Add GUC for temporarily disabling event triggers

In order to troubleshoot misbehaving or buggy event triggers, the
documented advice is to enter single-user mode.  In an attempt to
reduce the number of situations where single-user mode is required
(or even recommended) for non-extraordinary maintenance, this GUC
allows to temporarily suspend event triggers.

This was originally extracted from a larger patchset which aimed
at supporting event triggers on login events.

Reviewed-by: Ted Yu <yuzhihong@gmail.com>
Reviewed-by: Mikhail Gribkov <youzhick@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/9140106E-F9BF-4D85-8FC8-F2D3C094A6D9@yesql.se
Discussion: https://postgr.es/m/0d46d29f-4558-3af9-9c85-7774e14a7709@postgrespro.ru

6 files changed, 73 insertions, 7 deletions

diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index d4b00d1a828..bd812e42d94 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -72,6 +72,9 @@ typedef struct EventTriggerQueryState
 
 static EventTriggerQueryState *currentEventTriggerState = NULL;
 
+/* GUC parameter */
+bool		event_triggers = true;
+
 /* Support for dropped objects */
 typedef struct SQLDropObject
 {
@@ -657,8 +660,11 @@ EventTriggerDDLCommandStart(Node *parsetree)
 	 * wherein event triggers are disabled.  (Or we could implement
 	 * heapscan-and-sort logic for that case, but having disaster recovery
 	 * scenarios depend on code that's otherwise untested isn't appetizing.)
+	 *
+	 * Additionally, event triggers can be disabled with a superuser-only GUC
+	 * to make fixing database easier as per 1 above.
 	 */
-	if (!IsUnderPostmaster)
+	if (!IsUnderPostmaster || !event_triggers)
 		return;
 
 	runlist = EventTriggerCommonSetup(parsetree,
@@ -692,9 +698,9 @@ EventTriggerDDLCommandEnd(Node *parsetree)
 
 	/*
 	 * See EventTriggerDDLCommandStart for a discussion about why event
-	 * triggers are disabled in single user mode.
+	 * triggers are disabled in single user mode or via GUC.
 	 */
-	if (!IsUnderPostmaster)
+	if (!IsUnderPostmaster || !event_triggers)
 		return;
 
 	/*
@@ -740,9 +746,9 @@ EventTriggerSQLDrop(Node *parsetree)
 
 	/*
 	 * See EventTriggerDDLCommandStart for a discussion about why event
-	 * triggers are disabled in single user mode.
+	 * triggers are disabled in single user mode or via a GUC.
 	 */
-	if (!IsUnderPostmaster)
+	if (!IsUnderPostmaster || !event_triggers)
 		return;
 
 	/*
@@ -811,9 +817,9 @@ EventTriggerTableRewrite(Node *parsetree, Oid tableOid, int reason)
 
 	/*
 	 * See EventTriggerDDLCommandStart for a discussion about why event
-	 * triggers are disabled in single user mode.
+	 * triggers are disabled in single user mode or via a GUC.
 	 */
-	if (!IsUnderPostmaster)
+	if (!IsUnderPostmaster || !event_triggers)
 		return;
 
 	/*
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index bdb26e2b77d..16ec6c5ef02 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -37,6 +37,7 @@
 #include "catalog/namespace.h"
 #include "catalog/storage.h"
 #include "commands/async.h"
+#include "commands/event_trigger.h"
 #include "commands/tablespace.h"
 #include "commands/trigger.h"
 #include "commands/user.h"
@@ -2000,6 +2001,16 @@ struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"event_triggers", PGC_SUSET, CLIENT_CONN_STATEMENT,
+			gettext_noop("Enables event triggers."),
+			gettext_noop("When enabled, event triggers will fire for all applicable statements."),
+		},
+		&event_triggers,
+		true,
+		NULL, NULL, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, false, NULL, NULL, NULL
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 6bb39d39bad..d08d55c3fe4 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -706,6 +706,7 @@
 #xmloption = 'content'
 #gin_pending_list_limit = 4MB
 #createrole_self_grant = ''		# set and/or inherit
+#event_triggers = on
 
 # - Locale and Formatting -
 
diff --git a/src/include/commands/event_trigger.h b/src/include/commands/event_trigger.h
index 5ed6ece555d..1c925dbf257 100644
--- a/src/include/commands/event_trigger.h
+++ b/src/include/commands/event_trigger.h
@@ -29,6 +29,8 @@ typedef struct EventTriggerData
 	CommandTag	tag;
 } EventTriggerData;
 
+extern PGDLLIMPORT bool event_triggers;
+
 #define AT_REWRITE_ALTER_PERSISTENCE	0x01
 #define AT_REWRITE_DEFAULT_VAL			0x02
 #define AT_REWRITE_COLUMN_REWRITE		0x04
diff --git a/src/test/regress/expected/event_trigger.out b/src/test/regress/expected/event_trigger.out
index 2c8a6b22121..0b87a42d0a9 100644
--- a/src/test/regress/expected/event_trigger.out
+++ b/src/test/regress/expected/event_trigger.out
@@ -616,3 +616,25 @@ SELECT
 DROP EVENT TRIGGER start_rls_command;
 DROP EVENT TRIGGER end_rls_command;
 DROP EVENT TRIGGER sql_drop_command;
+-- Check the GUC for disabling event triggers
+CREATE FUNCTION test_event_trigger_guc() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+	obj record;
+BEGIN
+	FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
+	LOOP
+		RAISE NOTICE '% dropped %', tg_tag, obj.object_type;
+	END LOOP;
+END;
+$$;
+CREATE EVENT TRIGGER test_event_trigger_guc
+	ON sql_drop
+	WHEN TAG IN ('DROP POLICY') EXECUTE FUNCTION test_event_trigger_guc();
+SET event_triggers = 'on';
+CREATE POLICY pguc ON event_trigger_test USING (FALSE);
+DROP POLICY pguc ON event_trigger_test;
+NOTICE:  DROP POLICY dropped policy
+CREATE POLICY pguc ON event_trigger_test USING (FALSE);
+SET event_triggers = 'off';
+DROP POLICY pguc ON event_trigger_test;
diff --git a/src/test/regress/sql/event_trigger.sql b/src/test/regress/sql/event_trigger.sql
index 1aeaddbe715..6f0933b9e88 100644
--- a/src/test/regress/sql/event_trigger.sql
+++ b/src/test/regress/sql/event_trigger.sql
@@ -471,3 +471,27 @@ SELECT
 DROP EVENT TRIGGER start_rls_command;
 DROP EVENT TRIGGER end_rls_command;
 DROP EVENT TRIGGER sql_drop_command;
+
+-- Check the GUC for disabling event triggers
+CREATE FUNCTION test_event_trigger_guc() RETURNS event_trigger
+LANGUAGE plpgsql AS $$
+DECLARE
+	obj record;
+BEGIN
+	FOR obj IN SELECT * FROM pg_event_trigger_dropped_objects()
+	LOOP
+		RAISE NOTICE '% dropped %', tg_tag, obj.object_type;
+	END LOOP;
+END;
+$$;
+CREATE EVENT TRIGGER test_event_trigger_guc
+	ON sql_drop
+	WHEN TAG IN ('DROP POLICY') EXECUTE FUNCTION test_event_trigger_guc();
+
+SET event_triggers = 'on';
+CREATE POLICY pguc ON event_trigger_test USING (FALSE);
+DROP POLICY pguc ON event_trigger_test;
+
+CREATE POLICY pguc ON event_trigger_test USING (FALSE);
+SET event_triggers = 'off';
+DROP POLICY pguc ON event_trigger_test;