Add system view pg_wait_events

This new view, wrapped around a SRF, shows some information known about
wait events, as of:
- Name.
- Type (Activity, I/O, Extension, etc.).
- Description.

All the information retrieved comes from wait_event_names.txt, and the
description is the same as the documentation with filters applied to
remove any XML markups.  This view is useful when joined with
pg_stat_activity to get the description of a wait event reported.

Custom wait events for extensions are included in the view.

Original idea by Yves Colin.

Author: Bertrand Drouvot
Reviewed-by: Kyotaro Horiguchi, Masahiro Ikeda, Tom Lane, Michael
Paquier
Discussion: https://postgr.es/m/0e2ae164-dc89-03c3-cf7f-de86378053ac@gmail.com

doc/src/sgml/monitoring.sgml

@@ -1103,7 +1103,7 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
   &wait_event_types;
 
    <para>
-     Here is an example of how wait events can be viewed:
+     Here are examples of how wait events can be viewed:
 
 <programlisting>
 SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL;
@@ -1112,6 +1112,18 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
  2540 | Lock            | relation
  6644 | LWLock          | ProcArray
 (2 rows)
+</programlisting>
+
+<programlisting>
+SELECT a.pid, a.wait_event, w.description
+  FROM pg_stat_activity a JOIN
+       pg_wait_events w ON (a.wait_event_type = w.type AND
+                            a.wait_event = w.name)
+  WHERE wait_event is NOT NULL and a.state = 'active';
+-[ RECORD 1 ]------------------------------------------------------------------
+pid         | 686674
+wait_event  | WALInitSync
+description | Waiting for a newly initialized WAL file to reach durable storage
 </programlisting>
    </para>
 

doc/src/sgml/system-views.sgml

@@ -221,6 +221,11 @@
       <entry>views</entry>
      </row>
 
+     <row>
+      <entry><link linkend="view-pg-wait-events"><structname>pg_wait_events</structname></link></entry>
+      <entry>wait events</entry>
+     </row>
+
     </tbody>
    </tgroup>
   </table>
@@ -4825,4 +4830,63 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
   </table>
  </sect1>
 
+
+ <sect1 id="view-pg-wait-events">
+  <title><structname>pg_wait_events</structname></title>
+
+  <indexterm zone="view-pg-wait-events">
+   <primary>pg_wait_events</primary>
+  </indexterm>
+
+  <para>
+   The view <structname>pg_wait_events</structname> provides description about the
+   wait events.
+  </para>
+
+  <table>
+   <title><structname>pg_wait_events</structname> Columns</title>
+   <tgroup cols="1">
+    <thead>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       Column Type
+      </para>
+      <para>
+       Description
+      </para></entry>
+     </row>
+    </thead>
+
+    <tbody>
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>type</structfield> <type>text</type>
+      </para>
+      <para>
+       Wait event type
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>name</structfield> <type>text</type>
+      </para>
+      <para>
+       Wait event name
+      </para></entry>
+     </row>
+
+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>description</structfield> <type>text</type>
+      </para>
+      <para>
+       Wait event description
+      </para></entry>
+     </row>
+    </tbody>
+   </tgroup>
+  </table>
+ </sect1>
+
 </chapter>

src/backend/Makefile

@@ -134,7 +134,7 @@ storage/lmgr/lwlocknames.h: storage/lmgr/generate-lwlocknames.pl storage/lmgr/lw
 	$(MAKE) -C storage/lmgr lwlocknames.h lwlocknames.c
 
 utils/activity/wait_event_types.h: utils/activity/generate-wait_event_types.pl utils/activity/wait_event_names.txt
-	$(MAKE) -C utils/activity wait_event_types.h pgstat_wait_event.c
+	$(MAKE) -C utils/activity wait_event_types.h pgstat_wait_event.c wait_event_funcs_data.c
 
 # run this unconditionally to avoid needing to know its dependencies here:
 submake-catalog-headers:
@@ -311,6 +311,7 @@ maintainer-clean: distclean
 	      storage/lmgr/lwlocknames.c \
 	      storage/lmgr/lwlocknames.h \
 	      utils/activity/pgstat_wait_event.c \
+	      utils/activity/wait_event_funcs_data.c \
 	      utils/activity/wait_event_types.h \
 	      utils/adt/jsonpath_gram.c \
 	      utils/adt/jsonpath_gram.h \

src/backend/catalog/system_views.sql

@@ -1342,3 +1342,6 @@ CREATE VIEW pg_stat_subscription_stats AS
         ss.stats_reset
     FROM pg_subscription as s,
          pg_stat_get_subscription_stats(s.oid) as ss;
+
+CREATE VIEW pg_wait_events AS
+    SELECT * FROM pg_get_wait_events();

src/backend/utils/activity/.gitignore

@@ -1,2 +1,3 @@
 /pgstat_wait_event.c
 /wait_event_types.h
+/wait_event_funcs_data.c

src/backend/utils/activity/Makefile

@@ -32,10 +32,14 @@ OBJS = \
 	pgstat_subscription.o \
 	pgstat_wal.o \
 	pgstat_xact.o \
-	wait_event.o
+	wait_event.o \
+	wait_event_funcs.o
 
 include $(top_srcdir)/src/backend/common.mk
 
+wait_event_funcs.o: wait_event_funcs_data.c
+wait_event_funcs_data.c: wait_event_types.h
+
 wait_event.o: pgstat_wait_event.c
 pgstat_wait_event.c: wait_event_types.h
 	touch $@
@@ -44,4 +48,4 @@ wait_event_types.h: $(top_srcdir)/src/backend/utils/activity/wait_event_names.tx
 	$(PERL) $(srcdir)/generate-wait_event_types.pl --code $<
 
 maintainer-clean: clean
-	rm -f wait_event_types.h pgstat_wait_event.c
+	rm -f wait_event_types.h pgstat_wait_event.c wait_event_funcs_data.c

src/backend/utils/activity/generate-wait_event_types.pl

@@ -4,6 +4,7 @@
 # Generate wait events support files from wait_event_names.txt:
 # - wait_event_types.h (if --code is passed)
 # - pgstat_wait_event.c (if --code is passed)
+# - wait_event_funcs_data.c (if --code is passed)
 # - wait_event_types.sgml (if --docs is passed)
 #
 # Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
@@ -98,8 +99,10 @@ if ($gen_code)
 	# multiple times.
 	my $htmp = "$output_path/wait_event_types.h.tmp$$";
 	my $ctmp = "$output_path/pgstat_wait_event.c.tmp$$";
+	my $wctmp = "$output_path/wait_event_funcs_data.c.tmp$$";
 	open my $h, '>', $htmp or die "Could not open $htmp: $!";
 	open my $c, '>', $ctmp or die "Could not open $ctmp: $!";
+	open my $wc, '>', $wctmp or die "Could not open $wctmp: $!";
 
 	my $header_comment =
 	  '/*-------------------------------------------------------------------------
@@ -129,12 +132,14 @@ if ($gen_code)
 
 	printf $c $header_comment, 'pgstat_wait_event.c';
 
+	printf $wc $header_comment, 'wait_event_funcs_data.c';
+
+	# Generate the pgstat_wait_event.c and wait_event_types.h files
 	# uc() is being used to force the comparison to be case-insensitive.
 	foreach my $waitclass (sort { uc($a) cmp uc($b) } keys %hashwe)
 	{
-
+		# Don't generate the pgstat_wait_event.c and wait_event_types.h files
-		# Don't generate .c and .h files for Extension, LWLock and
+		# for Extension, LWLock and Lock, these are handled independently.
-		# Lock, these are handled independently.
 		next
 		  if ( $waitclass eq 'WaitEventExtension'
 			|| $waitclass eq 'WaitEventLWLock'
@@ -183,14 +188,57 @@ if ($gen_code)
 		printf $c "}\n\n";
 	}
 
+	# Generate wait_event_funcs_data.c, building the contents of a static
+	# C structure holding all the information about the wait events.
+	# uc() is being used to force the comparison to be case-insensitive,
+	# even though it is not required here.
+	foreach my $waitclass (sort { uc($a) cmp uc($b) } keys %hashwe)
+	{
+		my $last = $waitclass;
+		$last =~ s/^WaitEvent//;
+
+		foreach my $wev (@{ $hashwe{$waitclass} })
+		{
+			my $new_desc = substr $wev->[2], 1, -2;
+			# Escape single quotes.
+			$new_desc =~ s/'/\\'/g;
+
+			# Replace the "quote" markups by real ones.
+			$new_desc =~ s/<quote>(.*?)<\/quote>/\\"$1\\"/g;
+
+			# Remove SGML markups.
+			$new_desc =~ s/<.*?>(.*?)<.*?>/$1/g;
+
+			# Tweak contents about links <xref linkend="text"/>
+			# on GUCs,
+			while (my ($capture) =
+				$new_desc =~ m/<xref linkend="guc-(.*?)"\/>/g)
+			{
+				$capture =~ s/-/_/g;
+				$new_desc =~ s/<xref linkend="guc-.*?"\/>/$capture/g;
+			}
+			# Then remove any reference to
+			# "see <xref linkend="text"/>".
+			$new_desc =~ s/; see.*$//;
+
+			# Build one element of the C structure holding the
+			# wait event info, as of (type, name, description).
+			printf $wc "\t{\"%s\", \"%s\", \"%s\"},\n", $last, $wev->[1],
+			  $new_desc;
+		}
+	}
+
 	printf $h "#endif                          /* WAIT_EVENT_TYPES_H */\n";
 	close $h;
 	close $c;
+	close $wc;
 
 	rename($htmp, "$output_path/wait_event_types.h")
 	  || die "rename: $htmp to $output_path/wait_event_types.h: $!";
 	rename($ctmp, "$output_path/pgstat_wait_event.c")
 	  || die "rename: $ctmp to $output_path/pgstat_wait_event.c: $!";
+	rename($wctmp, "$output_path/wait_event_funcs_data.c")
+	  || die "rename: $wctmp to $output_path/wait_event_funcs_data.c: $!";
 }
 # Generate the .sgml file.
 elsif ($gen_docs)
@@ -249,7 +297,7 @@ Usage: perl  [--output <path>] [--code ] [ --sgml ] input_file
 
 Options:
     --outdir         Output directory (default '.')
-    --code           Generate wait_event_types.h and pgstat_wait_event.c.
+    --code           Generate C and header files.
     --sgml           Generate wait_event_types.sgml.
 
 generate-wait_event_types.pl generates the SGML documentation and code

src/backend/utils/activity/meson.build

@@ -23,6 +23,7 @@ backend_sources += files(
 # seems nicer to not add that as an include path for the whole backend.
 waitevent_sources = files(
   'wait_event.c',
+  'wait_event_funcs.c',
 )
 
 wait_event = static_library('wait_event_names',

src/backend/utils/activity/wait_event.c

@@ -264,6 +264,46 @@ GetWaitEventExtensionIdentifier(uint16 eventId)
 }
 
 
+/*
+ * Returns a list of currently defined custom wait event names for extensions.
+ * The result is a palloc'd array, with the number of elements saved in
+ * *nwaitevents.
+ */
+char	  **
+GetWaitEventExtensionNames(int *nwaitevents)
+{
+	char	  **waiteventnames;
+	WaitEventExtensionEntryByName *hentry;
+	HASH_SEQ_STATUS hash_seq;
+	int			index;
+	int			els;
+
+	LWLockAcquire(WaitEventExtensionLock, LW_SHARED);
+
+	/* Now we can safely count the number of entries */
+	els = hash_get_num_entries(WaitEventExtensionHashByName);
+
+	/* Allocate enough space for all entries */
+	waiteventnames = palloc(els * sizeof(char *));
+
+	/* Now scan the hash table to copy the data */
+	hash_seq_init(&hash_seq, WaitEventExtensionHashByName);
+
+	index = 0;
+	while ((hentry = (WaitEventExtensionEntryByName *) hash_seq_search(&hash_seq)) != NULL)
+	{
+		waiteventnames[index] = pstrdup(hentry->wait_event_name);
+		index++;
+	}
+
+	LWLockRelease(WaitEventExtensionLock);
+
+	Assert(index == els);
+
+	*nwaitevents = index;
+	return waiteventnames;
+}
+
 /*
  * 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()

src/backend/utils/activity/wait_event_funcs.c

@@ -0,0 +1,93 @@
+/*------------------------------------------------------------------------
+ *
+ * wait_event_funcs.c
+ *	  Functions for accessing wait event data.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/utils/activity/wait_event_funcs.c
+ *
+ *------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "funcapi.h"
+#include "utils/builtins.h"
+#include "utils/wait_event.h"
+
+/*
+ * Each wait event has one corresponding entry in this structure, fed to
+ * the SQL function of this file.
+ */
+static const struct
+{
+	const char *type;
+	const char *name;
+	const char *description;
+}
+
+			waitEventData[] =
+{
+#include "wait_event_funcs_data.c"
+	/* end of list */
+	{NULL, NULL, NULL}
+};
+
+
+/*
+ * pg_get_wait_events
+ *
+ * List information about wait events (type, name and description).
+ */
+Datum
+pg_get_wait_events(PG_FUNCTION_ARGS)
+{
+#define PG_GET_WAIT_EVENTS_COLS 3
+	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	char	  **waiteventnames;
+	int			nbextwaitevents;
+
+	/* Build tuplestore to hold the result rows */
+	InitMaterializedSRF(fcinfo, 0);
+
+	/* Iterate over the list of wait events */
+	for (int idx = 0; waitEventData[idx].type != NULL; idx++)
+	{
+		Datum		values[PG_GET_WAIT_EVENTS_COLS] = {0};
+		bool		nulls[PG_GET_WAIT_EVENTS_COLS] = {0};
+
+		values[0] = CStringGetTextDatum(waitEventData[idx].type);
+		values[1] = CStringGetTextDatum(waitEventData[idx].name);
+		values[2] = CStringGetTextDatum(waitEventData[idx].description);
+
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
+	}
+
+	/* Handle custom wait events for extensions */
+	waiteventnames = GetWaitEventExtensionNames(&nbextwaitevents);
+
+	for (int idx = 0; idx < nbextwaitevents; idx++)
+	{
+		StringInfoData buf;
+		Datum		values[PG_GET_WAIT_EVENTS_COLS] = {0};
+		bool		nulls[PG_GET_WAIT_EVENTS_COLS] = {0};
+
+
+		values[0] = CStringGetTextDatum("Extension");
+		values[1] = CStringGetTextDatum(waiteventnames[idx]);
+
+		initStringInfo(&buf);
+		appendStringInfo(&buf,
+						 "Waiting for custom wait event \"%s\" defined by extension module",
+						 waiteventnames[idx]);
+
+		values[2] = CStringGetTextDatum(buf.data);
+
+		tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc, values, nulls);
+	}
+
+	return (Datum) 0;
+}

src/include/catalog/pg_proc.dat

@@ -5417,6 +5417,12 @@
   proargmodes => '{i,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
   proargnames => '{pid,datid,pid,usesysid,application_name,state,query,wait_event_type,wait_event,xact_start,query_start,backend_start,state_change,client_addr,client_hostname,client_port,backend_xid,backend_xmin,backend_type,ssl,sslversion,sslcipher,sslbits,ssl_client_dn,ssl_client_serial,ssl_issuer_dn,gss_auth,gss_princ,gss_enc,gss_delegation,leader_pid,query_id}',
   prosrc => 'pg_stat_get_activity' },
+{ oid => '8403', descr => 'describe wait events',
+  proname => 'pg_get_wait_events', procost => '10', prorows => '250',
+  proretset => 't', provolatile => 'v', prorettype => 'record',
+  proargtypes => '', proallargtypes => '{text,text,text}',
+  proargmodes => '{o,o,o}', proargnames => '{type,name,description}',
+  prosrc => 'pg_get_wait_events' },
 { oid => '3318',
   descr => 'statistics: information about progress of backends running maintenance command',
   proname => 'pg_stat_get_progress_info', prorows => '100', proretset => 't',

src/include/utils/meson.build

@@ -1,6 +1,6 @@
 # Copyright (c) 2022-2023, PostgreSQL Global Development Group
 
-wait_event_output = ['wait_event_types.h', 'pgstat_wait_event.c']
+wait_event_output = ['wait_event_types.h', 'pgstat_wait_event.c', 'wait_event_funcs_data.c']
 wait_event_target = custom_target('wait_event_names',
   input: files('../../backend/utils/activity/wait_event_names.txt'),
   output: wait_event_output,
@@ -11,7 +11,7 @@ wait_event_target = custom_target('wait_event_names',
   ],
   build_by_default: true,
   install: true,
-  install_dir: [dir_include_server / 'utils', false],
+  install_dir: [dir_include_server / 'utils', false, false],
 )
 
 wait_event_types_h = wait_event_target[0]

src/include/utils/wait_event.h

@@ -63,6 +63,7 @@ extern void WaitEventExtensionShmemInit(void);
 extern Size WaitEventExtensionShmemSize(void);
 
 extern uint32 WaitEventExtensionNew(const char *wait_event_name);
+extern char **GetWaitEventExtensionNames(int *nwaitevents);
 
 /* ----------
  * pgstat_report_wait_start() -

src/test/modules/worker_spi/t/001_worker_spi.pl

@@ -47,6 +47,12 @@ $result = $node->poll_query_until(
 is($result, 1,
 	'dynamic bgworker has reported "worker_spi_main" as wait event');
 
+# Check the wait event used by the dynamic bgworker appears in pg_wait_events
+$result = $node->safe_psql('postgres',
+	q[SELECT count(*) > 0 from pg_wait_events where type = 'Extension' and name = 'worker_spi_main';]
+);
+is($result, 't', '"worker_spi_main" is reported in pg_wait_events');
+
 note "testing bgworkers loaded with shared_preload_libraries";
 
 # Create the database first so as the workers can connect to it when

src/test/regress/expected/rules.out

@@ -2631,6 +2631,10 @@ pg_views| SELECT n.nspname AS schemaname,
    FROM (pg_class c
      LEFT JOIN pg_namespace n ON ((n.oid = c.relnamespace)))
   WHERE (c.relkind = 'v'::"char");
+pg_wait_events| SELECT type,
+    name,
+    description
+   FROM pg_get_wait_events() pg_get_wait_events(type, name, description);
 SELECT tablename, rulename, definition FROM pg_rules
 WHERE schemaname = 'pg_catalog'
 ORDER BY tablename, rulename;

src/test/regress/expected/sysviews.out

@@ -134,6 +134,22 @@ select name, setting from pg_settings where name like 'enable%';
  enable_tidscan                 | on
 (21 rows)
 
+-- There are always wait event descriptions for various types.
+select type, count(*) > 0 as ok FROM pg_wait_events
+  group by type order by type COLLATE "C";
+   type    | ok 
+-----------+----
+ Activity  | t
+ BufferPin | t
+ Client    | t
+ Extension | t
+ IO        | t
+ IPC       | t
+ LWLock    | t
+ Lock      | t
+ Timeout   | t
+(9 rows)
+
 -- Test that the pg_timezone_names and pg_timezone_abbrevs views are
 -- more-or-less working.  We can't test their contents in any great detail
 -- without the outputs changing anytime IANA updates the underlying data,

src/test/regress/sql/sysviews.sql

@@ -55,6 +55,10 @@ select count(*) = 0 as ok from pg_stat_wal_receiver;
 -- a regression test run.
 select name, setting from pg_settings where name like 'enable%';
 
+-- There are always wait event descriptions for various types.
+select type, count(*) > 0 as ok FROM pg_wait_events
+  group by type order by type COLLATE "C";
+
 -- Test that the pg_timezone_names and pg_timezone_abbrevs views are
 -- more-or-less working.  We can't test their contents in any great detail
 -- without the outputs changing anytime IANA updates the underlying data,

src/tools/msvc/Solution.pm

@@ -588,7 +588,8 @@ sub GenerateFiles
 			'src/include/utils/wait_event_types.h',
 			'src/backend/utils/activity/wait_event_names.txt'))
 	{
-		print "Generating pgstat_wait_event.c and wait_event_types.h...\n";
+		print
+		  "Generating pgstat_wait_event.c, wait_event_types.h and wait_event_funcs_data.c...\n";
 		my $activ = 'src/backend/utils/activity';
 		system(
 			"perl $activ/generate-wait_event_types.pl --outdir $activ --code $activ/wait_event_names.txt"

src/tools/msvc/clean.bat

@@ -55,6 +55,7 @@ if exist src\include\catalog\header-stamp del /q src\include\catalog\header-stam
 if exist doc\src\sgml\version.sgml del /q doc\src\sgml\version.sgml
 
 if %DIST%==1 if exist src\backend\utils\activity\pgstat_wait_event.c del /q src\backend\utils\activity\pgstat_wait_event.c
+if %DIST%==1 if exist src\backend\utils\activity\wait_event_funcs_data.c del /q src\backend\utils\activity\wait_event_funcs_data.c
 if %DIST%==1 if exist src\backend\utils\activity\wait_event_types.h del /q src\backend\utils\activity\wait_event_types.h
 if %DIST%==1 if exist src\backend\utils\fmgroids.h del /q src\backend\utils\fmgroids.h
 if %DIST%==1 if exist src\backend\utils\fmgrprotos.h del /q src\backend\utils\fmgrprotos.h