Add flush option to pg_logical_emit_message()

Since its introduction, LogLogicalMessage() (via the SQL interface
pg_logical_emit_message()) has never included a call to XLogFlush(),
causing it to potentially lose messages on a crash when used in
non-transactional mode.  This has come up to me as a problem while
playing with ideas to design a test suite for what has become
039_end_of_wal.pl introduced in bae868caf222 by Thomas Munro, because
there are no direct ways to force a WAL flush via SQL.

The default is false, to not flush messages and influence existing
use-cases where this function could be used.  If set to true, the
message emitted is flushed before returning back to the caller, making
the message durable on crash.  This new option has no effect when using
pg_logical_emit_message() in transactional mode, as the record's flush
is guaranteed by the WAL record generated by the transaction committed.

Two queries of test_decoding are tweaked to cover the new code path for
the flush.

Bump catalog version.

Author: Michael Paquier
Reviewed-by: Andres Freund, Amit Kapila, Fujii Masao, Tung Nguyen, Tomas
Vondra
Discussion: https://postgr.es/m/ZNsdThSe2qgsfs7R@paquier.xyz

6 files changed, 38 insertions, 7 deletions

diff --git a/src/backend/catalog/system_functions.sql b/src/backend/catalog/system_functions.sql
index 07c0d89c4f8..35d738d5763 100644
--- a/src/backend/catalog/system_functions.sql
+++ b/src/backend/catalog/system_functions.sql
@@ -446,6 +446,26 @@ LANGUAGE INTERNAL
 VOLATILE ROWS 1000 COST 1000
 AS 'pg_logical_slot_peek_binary_changes';
 
+CREATE OR REPLACE FUNCTION pg_logical_emit_message(
+    transactional boolean,
+    prefix text,
+    message text,
+    flush boolean DEFAULT false)
+RETURNS pg_lsn
+LANGUAGE INTERNAL
+STRICT VOLATILE
+AS 'pg_logical_emit_message_text';
+
+CREATE OR REPLACE FUNCTION pg_logical_emit_message(
+    transactional boolean,
+    prefix text,
+    message bytea,
+    flush boolean DEFAULT false)
+RETURNS pg_lsn
+LANGUAGE INTERNAL
+STRICT VOLATILE
+AS 'pg_logical_emit_message_bytea';
+
 CREATE OR REPLACE FUNCTION pg_create_physical_replication_slot(
     IN slot_name name, IN immediately_reserve boolean DEFAULT false,
     IN temporary boolean DEFAULT false,
diff --git a/src/backend/replication/logical/logicalfuncs.c b/src/backend/replication/logical/logicalfuncs.c
index 197169d6b0d..1067aca08fc 100644
--- a/src/backend/replication/logical/logicalfuncs.c
+++ b/src/backend/replication/logical/logicalfuncs.c
@@ -362,10 +362,11 @@ pg_logical_emit_message_bytea(PG_FUNCTION_ARGS)
 	bool		transactional = PG_GETARG_BOOL(0);
 	char	   *prefix = text_to_cstring(PG_GETARG_TEXT_PP(1));
 	bytea	   *data = PG_GETARG_BYTEA_PP(2);
+	bool		flush = PG_GETARG_BOOL(3);
 	XLogRecPtr	lsn;
 
 	lsn = LogLogicalMessage(prefix, VARDATA_ANY(data), VARSIZE_ANY_EXHDR(data),
-							transactional);
+							transactional, flush);
 	PG_RETURN_LSN(lsn);
 }
 
diff --git a/src/backend/replication/logical/message.c b/src/backend/replication/logical/message.c
index c5de14afc65..b5d29382f54 100644
--- a/src/backend/replication/logical/message.c
+++ b/src/backend/replication/logical/message.c
@@ -44,9 +44,10 @@
  */
 XLogRecPtr
 LogLogicalMessage(const char *prefix, const char *message, size_t size,
-				  bool transactional)
+				  bool transactional, bool flush)
 {
 	xl_logical_message xlrec;
+	XLogRecPtr	lsn;
 
 	/*
 	 * Force xid to be allocated if we're emitting a transactional message.
@@ -71,7 +72,15 @@ LogLogicalMessage(const char *prefix, const char *message, size_t size,
 	/* allow origin filtering */
 	XLogSetRecordFlags(XLOG_INCLUDE_ORIGIN);
 
-	return XLogInsert(RM_LOGICALMSG_ID, XLOG_LOGICAL_MESSAGE);
+	lsn = XLogInsert(RM_LOGICALMSG_ID, XLOG_LOGICAL_MESSAGE);
+
+	/*
+	 * Make sure that the message hits disk before leaving if emitting a
+	 * non-transactional message when flush is requested.
+	 */
+	if (!transactional && flush)
+		XLogFlush(lsn);
+	return lsn;
 }
 
 /*
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index c5f4af24dc1..2f46fdc7391 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202310161
+#define CATALOG_VERSION_NO	202310181
 
 #endif
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 72ea4aa8b8c..c92d0631a01 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -11167,11 +11167,11 @@
   prosrc => 'pg_replication_slot_advance' },
 { oid => '3577', descr => 'emit a textual logical decoding message',
   proname => 'pg_logical_emit_message', provolatile => 'v', proparallel => 'u',
-  prorettype => 'pg_lsn', proargtypes => 'bool text text',
+  prorettype => 'pg_lsn', proargtypes => 'bool text text bool',
   prosrc => 'pg_logical_emit_message_text' },
 { oid => '3578', descr => 'emit a binary logical decoding message',
   proname => 'pg_logical_emit_message', provolatile => 'v', proparallel => 'u',
-  prorettype => 'pg_lsn', proargtypes => 'bool text bytea',
+  prorettype => 'pg_lsn', proargtypes => 'bool text bytea bool',
   prosrc => 'pg_logical_emit_message_bytea' },
 
 # event triggers
diff --git a/src/include/replication/message.h b/src/include/replication/message.h
index 6ce7f2038b2..0f168d572c1 100644
--- a/src/include/replication/message.h
+++ b/src/include/replication/message.h
@@ -30,7 +30,8 @@ typedef struct xl_logical_message
 #define SizeOfLogicalMessage	(offsetof(xl_logical_message, message))
 
 extern XLogRecPtr LogLogicalMessage(const char *prefix, const char *message,
-									size_t size, bool transactional);
+									size_t size, bool transactional,
+									bool flush);
 
 /* RMGR API */
 #define XLOG_LOGICAL_MESSAGE	0x00