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

Author: danielgustafsson

doc/src/sgml/config.sgml

@@ -9425,6 +9425,25 @@ SET XML OPTION { DOCUMENT | CONTENT };
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-event-triggers" xreflabel="event_triggers">
+      <term><varname>event_triggers</varname> (<type>boolean</type>)
+      <indexterm>
+       <primary><varname>event_triggers</varname></primary>
+       <secondary>configuration parameter</secondary>
+     </indexterm>
+     </term>
+     <listitem>
+      <para>
+       Allow temporarily disabling execution of event triggers in order to
+       troubleshoot and repair faulty event triggers. All event triggers will
+       be disabled by setting it to <literal>false</literal>. Setting the value
+       to <literal>true</literal> allows all event triggers to fire, this
+       is the default value. Only superusers and users with the appropriate
+       <literal>SET</literal> privilege can change this setting.
+      </para>
+     </listitem>
+     </varlistentry>
+
      </variablelist>
     </sect2>
      <sect2 id="runtime-config-client-format">

doc/src/sgml/ref/create_event_trigger.sgml

@@ -121,9 +121,12 @@ CREATE EVENT TRIGGER <replaceable class="parameter">name</replaceable>
 
   <para>
    Event triggers are disabled in single-user mode (see <xref
-   linkend="app-postgres"/>).  If an erroneous event trigger disables the
-   database so much that you can't even drop the trigger, restart in
-   single-user mode and you'll be able to do that.
+   linkend="app-postgres"/>) as well as when
+   <xref linkend="guc-event-triggers"/> is set to <literal>false</literal>.
+   If an erroneous event trigger disables the database so much that you can't
+   even drop the trigger, restart with <xref linkend="guc-event-triggers"/>
+   set to <literal>false</literal> to temporarily disable event triggers, or
+   in single-user mode, and you'll be able to do that.
   </para>
  </refsect1>
 

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;
 
 	/*

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

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 -
 

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

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;

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;