From: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Date: Fri, 31 Jul 2020 18:07:38 +0200
Subject: [PATCH 3/4] demote: add pg_demote() function

---
 src/backend/access/transam/xlogfuncs.c | 94 ++++++++++++++++++++++++++
 src/backend/catalog/system_views.sql   |  6 ++
 src/include/catalog/pg_proc.dat        |  4 ++
 3 files changed, 104 insertions(+)

diff --git a/src/backend/access/transam/xlogfuncs.c b/src/backend/access/transam/xlogfuncs.c
index 290658b22c..733f465d38 100644
--- a/src/backend/access/transam/xlogfuncs.c
+++ b/src/backend/access/transam/xlogfuncs.c
@@ -784,3 +784,97 @@ pg_promote(PG_FUNCTION_ARGS)
 			(errmsg("server did not promote within %d seconds", wait_seconds)));
 	PG_RETURN_BOOL(false);
 }
+
+/*
+ * Demotes a production server.
+ *
+ * A result of "true" means that demotion has been completed if "wait" is
+ * "true", or initiated if "wait" is false.
+ */
+Datum
+pg_demote(PG_FUNCTION_ARGS)
+{
+	bool		fast = PG_GETARG_BOOL(0);
+	bool		wait = PG_GETARG_BOOL(1);
+	int			wait_seconds = PG_GETARG_INT32(2);
+	char		demote_filename[] = "demote_fast";
+	FILE	   *demote_file;
+	int			i;
+
+	if (RecoveryInProgress())
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("recovery in progress"),
+				 errhint("you can not demote while already in recovery.")));
+
+	if (!EnableHotStandby)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("function pg_demote() requires hot_standby parameter to be enabled"),
+				 errhint("The function can not return its status from a non hot_standby-enabled standby")));
+
+	if (wait_seconds <= 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+				 errmsg("\"wait_seconds\" must not be negative or zero")));
+
+	if (!fast)
+		demote_filename[6] = '\0';
+
+	/* create the demote signal file */
+	demote_file = AllocateFile(demote_filename, "w");
+	if (!demote_file)
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not create file \"%s\": %m",
+						demote_filename)));
+
+	if (FreeFile(demote_file))
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not write file \"%s\": %m",
+						demote_filename)));
+
+	/* signal the postmaster */
+	if (kill(PostmasterPid, SIGUSR1) != 0)
+	{
+		ereport(WARNING,
+				(errmsg("failed to send signal to postmaster: %m")));
+		(void) unlink(demote_filename);
+		PG_RETURN_BOOL(false);
+	}
+
+	/* return immediately if waiting was not requested */
+	if (!wait)
+		PG_RETURN_BOOL(true);
+
+	/* wait for the amount of time wanted until demotion */
+#define WAITS_PER_SECOND 10
+	for (i = 0; i < WAITS_PER_SECOND * wait_seconds; i++)
+	{
+		int			rc;
+
+		ResetLatch(MyLatch);
+
+		if (RecoveryInProgress())
+			PG_RETURN_BOOL(true);
+
+		CHECK_FOR_INTERRUPTS();
+
+		rc = WaitLatch(MyLatch,
+					   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
+					   1000L / WAITS_PER_SECOND,
+					   WAIT_EVENT_DEMOTE);
+
+		/*
+		 * Emergency bailout if postmaster has died.  This is to avoid the
+		 * necessity for manual cleanup of all postmaster children.
+		 */
+		if (rc & WL_POSTMASTER_DEATH)
+			PG_RETURN_BOOL(false);
+	}
+
+	ereport(WARNING,
+			(errmsg("server did not demote within %d seconds", wait_seconds)));
+	PG_RETURN_BOOL(false);
+}
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 8625cbeab6..573d7b46eb 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1219,6 +1219,11 @@ CREATE OR REPLACE FUNCTION
   RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_promote'
   PARALLEL SAFE;
 
+CREATE OR REPLACE FUNCTION
+  pg_demote(fast boolean DEFAULT true, wait boolean DEFAULT true, wait_seconds integer DEFAULT 60)
+  RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_demote'
+  PARALLEL SAFE;
+
 -- legacy definition for compatibility with 9.3
 CREATE OR REPLACE FUNCTION
   json_populate_record(base anyelement, from_json json, use_json_as_text boolean DEFAULT false)
@@ -1435,6 +1440,7 @@ REVOKE EXECUTE ON FUNCTION pg_reload_conf() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_current_logfile() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_current_logfile(text) FROM public;
 REVOKE EXECUTE ON FUNCTION pg_promote(boolean, integer) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_demote(boolean, boolean, integer) FROM public;
 
 REVOKE EXECUTE ON FUNCTION pg_stat_reset() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_stat_reset_shared(text) FROM public;
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 082a11f270..9e4d000d00 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -6084,6 +6084,10 @@
   proname => 'pg_promote', provolatile => 'v', prorettype => 'bool',
   proargtypes => 'bool int4', proargnames => '{wait,wait_seconds}',
   prosrc => 'pg_promote' },
+{ oid => '8967', descr => 'demote production server',
+  proname => 'pg_demote', provolatile => 'v', prorettype => 'bool',
+  proargtypes => 'bool bool int4', proargnames => '{fast,wait,wait_seconds}',
+  prosrc => 'pg_demote' },
 { oid => '2848', descr => 'switch to new wal file',
   proname => 'pg_switch_wal', provolatile => 'v', prorettype => 'pg_lsn',
   proargtypes => '', prosrc => 'pg_switch_wal' },
-- 
2.20.1


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