw_pid;
+	notify_pid = rw->rw_worker.bgw_notify_pid;
+
+	/*
+	 * If this worker is slated for deregistration, do that before notifying
+	 * the process which started it.  Otherwise, if that process tries to
+	 * reuse the slot immediately, it might not be available yet.  In theory
+	 * that could happen anyway if the process checks slot->pid at just the
+	 * wrong moment, but this makes the window narrower.
+	 */
+	if (rw->rw_terminate ||
+		rw->rw_worker.bgw_restart_time == BGW_NEVER_RESTART)
+		ForgetBackgroundWorker(cur);
+
+	if (notify_pid != 0)
+		kill(notify_pid, SIGUSR1);
+}
+
+/*
+ * Cancel SIGUSR1 notifications for a PID belonging to an exiting backend.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+BackgroundWorkerStopNotifications(pid_t pid)
+{
+	slist_iter	siter;
+
+	slist_foreach(siter, &BackgroundWorkerList)
+	{
+		RegisteredBgWorker *rw;
+
+		rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
+		if (rw->rw_worker.bgw_notify_pid == pid)
+			rw->rw_worker.bgw_notify_pid = 0;
+	}
+}
+
+/*
+ * Cancel any not-yet-started worker requests that have waiting processes.
+ *
+ * This is called during a normal ("smart" or "fast") database shutdown.
+ * After this point, no new background workers will be started, so anything
+ * that might be waiting for them needs to be kicked off its wait.  We do
+ * that by cancelling the bgworker registration entirely, which is perhaps
+ * overkill, but since we're shutting down it does not matter whether the
+ * registration record sticks around.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+ForgetUnstartedBackgroundWorkers(void)
+{
+	slist_mutable_iter iter;
+
+	slist_foreach_modify(iter, &BackgroundWorkerList)
+	{
+		RegisteredBgWorker *rw;
+		BackgroundWorkerSlot *slot;
+
+		rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
+		Assert(rw->rw_shmem_slot < max_worker_processes);
+		slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
+
+		/* If it's not yet started, and there's someone waiting ... */
+		if (slot->pid == InvalidPid &&
+			rw->rw_worker.bgw_notify_pid != 0)
+		{
+			/* ... then zap it, and notify the waiter */
+			int			notify_pid = rw->rw_worker.bgw_notify_pid;
+
+			ForgetBackgroundWorker(&iter);
+			if (notify_pid != 0)
+				kill(notify_pid, SIGUSR1);
+		}
+	}
+}
+
+/*
+ * Reset background worker crash state.
+ *
+ * We assume that, after a crash-and-restart cycle, background workers without
+ * the never-restart flag should be restarted immediately, instead of waiting
+ * for bgw_restart_time to elapse.  On the other hand, workers with that flag
+ * should be forgotten immediately, since we won't ever restart them.
+ *
+ * This function should only be called from the postmaster.
+ */
+void
+ResetBackgroundWorkerCrashTimes(void)
+{
+	slist_mutable_iter iter;
+
+	slist_foreach_modify(iter, &BackgroundWorkerList)
+	{
+		RegisteredBgWorker *rw;
+
+		rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
+
+		if (rw->rw_worker.bgw_restart_time == BGW_NEVER_RESTART)
+		{
+			/*
+			 * Workers marked BGW_NEVER_RESTART shouldn't get relaunched after
+			 * the crash, so forget about them.  (If we wait until after the
+			 * crash to forget about them, and they are parallel workers,
+			 * parallel_terminate_count will get incremented after we've
+			 * already zeroed parallel_register_count, which would be bad.)
+			 */
+			ForgetBackgroundWorker(&iter);
+		}
+		else
+		{
+			/*
+			 * The accounting which we do via parallel_register_count and
+			 * parallel_terminate_count would get messed up if a worker marked
+			 * parallel could survive a crash and restart cycle. All such
+			 * workers should be marked BGW_NEVER_RESTART, and thus control
+			 * should never reach this branch.
+			 */
+			Assert((rw->rw_worker.bgw_flags & BGWORKER_CLASS_PARALLEL) == 0);
+
+			/*
+			 * Allow this worker to be restarted immediately after we finish
+			 * resetting.
+			 */
+			rw->rw_crashed_at = 0;
+
+			/*
+			 * If there was anyone waiting for it, they're history.
+			 */
+			rw->rw_worker.bgw_notify_pid = 0;
+		}
+	}
+}
+
+#ifdef EXEC_BACKEND
+/*
+ * In EXEC_BACKEND mode, workers use this to retrieve their details from
+ * shared memory.
+ */
+BackgroundWorker *
+BackgroundWorkerEntry(int slotno)
+{
+	static BackgroundWorker myEntry;
+	BackgroundWorkerSlot *slot;
+
+	Assert(slotno < BackgroundWorkerData->total_slots);
+	slot = &BackgroundWorkerData->slot[slotno];
+	Assert(slot->in_use);
+
+	/* must copy this in case we don't intend to retain shmem access */
+	memcpy(&myEntry, &slot->worker, sizeof myEntry);
+	return &myEntry;
+}
+#endif
+
+/*
+ * Complain about the BackgroundWorker definition using error level elevel.
+ * Return true if it looks ok, false if not (unless elevel >= ERROR, in
+ * which case we won't return at all in the not-OK case).
+ */
+static bool
+SanityCheckBackgroundWorker(BackgroundWorker *worker, int elevel)
+{
+	/* sanity check for flags */
+	if (worker->bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
+	{
+		if (!(worker->bgw_flags & BGWORKER_SHMEM_ACCESS))
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("background worker \"%s\": must attach to shared memory in order to request a database connection",
+							worker->bgw_name)));
+			return false;
+		}
+
+		if (worker->bgw_start_time == BgWorkerStart_PostmasterStart)
+		{
+			ereport(elevel,
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("background worker \"%s\": cannot request database access if starting at postmaster start",
+							worker->bgw_name)));
+			return false;
+		}
+
+		/* XXX other checks? */
+	}
+
+	if ((worker->bgw_restart_time < 0 &&
+		 worker->bgw_restart_time != BGW_NEVER_RESTART) ||
+		(worker->bgw_restart_time > USECS_PER_DAY / 1000))
+	{
+		ereport(elevel,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("background worker \"%s\": invalid restart interval",
+						worker->bgw_name)));
+		return false;
+	}
+
+	/*
+	 * Parallel workers may not be configured for restart, because the
+	 * parallel_register_count/parallel_terminate_count accounting can't
+	 * handle parallel workers lasting through a crash-and-restart cycle.
+	 */
+	if (worker->bgw_restart_time != BGW_NEVER_RESTART &&
+		(worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
+	{
+		ereport(elevel,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("background worker \"%s\": parallel workers may not be configured for restart",
+						worker->bgw_name)));
+		return false;
+	}
+
+	/*
+	 * If bgw_type is not filled in, use bgw_name.
+	 */
+	if (strcmp(worker->bgw_type, "") == 0)
+		strcpy(worker->bgw_type, worker->bgw_name);
+
+	return true;
+}
+
+/*
+ * Standard SIGTERM handler for background workers
+ */
+static void
+bgworker_die(SIGNAL_ARGS)
+{
+	PG_SETMASK(&BlockSig);
+
+	ereport(FATAL,
+			(errcode(ERRCODE_ADMIN_SHUTDOWN),
+			 errmsg("terminating background worker \"%s\" due to administrator command",
+					MyBgworkerEntry->bgw_type)));
+}
+
+/*
+ * Start a new background worker
+ *
+ * This is the main entry point for background worker, to be called from
+ * postmaster.
+ */
+void
+StartBackgroundWorker(void)
+{
+	sigjmp_buf	local_sigjmp_buf;
+	BackgroundWorker *worker = MyBgworkerEntry;
+	bgworker_main_type entrypt;
+
+	if (worker == NULL)
+		elog(FATAL, "unable to find bgworker entry");
+
+	IsBackgroundWorker = true;
+
+	MyBackendType = B_BG_WORKER;
+	init_ps_display(worker->bgw_name);
+
+	/*
+	 * If we're not supposed to have shared memory access, then detach from
+	 * shared memory.  If we didn't request shared memory access, the
+	 * postmaster won't force a cluster-wide restart if we exit unexpectedly,
+	 * so we'd better make sure that we don't mess anything up that would
+	 * require that sort of cleanup.
+	 */
+	if ((worker->bgw_flags & BGWORKER_SHMEM_ACCESS) == 0)
+	{
+		ShutdownLatchSupport();
+		dsm_detach_all();
+		PGSharedMemoryDetach();
+	}
+
+	SetProcessingMode(InitProcessing);
+
+	/* Apply PostAuthDelay */
+	if (PostAuthDelay > 0)
+		pg_usleep(PostAuthDelay * 1000000L);
+
+	/*
+	 * Set up signal handlers.
+	 */
+	if (worker->bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
+	{
+		/*
+		 * SIGINT is used to signal canceling the current action
+		 */
+		pqsignal(SIGINT, StatementCancelHandler);
+		pqsignal(SIGUSR1, procsignal_sigusr1_handler);
+		pqsignal(SIGFPE, FloatExceptionHandler);
+
+		/* XXX Any other handlers needed here? */
+	}
+	else
+	{
+		pqsignal(SIGINT, SIG_IGN);
+		pqsignal(SIGUSR1, SIG_IGN);
+		pqsignal(SIGFPE, SIG_IGN);
+	}
+	pqsignal(SIGTERM, bgworker_die);
+	/* SIGQUIT handler was already set up by InitPostmasterChild */
+	pqsignal(SIGHUP, SIG_IGN);
+
+	InitializeTimeouts();		/* establishes SIGALRM handler */
+
+	pqsignal(SIGPIPE, SIG_IGN);
+	pqsignal(SIGUSR2, SIG_IGN);
+	pqsignal(SIGCHLD, SIG_DFL);
+
+	/*
+	 * If an exception is encountered, processing resumes here.
+	 *
+	 * We just need to clean up, report the error, and go away.
+	 */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		/* Since not using PG_TRY, must reset error stack by hand */
+		error_context_stack = NULL;
+
+		/* Prevent interrupts while cleaning up */
+		HOLD_INTERRUPTS();
+
+		/*
+		 * sigsetjmp will have blocked all signals, but we may need to accept
+		 * signals while communicating with our parallel leader.  Once we've
+		 * done HOLD_INTERRUPTS() it should be safe to unblock signals.
+		 */
+		BackgroundWorkerUnblockSignals();
+
+		/* Report the error to the parallel leader and the server log */
+		EmitErrorReport();
+
+		/*
+		 * Do we need more cleanup here?  For shmem-connected bgworkers, we
+		 * will call InitProcess below, which will install ProcKill as exit
+		 * callback.  That will take care of releasing locks, etc.
+		 */
+
+		/* and go away */
+		proc_exit(1);
+	}
+
+	/* We can now handle ereport(ERROR) */
+	PG_exception_stack = &local_sigjmp_buf;
+
+	/*
+	 * If the background worker request shared memory access, set that up now;
+	 * else, detach all shared memory segments.
+	 */
+	if (worker->bgw_flags & BGWORKER_SHMEM_ACCESS)
+	{
+		/*
+		 * Early initialization.  Some of this could be useful even for
+		 * background workers that aren't using shared memory, but they can
+		 * call the individual startup routines for those subsystems if
+		 * needed.
+		 */
+		BaseInit();
+
+		/*
+		 * Create a per-backend PGPROC struct in shared memory, except in the
+		 * EXEC_BACKEND case where this was done in SubPostmasterMain. We must
+		 * do this before we can use LWLocks (and in the EXEC_BACKEND case we
+		 * already had to do some stuff with LWLocks).
+		 */
+#ifndef EXEC_BACKEND
+		InitProcess();
+#endif
+	}
+
+	/*
+	 * Look up the entry point function, loading its library if necessary.
+	 */
+	entrypt = LookupBackgroundWorkerFunction(worker->bgw_library_name,
+											 worker->bgw_function_name);
+
+	/*
+	 * Note that in normal processes, we would call InitPostgres here.  For a
+	 * worker, however, we don't know what database to connect to, yet; so we
+	 * need to wait until the user code does it via
+	 * BackgroundWorkerInitializeConnection().
+	 */
+
+	/*
+	 * Now invoke the user-defined worker code
+	 */
+	entrypt(worker->bgw_main_arg);
+
+	/* ... and if it returns, we're done */
+	proc_exit(0);
+}
+
+/*
+ * Register a new static background worker.
+ *
+ * This can only be called directly from postmaster or in the _PG_init
+ * function of a module library that's loaded by shared_preload_libraries;
+ * otherwise it will have no effect.
+ */
+void
+RegisterBackgroundWorker(BackgroundWorker *worker)
+{
+	RegisteredBgWorker *rw;
+	static int	numworkers = 0;
+
+	if (!IsUnderPostmaster)
+		ereport(DEBUG1,
+				(errmsg_internal("registering background worker \"%s\"", worker->bgw_name)));
+
+	if (!process_shared_preload_libraries_in_progress &&
+		strcmp(worker->bgw_library_name, "postgres") != 0)
+	{
+		if (!IsUnderPostmaster)
+			ereport(LOG,
+					(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+					 errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
+							worker->bgw_name)));
+		return;
+	}
+
+	if (!SanityCheckBackgroundWorker(worker, LOG))
+		return;
+
+	if (worker->bgw_notify_pid != 0)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("background worker \"%s\": only dynamic background workers can request notification",
+						worker->bgw_name)));
+		return;
+	}
+
+	/*
+	 * Enforce maximum number of workers.  Note this is overly restrictive: we
+	 * could allow more non-shmem-connected workers, because these don't count
+	 * towards the MAX_BACKENDS limit elsewhere.  For now, it doesn't seem
+	 * important to relax this restriction.
+	 */
+	if (++numworkers > max_worker_processes)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
+				 errmsg("too many background workers"),
+				 errdetail_plural("Up to %d background worker can be registered with the current settings.",
+								  "Up to %d background workers can be registered with the current settings.",
+								  max_worker_processes,
+								  max_worker_processes),
+				 errhint("Consider increasing the configuration parameter \"max_worker_processes\".")));
+		return;
+	}
+
+	/*
+	 * Copy the registration data into the registered workers list.
+	 */
+	rw = malloc(sizeof(RegisteredBgWorker));
+	if (rw == NULL)
+	{
+		ereport(LOG,
+				(errcode(ERRCODE_OUT_OF_MEMORY),
+				 errmsg("out of memory")));
+		return;
+	}
+
+	rw->rw_worker = *worker;
+	rw->rw_backend = NULL;
+	rw->rw_pid = 0;
+	rw->rw_child_slot = 0;
+	rw->rw_crashed_at = 0;
+	rw->rw_terminate = false;
+
+	slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
+}
+
+/*
+ * Register a new background worker from a regular backend.
+ *
+ * Returns true on success and false on failure.  Failure typically indicates
+ * that no background worker slots are currently available.
+ *
+ * If handle != NULL, we'll set *handle to a pointer that can subsequently
+ * be used as an argument to GetBackgroundWorkerPid().  The caller can
+ * free this pointer using pfree(), if desired.
+ */
+bool
+RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
+								BackgroundWorkerHandle **handle)
+{
+	int			slotno;
+	bool		success = false;
+	bool		parallel;
+	uint64		generation = 0;
+
+	/*
+	 * We can't register dynamic background workers from the postmaster. If
+	 * this is a standalone backend, we're the only process and can't start
+	 * any more.  In a multi-process environment, it might be theoretically
+	 * possible, but we don't currently support it due to locking
+	 * considerations; see comments on the BackgroundWorkerSlot data
+	 * structure.
+	 */
+	if (!IsUnderPostmaster)
+		return false;
+
+	if (!SanityCheckBackgroundWorker(worker, ERROR))
+		return false;
+
+	parallel = (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0;
+
+	LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
+
+	/*
+	 * If this is a parallel worker, check whether there are already too many
+	 * parallel workers; if so, don't register another one.  Our view of
+	 * parallel_terminate_count may be slightly stale, but that doesn't really
+	 * matter: we would have gotten the same result if we'd arrived here
+	 * slightly earlier anyway.  There's no help for it, either, since the
+	 * postmaster must not take locks; a memory barrier wouldn't guarantee
+	 * anything useful.
+	 */
+	if (parallel && (BackgroundWorkerData->parallel_register_count -
+					 BackgroundWorkerData->parallel_terminate_count) >=
+		max_parallel_workers)
+	{
+		Assert(BackgroundWorkerData->parallel_register_count -
+			   BackgroundWorkerData->parallel_terminate_count <=
+			   MAX_PARALLEL_WORKER_LIMIT);
+		LWLockRelease(BackgroundWorkerLock);
+		return false;
+	}
+
+	/*
+	 * Look for an unused slot.  If we find one, grab it.
+	 */
+	for (slotno = 0; slotno < BackgroundWorkerData->total_slots; ++slotno)
+	{
+		BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
+
+		if (!slot->in_use)
+		{
+			memcpy(&slot->worker, worker, sizeof(BackgroundWorker));
+			slot->pid = InvalidPid; /* indicates not started yet */
+			slot->generation++;
+			slot->terminate = false;
+			generation = slot->generation;
+			if (parallel)
+				BackgroundWorkerData->parallel_register_count++;
+
+			/*
+			 * Make sure postmaster doesn't see the slot as in use before it
+			 * sees the new contents.
+			 */
+			pg_write_barrier();
+
+			slot->in_use = true;
+			success = true;
+			break;
+		}
+	}
+
+	LWLockRelease(BackgroundWorkerLock);
+
+	/* If we found a slot, tell the postmaster to notice the change. */
+	if (success)
+		SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
+
+	/*
+	 * If we found a slot and the user has provided a handle, initialize it.
+	 */
+	if (success && handle)
+	{
+		*handle = palloc(sizeof(BackgroundWorkerHandle));
+		(*handle)->slot = slotno;
+		(*handle)->generation = generation;
+	}
+
+	return success;
+}
+
+/*
+ * Get the PID of a dynamically-registered background worker.
+ *
+ * If the worker is determined to be running, the return value will be
+ * BGWH_STARTED and *pidp will get the PID of the worker process.  If the
+ * postmaster has not yet attempted to start the worker, the return value will
+ * be BGWH_NOT_YET_STARTED.  Otherwise, the return value is BGWH_STOPPED.
+ *
+ * BGWH_STOPPED can indicate either that the worker is temporarily stopped
+ * (because it is configured for automatic restart and exited non-zero),
+ * or that the worker is permanently stopped (because it exited with exit
+ * code 0, or was not configured for automatic restart), or even that the
+ * worker was unregistered without ever starting (either because startup
+ * failed and the worker is not configured for automatic restart, or because
+ * TerminateBackgroundWorker was used before the worker was successfully
+ * started).
+ */
+BgwHandleStatus
+GetBackgroundWorkerPid(BackgroundWorkerHandle *handle, pid_t *pidp)
+{
+	BackgroundWorkerSlot *slot;
+	pid_t		pid;
+
+	Assert(handle->slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[handle->slot];
+
+	/*
+	 * We could probably arrange to synchronize access to data using memory
+	 * barriers only, but for now, let's just keep it simple and grab the
+	 * lock.  It seems unlikely that there will be enough traffic here to
+	 * result in meaningful contention.
+	 */
+	LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
+
+	/*
+	 * The generation number can't be concurrently changed while we hold the
+	 * lock.  The pid, which is updated by the postmaster, can change at any
+	 * time, but we assume such changes are atomic.  So the value we read
+	 * won't be garbage, but it might be out of date by the time the caller
+	 * examines it (but that's unavoidable anyway).
+	 *
+	 * The in_use flag could be in the process of changing from true to false,
+	 * but if it is already false then it can't change further.
+	 */
+	if (handle->generation != slot->generation || !slot->in_use)
+		pid = 0;
+	else
+		pid = slot->pid;
+
+	/* All done. */
+	LWLockRelease(BackgroundWorkerLock);
+
+	if (pid == 0)
+		return BGWH_STOPPED;
+	else if (pid == InvalidPid)
+		return BGWH_NOT_YET_STARTED;
+	*pidp = pid;
+	return BGWH_STARTED;
+}
+
+/*
+ * Wait for a background worker to start up.
+ *
+ * This is like GetBackgroundWorkerPid(), except that if the worker has not
+ * yet started, we wait for it to do so; thus, BGWH_NOT_YET_STARTED is never
+ * returned.  However, if the postmaster has died, we give up and return
+ * BGWH_POSTMASTER_DIED, since it that case we know that startup will not
+ * take place.
+ *
+ * The caller *must* have set our PID as the worker's bgw_notify_pid,
+ * else we will not be awoken promptly when the worker's state changes.
+ */
+BgwHandleStatus
+WaitForBackgroundWorkerStartup(BackgroundWorkerHandle *handle, pid_t *pidp)
+{
+	BgwHandleStatus status;
+	int			rc;
+
+	for (;;)
+	{
+		pid_t		pid;
+
+		CHECK_FOR_INTERRUPTS();
+
+		status = GetBackgroundWorkerPid(handle, &pid);
+		if (status == BGWH_STARTED)
+			*pidp = pid;
+		if (status != BGWH_NOT_YET_STARTED)
+			break;
+
+		rc = WaitLatch(MyLatch,
+					   WL_LATCH_SET | WL_POSTMASTER_DEATH, 0,
+					   WAIT_EVENT_BGWORKER_STARTUP);
+
+		if (rc & WL_POSTMASTER_DEATH)
+		{
+			status = BGWH_POSTMASTER_DIED;
+			break;
+		}
+
+		ResetLatch(MyLatch);
+	}
+
+	return status;
+}
+
+/*
+ * Wait for a background worker to stop.
+ *
+ * If the worker hasn't yet started, or is running, we wait for it to stop
+ * and then return BGWH_STOPPED.  However, if the postmaster has died, we give
+ * up and return BGWH_POSTMASTER_DIED, because it's the postmaster that
+ * notifies us when a worker's state changes.
+ *
+ * The caller *must* have set our PID as the worker's bgw_notify_pid,
+ * else we will not be awoken promptly when the worker's state changes.
+ */
+BgwHandleStatus
+WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
+{
+	BgwHandleStatus status;
+	int			rc;
+
+	for (;;)
+	{
+		pid_t		pid;
+
+		CHECK_FOR_INTERRUPTS();
+
+		status = GetBackgroundWorkerPid(handle, &pid);
+		if (status == BGWH_STOPPED)
+			break;
+
+		rc = WaitLatch(MyLatch,
+					   WL_LATCH_SET | WL_POSTMASTER_DEATH, 0,
+					   WAIT_EVENT_BGWORKER_SHUTDOWN);
+
+		if (rc & WL_POSTMASTER_DEATH)
+		{
+			status = BGWH_POSTMASTER_DIED;
+			break;
+		}
+
+		ResetLatch(MyLatch);
+	}
+
+	return status;
+}
+
+/*
+ * Instruct the postmaster to terminate a background worker.
+ *
+ * Note that it's safe to do this without regard to whether the worker is
+ * still running, or even if the worker may already have exited and been
+ * unregistered.
+ */
+void
+TerminateBackgroundWorker(BackgroundWorkerHandle *handle)
+{
+	BackgroundWorkerSlot *slot;
+	bool		signal_postmaster = false;
+
+	Assert(handle->slot < max_worker_processes);
+	slot = &BackgroundWorkerData->slot[handle->slot];
+
+	/* Set terminate flag in shared memory, unless slot has been reused. */
+	LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
+	if (handle->generation == slot->generation)
+	{
+		slot->terminate = true;
+		signal_postmaster = true;
+	}
+	LWLockRelease(BackgroundWorkerLock);
+
+	/* Make sure the postmaster notices the change to shared memory. */
+	if (signal_postmaster)
+		SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
+}
+
+/*
+ * Look up (and possibly load) a bgworker entry point function.
+ *
+ * For functions contained in the core code, we use library name "postgres"
+ * and consult the InternalBGWorkers 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 InternalBGWorkers[]
+ * in favor of applying load_external_function() for core functions too;
+ * but that raises portability issues that are not worth addressing now.
+ */
+static bgworker_main_type
+LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname)
+{
+	/*
+	 * If the function is to be loaded from postgres itself, search the
+	 * InternalBGWorkers array.
+	 */
+	if (strcmp(libraryname, "postgres") == 0)
+	{
+		int			i;
+
+		for (i = 0; i < lengthof(InternalBGWorkers); i++)
+		{
+			if (strcmp(InternalBGWorkers[i].fn_name, funcname) == 0)
+				return InternalBGWorkers[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 (bgworker_main_type)
+		load_external_function(libraryname, funcname, true, NULL);
+}
+
+/*
+ * Given a PID, get the bgw_type of the background worker.  Returns NULL if
+ * not a valid background worker.
+ *
+ * The return value is in static memory belonging to this function, so it has
+ * to be used before calling this function again.  This is so that the caller
+ * doesn't have to worry about the background worker locking protocol.
+ */
+const char *
+GetBackgroundWorkerTypeByPid(pid_t pid)
+{
+	int			slotno;
+	bool		found = false;
+	static char result[BGW_MAXLEN];
+
+	LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
+
+	for (slotno = 0; slotno < BackgroundWorkerData->total_slots; slotno++)
+	{
+		BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
+
+		if (slot->pid > 0 && slot->pid == pid)
+		{
+			strcpy(result, slot->worker.bgw_type);
+			found = true;
+			break;
+		}
+	}
+
+	LWLockRelease(BackgroundWorkerLock);
+
+	if (!found)
+		return NULL;
+
+	return result;
+}