v5-0001-Support-custom-wait-events-for-extensions.patch

From 630f78822534171bb8839c79d0fdca3bb8268a95 Mon Sep 17 00:00:00 2001
From: Michael Paquier <michael@paquier.xyz>
Date: Tue, 11 Jul 2023 16:41:35 +0900
Subject: [PATCH v5] Support custom wait events for extensions.

To support custom wait events, it add 2 APIs to allocate new wait events
dynamically.

The APIs are
* ExtensionWaitEventNewTranche()
* ExtensionWaitEventRegisterTranche()

These are similar to the existing LWLockNewTrancheId() and
LWLockRegisterTranche().

First, extension should call ExtensionWaitEventNewTranche() to get one
or more wait event "tranches", which are ID is allocated from a shared
counter.  Next, each individual process can use the tranche numbers with
ExtensionWaitEventRegisterTranche() to associate that a wait event
string to the associated name.

Note that this includes a test module, which perhaps should be split
later into its own commit.
---
 src/include/utils/wait_event.h                |  31 +++
 src/backend/storage/ipc/ipci.c                |   3 +
 src/backend/storage/ipc/shmem.c               |   3 +-
 .../activity/generate-wait_event_types.pl     |  12 +-
 src/backend/utils/activity/wait_event.c       | 149 +++++++++++++-
 .../utils/activity/wait_event_names.txt       |   2 +-
 src/test/modules/Makefile                     |   1 +
 src/test/modules/meson.build                  |   1 +
 .../test_custom_wait_events/.gitignore        |   2 +
 .../modules/test_custom_wait_events/Makefile  |  23 +++
 .../test_custom_wait_events/meson.build       |  33 +++
 .../test_custom_wait_events/t/001_basic.pl    |  34 ++++
 .../test_custom_wait_events--1.0.sql          |  14 ++
 .../test_custom_wait_events.c                 | 188 ++++++++++++++++++
 .../test_custom_wait_events.control           |   3 +
 15 files changed, 484 insertions(+), 15 deletions(-)
 create mode 100644 src/test/modules/test_custom_wait_events/.gitignore
 create mode 100644 src/test/modules/test_custom_wait_events/Makefile
 create mode 100644 src/test/modules/test_custom_wait_events/meson.build
 create mode 100644 src/test/modules/test_custom_wait_events/t/001_basic.pl
 create mode 100644 src/test/modules/test_custom_wait_events/test_custom_wait_events--1.0.sql
 create mode 100644 src/test/modules/test_custom_wait_events/test_custom_wait_events.c
 create mode 100644 src/test/modules/test_custom_wait_events/test_custom_wait_events.control

diff --git a/src/include/utils/wait_event.h b/src/include/utils/wait_event.h
index 4517425f84..dfcaf8c506 100644
--- a/src/include/utils/wait_event.h
+++ b/src/include/utils/wait_event.h
@@ -67,6 +67,37 @@ pgstat_report_wait_start(uint32 wait_event_info)
 	*(volatile uint32 *) my_wait_event_info = wait_event_info;
 }
 
+/* ----------
+ * Wait Events - Extension
+ *
+ * Use this category when the server process is waiting for some condition
+ * defined by an extension module.
+ *
+ * Extensions can define custom wait events.  First, call
+ * WaitEventExtensionNewTranche() just once to obtain a new wait event
+ * tranche.  The ID is allocated from a shared counter.  Next, each
+ * individual process using the tranche should call
+ * WaitEventExtensionRegisterTranche() to associate that wait event with
+ * a name.
+ *
+ * It may seem strange that each process using the tranche must register it
+ * separately, but dynamic shared memory segments aren't guaranteed to be
+ * mapped at the same address in all coordinating backends, so storing the
+ * registration in the main shared memory segment wouldn't work for that case.
+ */
+typedef enum
+{
+	WAIT_EVENT_EXTENSION = PG_WAIT_EXTENSION,
+	WAIT_EVENT_EXTENSION_FIRST_USER_DEFINED
+} WaitEventExtension;
+
+extern void WaitEventExtensionShmemInit(void);
+extern Size WaitEventExtensionShmemSize(void);
+
+extern uint32 WaitEventExtensionNewTranche(void);
+extern void WaitEventExtensionRegisterTranche(uint32 wait_event_info,
+											  const char *tranche_name);
+
 /* ----------
  * pgstat_report_wait_end() -
  *
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index cc387c00a1..5551afffc0 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -49,6 +49,7 @@
 #include "storage/spin.h"
 #include "utils/guc.h"
 #include "utils/snapmgr.h"
+#include "utils/wait_event.h"
 
 /* GUCs */
 int			shared_memory_type = DEFAULT_SHARED_MEMORY_TYPE;
@@ -142,6 +143,7 @@ CalculateShmemSize(int *num_semaphores)
 	size = add_size(size, SyncScanShmemSize());
 	size = add_size(size, AsyncShmemSize());
 	size = add_size(size, StatsShmemSize());
+	size = add_size(size, WaitEventExtensionShmemSize());
 #ifdef EXEC_BACKEND
 	size = add_size(size, ShmemBackendArraySize());
 #endif
@@ -301,6 +303,7 @@ CreateSharedMemoryAndSemaphores(void)
 	SyncScanShmemInit();
 	AsyncShmemInit();
 	StatsShmemInit();
+	WaitEventExtensionShmemInit();
 
 #ifdef EXEC_BACKEND
 
diff --git a/src/backend/storage/ipc/shmem.c b/src/backend/storage/ipc/shmem.c
index 5465fa1964..02c72ebbb1 100644
--- a/src/backend/storage/ipc/shmem.c
+++ b/src/backend/storage/ipc/shmem.c
@@ -85,7 +85,8 @@ static void *ShmemBase;			/* start address of shared memory */
 
 static void *ShmemEnd;			/* end+1 address of shared memory */
 
-slock_t    *ShmemLock;			/* spinlock for shared memory and LWLock
+slock_t    *ShmemLock;			/* spinlock for shared memory, LWLock
+								 * allocation, and named extension wait event
 								 * allocation */
 
 static HTAB *ShmemIndex = NULL; /* primary index hashtable for shmem */
diff --git a/src/backend/utils/activity/generate-wait_event_types.pl b/src/backend/utils/activity/generate-wait_event_types.pl
index 6d1a2af42a..2cad28fbfe 100644
--- a/src/backend/utils/activity/generate-wait_event_types.pl
+++ b/src/backend/utils/activity/generate-wait_event_types.pl
@@ -85,10 +85,11 @@ foreach my $line (@lines_sorted)
 	my $trimmedwaiteventname = $waiteventenumname;
 	$trimmedwaiteventname =~ s/^WAIT_EVENT_//;
 
-	# An exception is required for LWLock and Lock as these don't require
-	# any C and header files generated.
+	# An exception is required for Extension, LWLock and Lock as these don't
+	# require any C and header files generated.
 	die "wait event names must start with 'WAIT_EVENT_'"
 	  if ( $trimmedwaiteventname eq $waiteventenumname
+		&& $waiteventenumname !~ /^Extension/
 		&& $waiteventenumname !~ /^LWLock/
 		&& $waiteventenumname !~ /^Lock/);
 	$continue = ",\n";
@@ -138,11 +139,12 @@ if ($gen_code)
 	foreach my $waitclass (sort { uc($a) cmp uc($b) } keys %hashwe)
 	{
 
-		# Don't generate .c and .h files for LWLock and Lock, these are
-		# handled independently.
+		# Don't generate .c and .h files for Extension, LWLock and
+		# Lock, these are handled independently.
 		next
 		  if ( $waitclass =~ /^WaitEventLWLock$/
-			|| $waitclass =~ /^WaitEventLock$/);
+			|| $waitclass =~ /^WaitEventLock$/
+			|| $waitclass =~ /^WaitEventExtension$/);
 
 		my $last = $waitclass;
 		$last =~ s/^WaitEvent//;
diff --git a/src/backend/utils/activity/wait_event.c b/src/backend/utils/activity/wait_event.c
index 4aad11c111..9d4652fcc9 100644
--- a/src/backend/utils/activity/wait_event.c
+++ b/src/backend/utils/activity/wait_event.c
@@ -22,15 +22,21 @@
  */
 #include "postgres.h"
 
+#include "miscadmin.h"
+#include "port/pg_bitutils.h"
 #include "storage/lmgr.h"		/* for GetLockNameFromTagType */
 #include "storage/lwlock.h"		/* for GetLWLockIdentifier */
+#include "storage/spin.h"
+#include "utils/memutils.h"
 #include "utils/wait_event.h"
 
 
+/* We use the ShmemLock spinlock to protect WaitEventExtensionCounter */
+extern slock_t *ShmemLock;
+
 static const char *pgstat_get_wait_activity(WaitEventActivity w);
 static const char *pgstat_get_wait_bufferpin(WaitEventBufferPin w);
 static const char *pgstat_get_wait_client(WaitEventClient w);
-static const char *pgstat_get_wait_extension(WaitEventExtension w);
 static const char *pgstat_get_wait_ipc(WaitEventIPC w);
 static const char *pgstat_get_wait_timeout(WaitEventTimeout w);
 static const char *pgstat_get_wait_io(WaitEventIO w);
@@ -40,6 +46,137 @@ static uint32 local_my_wait_event_info;
 uint32	   *my_wait_event_info = &local_my_wait_event_info;
 
 
+/* dynamic allocation counter in shared memory */
+static uint16 *WaitEventExtensionCounter;
+
+/* first event ID for named extension wait event tranche */
+#define NUM_BUILTIN_WAIT_EVENT_EXTENSION	\
+	(WAIT_EVENT_EXTENSION_FIRST_USER_DEFINED - WAIT_EVENT_EXTENSION)
+
+/*
+ * This is indexed by event ID minus NUM_BUILTIN_WAIT_EVENT_EXTENSION, and
+ * stores the names of all dynamically-created event ID known to the current
+ * process.  Any unused entries in the array will contain NULL.
+ */
+static const char **WaitEventExtensionTrancheNames = NULL;
+static int	WaitEventExtensionTrancheNamesAllocated = 0;
+
+static const char *GetWaitEventExtensionIdentifier(uint16 eventId);
+
+/*
+ *  Return the space for dynamic allocation counter.
+ */
+Size
+WaitEventExtensionShmemSize(void)
+{
+	return sizeof(*WaitEventExtensionCounter);
+}
+
+/*
+ * Allocate shmem space for dynamic allocation counter.
+ */
+void
+WaitEventExtensionShmemInit(void)
+{
+	if (!IsUnderPostmaster)
+	{
+		Size		space = WaitEventExtensionShmemSize();
+
+		/* Initialize the dynamic-allocation counter */
+		WaitEventExtensionCounter = (uint16 *) ShmemAlloc(space);
+		*WaitEventExtensionCounter = NUM_BUILTIN_WAIT_EVENT_EXTENSION;
+	}
+}
+
+/*
+ * Allocate a new event ID and return the wait event info.
+ */
+uint32
+WaitEventExtensionNewTranche(void)
+{
+	uint16		eventId;
+
+	SpinLockAcquire(ShmemLock);
+	eventId = (*WaitEventExtensionCounter)++;
+	SpinLockRelease(ShmemLock);
+
+	return PG_WAIT_EXTENSION | eventId;
+}
+
+/*
+ * Register a dynamic tranche name in the lookup table of the current process.
+ *
+ * This routine will save a pointer to the wait event tranche name passed
+ * as an argument, so the name should be allocated in a backend-lifetime context
+ * (shared memory, TopMemoryContext, static constant, or similar).
+ *
+ * The "wait_event_name" will be user-visible as a wait event name, so try to
+ * use a name that fits the style for those.
+ */
+void
+WaitEventExtensionRegisterTranche(uint32 wait_event_info,
+								  const char *wait_event_name)
+{
+	uint16		eventId;
+
+	/* Check wait event class. */
+	Assert((wait_event_info & 0xFF000000) == PG_WAIT_EXTENSION);
+
+	eventId = wait_event_info & 0x0000FFFF;
+
+	/* This should only be called for user-defined tranches. */
+	if (eventId < NUM_BUILTIN_WAIT_EVENT_EXTENSION)
+		return;
+
+	/* Convert to array index. */
+	eventId -= NUM_BUILTIN_WAIT_EVENT_EXTENSION;
+
+	/* If necessary, create or enlarge array. */
+	if (eventId >= WaitEventExtensionTrancheNamesAllocated)
+	{
+		int			newalloc;
+
+		newalloc = pg_nextpower2_32(Max(8, eventId + 1));
+
+		if (WaitEventExtensionTrancheNames == NULL)
+			WaitEventExtensionTrancheNames = (const char **)
+				MemoryContextAllocZero(TopMemoryContext,
+									   newalloc * sizeof(char *));
+		else
+			WaitEventExtensionTrancheNames =
+				repalloc0_array(WaitEventExtensionTrancheNames, const char *, WaitEventExtensionTrancheNamesAllocated, newalloc);
+		WaitEventExtensionTrancheNamesAllocated = newalloc;
+	}
+
+	WaitEventExtensionTrancheNames[eventId] = wait_event_name;
+}
+
+/*
+ * Return the name of an Extension wait event ID.
+ */
+static const char *
+GetWaitEventExtensionIdentifier(uint16 eventId)
+{
+	/* Build-in tranche? */
+	if (eventId < NUM_BUILTIN_WAIT_EVENT_EXTENSION)
+		return "Extension";
+
+	/*
+	 * It's an extension tranche, so look in WaitEventExtensionTrancheNames[].
+	 * However, it's possible that the tranche has never been registered by
+	 * calling WaitEventExtensionRegisterTranche() in the current process, in
+	 * which case give up and return "Extension".
+	 */
+	eventId -= NUM_BUILTIN_WAIT_EVENT_EXTENSION;
+
+	if (eventId >= WaitEventExtensionTrancheNamesAllocated ||
+		WaitEventExtensionTrancheNames[eventId] == NULL)
+		return "Extension";
+
+	return WaitEventExtensionTrancheNames[eventId];
+}
+
+
 /*
  * Configure wait event reporting to report wait events to *wait_event_info.
  * *wait_event_info needs to be valid until pgstat_reset_wait_event_storage()
@@ -149,6 +286,9 @@ pgstat_get_wait_event(uint32 wait_event_info)
 		case PG_WAIT_LOCK:
 			event_name = GetLockNameFromTagType(eventId);
 			break;
+		case PG_WAIT_EXTENSION:
+			event_name = GetWaitEventExtensionIdentifier(eventId);
+			break;
 		case PG_WAIT_BUFFERPIN:
 			{
 				WaitEventBufferPin w = (WaitEventBufferPin) wait_event_info;
@@ -170,13 +310,6 @@ pgstat_get_wait_event(uint32 wait_event_info)
 				event_name = pgstat_get_wait_client(w);
 				break;
 			}
-		case PG_WAIT_EXTENSION:
-			{
-				WaitEventExtension w = (WaitEventExtension) wait_event_info;
-
-				event_name = pgstat_get_wait_extension(w);
-				break;
-			}
 		case PG_WAIT_IPC:
 			{
 				WaitEventIPC w = (WaitEventIPC) wait_event_info;
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index 7af1a61f95..d3cddde95f 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -261,7 +261,7 @@ WAIT_EVENT_BUFFER_PIN	"BufferPin"	"Waiting to acquire an exclusive pin on a buff
 
 Section: ClassName - WaitEventExtension
 
-WAIT_EVENT_EXTENSION	"Extension"	"Waiting in an extension."
+WAIT_EVENT_DOCONLY	"Extension"	"Waiting in an extension."
 
 #
 # Wait events - LWLock
diff --git a/src/test/modules/Makefile b/src/test/modules/Makefile
index 6331c976dc..84312b7e57 100644
--- a/src/test/modules/Makefile
+++ b/src/test/modules/Makefile
@@ -17,6 +17,7 @@ SUBDIRS = \
 		  test_bloomfilter \
 		  test_copy_callbacks \
 		  test_custom_rmgrs \
+		  test_custom_wait_events \
 		  test_ddl_deparse \
 		  test_extensions \
 		  test_ginpostinglist \
diff --git a/src/test/modules/meson.build b/src/test/modules/meson.build
index 17d369e378..83a1d8fd19 100644
--- a/src/test/modules/meson.build
+++ b/src/test/modules/meson.build
@@ -14,6 +14,7 @@ subdir('ssl_passphrase_callback')
 subdir('test_bloomfilter')
 subdir('test_copy_callbacks')
 subdir('test_custom_rmgrs')
+subdir('test_custom_wait_events')
 subdir('test_ddl_deparse')
 subdir('test_extensions')
 subdir('test_ginpostinglist')
diff --git a/src/test/modules/test_custom_wait_events/.gitignore b/src/test/modules/test_custom_wait_events/.gitignore
new file mode 100644
index 0000000000..716e17f5a2
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/.gitignore
@@ -0,0 +1,2 @@
+# Generated subdirectories
+/tmp_check/
diff --git a/src/test/modules/test_custom_wait_events/Makefile b/src/test/modules/test_custom_wait_events/Makefile
new file mode 100644
index 0000000000..dda365d523
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/Makefile
@@ -0,0 +1,23 @@
+# src/test/modules/test_custom_wait_events/Makefile
+
+MODULE_big = test_custom_wait_events
+OBJS = \
+	$(WIN32RES) \
+	test_custom_wait_events.o
+PGFILEDESC = "test_custom_wait_events - test custom wait events"
+
+EXTENSION = test_custom_wait_events
+DATA = test_custom_wait_events--1.0.sql
+
+TAP_TESTS = 1
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = src/test/modules/test_custom_wait_events
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/src/test/modules/test_custom_wait_events/meson.build b/src/test/modules/test_custom_wait_events/meson.build
new file mode 100644
index 0000000000..8ad073e577
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/meson.build
@@ -0,0 +1,33 @@
+# Copyright (c) PostgreSQL Global Development Group
+
+test_custom_wait_events = files(
+  'test_custom_wait_events.c',
+)
+
+if host_system == 'windows'
+  test_custom_wait_events += rc_lib_gen.process(win32ver_rc, extra_args: [
+    '--NAME', 'test_custom_wait_events',
+    '--FILEDESC', 'test_custom_wait_events - test custom wait events',])
+endif
+
+test_custom_wait_events = shared_module('test_custom_wait_events',
+  test_custom_wait_events,
+  kwargs: pg_test_mod_args,
+)
+test_install_libs += test_custom_wait_events
+
+test_install_data += files(
+  'test_custom_wait_events.control',
+  'test_custom_wait_events--1.0.sql',
+)
+
+tests += {
+  'name': 'test_custom_wait_events',
+  'sd': meson.current_source_dir(),
+  'bd': meson.current_build_dir(),
+  'tap': {
+    'tests': [
+      't/001_basic.pl',
+    ],
+  },
+}
diff --git a/src/test/modules/test_custom_wait_events/t/001_basic.pl b/src/test/modules/test_custom_wait_events/t/001_basic.pl
new file mode 100644
index 0000000000..339a9f66ba
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/t/001_basic.pl
@@ -0,0 +1,34 @@
+# Copyright (c) 2023, PostgreSQL Global Development Group
+
+use strict;
+use warnings;
+
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('main');
+
+$node->init;
+$node->append_conf(
+	'postgresql.conf',
+	"shared_preload_libraries = 'test_custom_wait_events'"
+);
+$node->start;
+
+# setup
+$node->safe_psql('postgres', 'CREATE EXTENSION test_custom_wait_events;');
+
+$node->safe_psql('postgres', 'SELECT wait_worker_launch();');
+
+ok($node->poll_query_until('postgres',
+	qq[SELECT
+		(SELECT count(*) FROM pg_stat_activity
+			WHERE backend_type = 'test_custom_wait_events'
+			  AND wait_event_type = 'Extension'
+			  AND wait_event = 'custom_wait_event'
+		) > 0;])
+) or die "Timed out waiting the custom wait event to be showed up";
+
+$node->stop;
+done_testing();
diff --git a/src/test/modules/test_custom_wait_events/test_custom_wait_events--1.0.sql b/src/test/modules/test_custom_wait_events/test_custom_wait_events--1.0.sql
new file mode 100644
index 0000000000..1fc2130f84
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/test_custom_wait_events--1.0.sql
@@ -0,0 +1,14 @@
+/* src/test/modules/test_custom_wait_events/test_custom_wait_events--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION test_custom_wait_events" to load this file. \quit
+
+--
+-- wait_worker_launch()
+--
+-- Launch a background worker to wait forever with a custom wait event.
+--
+CREATE FUNCTION wait_worker_launch()
+RETURNS int
+AS 'MODULE_PATHNAME', 'wait_worker_launch'
+LANGUAGE C STRICT PARALLEL SAFE;
diff --git a/src/test/modules/test_custom_wait_events/test_custom_wait_events.c b/src/test/modules/test_custom_wait_events/test_custom_wait_events.c
new file mode 100644
index 0000000000..f6d3739411
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/test_custom_wait_events.c
@@ -0,0 +1,188 @@
+/*--------------------------------------------------------------------------
+ *
+ * test_custom_wait_events.c
+ *		Code for testing custom wait events
+ *
+ * This code initializes a custom wait event in shmem_request_hook() and
+ * provide a function to launch a background worker waiting forever
+ * with the custom wait event.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/test_custom_wait_events/test_custom_wait_events.c
+ *
+ * -------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "fmgr.h"
+#include "miscadmin.h"
+#include "postmaster/bgworker.h"
+#include "postmaster/interrupt.h"
+#include "storage/ipc.h"
+#include "storage/latch.h"
+#include "storage/lwlock.h"
+#include "storage/proc.h"
+#include "storage/shmem.h"
+#include "tcop/utility.h"
+#include "utils/wait_event.h"
+
+PG_MODULE_MAGIC;
+
+PG_FUNCTION_INFO_V1(wait_worker_launch);
+
+PGDLLEXPORT void custom_we_worker_main(Datum main_arg);
+
+/* GUC variables */
+typedef struct CustomWaitEventState
+{
+	uint32			wait_event_info;  /* a allocated wait event */
+}			CustomWaitEventState;
+
+static shmem_request_hook_type prev_shmem_request_hook = NULL;
+static shmem_request_hook_type prev_shmem_startup_hook = NULL;
+
+static char *worker_database = "postgres";   /* connected by a background worker */
+static CustomWaitEventState *ss = NULL;
+
+static void custom_we_shmem_request(void);
+static void custom_we_shmem_startup(void);
+static Size custom_we_memsize(void);
+
+void
+_PG_init(void)
+{
+	if (!process_shared_preload_libraries_in_progress)
+		return;
+
+	prev_shmem_request_hook = shmem_request_hook;
+	shmem_request_hook = custom_we_shmem_request;
+	prev_shmem_startup_hook = shmem_startup_hook;
+	shmem_startup_hook = custom_we_shmem_startup;
+}
+
+static void
+custom_we_shmem_request(void)
+{
+	if (prev_shmem_request_hook)
+		prev_shmem_request_hook();
+
+	RequestAddinShmemSpace(custom_we_memsize());
+}
+
+static Size
+custom_we_memsize(void)
+{
+	return sizeof(CustomWaitEventState);
+}
+
+/*
+ * Allocate and register a new wait event.
+ *
+ * This routine allocate a wait event id when it's called first time.
+ * After that, the name of the wait event is associated with the id. It's
+ * registered in the lookup table of each processes dynamically.
+ */
+static void
+custom_we_shmem_startup(void)
+{
+	bool		found;
+
+	if (prev_shmem_startup_hook)
+		prev_shmem_startup_hook();
+
+	ss = NULL;
+
+	/* Create or attach to the shared memory state */
+	LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
+	ss = ShmemInitStruct("custom_wait_event",
+						 sizeof(CustomWaitEventState),
+						 &found);
+
+	/* Allocate a new wait event. */
+	if (!found)
+		ss->wait_event_info = WaitEventExtensionNewTranche();
+
+	LWLockRelease(AddinShmemInitLock);
+
+	/*
+	 * Register the wait event in the lookup table of the current process.
+	 */
+	WaitEventExtensionRegisterTranche(ss->wait_event_info,
+									  "custom_wait_event");
+
+	return;
+}
+
+/*
+ * Connect to the database and wait forever with the custom wait event
+ */
+void
+custom_we_worker_main(Datum main_arg)
+{
+	pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
+	BackgroundWorkerUnblockSignals();
+
+	/* Connect to our database to show up a wait event in pg_stat_activity */
+	BackgroundWorkerInitializeConnection(worker_database, NULL, 0);
+
+	/* Wait forever with the custom wait event */
+	while (!ShutdownRequestPending)
+	{
+		(void) WaitLatch(MyLatch,
+						 WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
+						 -1L,
+						 ss->wait_event_info);  /* the custom wait event */
+		ResetLatch(MyLatch);
+	}
+}
+
+/*
+ * Dynamically launch a background worker waiting forever
+ */
+Datum
+wait_worker_launch(PG_FUNCTION_ARGS)
+{
+	BackgroundWorker worker;
+	BackgroundWorkerHandle *handle;
+	BgwHandleStatus status;
+	pid_t		pid;
+
+	if (ss == NULL)
+		ereport(ERROR,
+				(errmsg("cannot use \"%s\" if \"%s\" has not been loaded with shared_preload_libraries",
+						"wait_worker_launch", "test_custom_wait_events")));
+
+	memset(&worker, 0, sizeof(worker));
+	worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
+		BGWORKER_BACKEND_DATABASE_CONNECTION;
+	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
+	worker.bgw_restart_time = BGW_NEVER_RESTART;
+	sprintf(worker.bgw_function_name, "custom_we_worker_main");
+	sprintf(worker.bgw_library_name, "test_custom_wait_events");
+	snprintf(worker.bgw_name, BGW_MAXLEN, "test_custom_wait_events worker");
+	snprintf(worker.bgw_type, BGW_MAXLEN, "test_custom_wait_events");
+	worker.bgw_notify_pid = MyProcPid;
+
+	if (!RegisterDynamicBackgroundWorker(&worker, &handle))
+		PG_RETURN_NULL();
+
+	status = WaitForBackgroundWorkerStartup(handle, &pid);
+
+	if (status == BGWH_STOPPED)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("could not start background process"),
+				 errhint("More details may be available in the server log.")));
+	if (status == BGWH_POSTMASTER_DIED)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
+				 errmsg("cannot start background processes without postmaster"),
+				 errhint("Kill all remaining database processes and restart the database.")));
+	Assert(status == BGWH_STARTED);
+
+	PG_RETURN_INT32(pid);
+}
diff --git a/src/test/modules/test_custom_wait_events/test_custom_wait_events.control b/src/test/modules/test_custom_wait_events/test_custom_wait_events.control
new file mode 100644
index 0000000000..8dd875a252
--- /dev/null
+++ b/src/test/modules/test_custom_wait_events/test_custom_wait_events.control
@@ -0,0 +1,3 @@
+comment = 'Test code for custom wait events'
+default_version = '1.0'
+module_pathname = '$libdir/test_custom_wait_events'
-- 
2.40.1