Allow pgoutput to send logical decoding messages.

The output plugin accepts a new parameter (messages) that controls if
logical decoding messages are written into the replication stream. It is
useful for those clients that use pgoutput as an output plugin and needs
to process messages that were written by pg_logical_emit_message().

Although logical streaming replication protocol supports logical
decoding messages now, logical replication does not use this feature yet.

Author: David Pirotte, Euler Taveira
Reviewed-by: Euler Taveira, Andres Freund, Ashutosh Bapat, Amit Kapila
Discussion: https://postgr.es/m/CADK3HHJ-+9SO7KuRLH=9Wa1rAo60Yreq1GFNkH_kd0=CdaWM+A@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -6433,6 +6433,82 @@ Begin
 </listitem>
 </varlistentry>
 
+<varlistentry>
+<term>
+Message
+</term>
+<listitem>
+<para>
+
+<variablelist>
+<varlistentry>
+<term>
+        Byte1('M')
+</term>
+<listitem>
+<para>
+                Identifies the message as a logical decoding message.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+        Int32
+</term>
+<listitem>
+<para>
+                Xid of the transaction. The XID is sent only when user has
+                requested streaming of in-progress transactions.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+        Int8
+</term>
+<listitem>
+<para>
+                Flags; Either 0 for no flags or 1 if the logical decoding
+                message is transactional.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+        Int64
+</term>
+<listitem>
+<para>
+                The LSN of the logical decoding message.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+        String
+</term>
+<listitem>
+<para>
+                The prefix of the logical decoding message.
+</para>
+</listitem>
+</varlistentry>
+<varlistentry>
+<term>
+        Byte<replaceable>n</replaceable>
+</term>
+<listitem>
+<para>
+                The content of the logical decoding message.
+</para>
+</listitem>
+</varlistentry>
+
+</variablelist>
+</para>
+</listitem>
+</varlistentry>
+
 <varlistentry>
 <term>
 Commit

src/backend/replication/logical/proto.c

@@ -25,6 +25,7 @@
  */
 #define LOGICALREP_IS_REPLICA_IDENTITY 1
 
+#define MESSAGE_TRANSACTIONAL (1<<0)
 #define TRUNCATE_CASCADE		(1<<0)
 #define TRUNCATE_RESTART_SEQS	(1<<1)
 
@@ -361,6 +362,33 @@ logicalrep_read_truncate(StringInfo in,
 	return relids;
 }
 
+/*
+ * Write MESSAGE to stream
+ */
+void
+logicalrep_write_message(StringInfo out, TransactionId xid, XLogRecPtr lsn,
+						 bool transactional, const char *prefix, Size sz,
+						 const char *message)
+{
+	uint8		flags = 0;
+
+	pq_sendbyte(out, LOGICAL_REP_MSG_MESSAGE);
+
+	/* encode and send message flags */
+	if (transactional)
+		flags |= MESSAGE_TRANSACTIONAL;
+
+	/* transaction ID (if not valid, we're not streaming) */
+	if (TransactionIdIsValid(xid))
+		pq_sendint32(out, xid);
+
+	pq_sendint8(out, flags);
+	pq_sendint64(out, lsn);
+	pq_sendstring(out, prefix);
+	pq_sendint32(out, sz);
+	pq_sendbytes(out, message, sz);
+}
+
 /*
  * Write relation description to the output stream.
  */

src/backend/replication/logical/worker.c

@@ -1939,6 +1939,15 @@ apply_dispatch(StringInfo s)
 			apply_handle_origin(s);
 			return;
 
+		case LOGICAL_REP_MSG_MESSAGE:
+
+			/*
+			 * Logical replication does not use generic logical messages yet.
+			 * Although, it could be used by other applications that use this
+			 * output plugin.
+			 */
+			return;
+
 		case LOGICAL_REP_MSG_STREAM_START:
 			apply_handle_stream_start(s);
 			return;

src/backend/replication/pgoutput/pgoutput.c

@@ -45,6 +45,10 @@ static void pgoutput_change(LogicalDecodingContext *ctx,
 static void pgoutput_truncate(LogicalDecodingContext *ctx,
 							  ReorderBufferTXN *txn, int nrelations, Relation relations[],
 							  ReorderBufferChange *change);
+static void pgoutput_message(LogicalDecodingContext *ctx,
+							 ReorderBufferTXN *txn, XLogRecPtr message_lsn,
+							 bool transactional, const char *prefix,
+							 Size sz, const char *message);
 static bool pgoutput_origin_filter(LogicalDecodingContext *ctx,
 								   RepOriginId origin_id);
 static void pgoutput_stream_start(struct LogicalDecodingContext *ctx,
@@ -142,6 +146,7 @@ _PG_output_plugin_init(OutputPluginCallbacks *cb)
 	cb->begin_cb = pgoutput_begin_txn;
 	cb->change_cb = pgoutput_change;
 	cb->truncate_cb = pgoutput_truncate;
+	cb->message_cb = pgoutput_message;
 	cb->commit_cb = pgoutput_commit_txn;
 	cb->filter_by_origin_cb = pgoutput_origin_filter;
 	cb->shutdown_cb = pgoutput_shutdown;
@@ -152,6 +157,7 @@ _PG_output_plugin_init(OutputPluginCallbacks *cb)
 	cb->stream_abort_cb = pgoutput_stream_abort;
 	cb->stream_commit_cb = pgoutput_stream_commit;
 	cb->stream_change_cb = pgoutput_change;
+	cb->stream_message_cb = pgoutput_message;
 	cb->stream_truncate_cb = pgoutput_truncate;
 }
 
@@ -162,10 +168,12 @@ parse_output_parameters(List *options, PGOutputData *data)
 	bool		protocol_version_given = false;
 	bool		publication_names_given = false;
 	bool		binary_option_given = false;
+	bool		messages_option_given = false;
 	bool		streaming_given = false;
 
 	data->binary = false;
 	data->streaming = false;
+	data->messages = false;
 
 	foreach(lc, options)
 	{
@@ -221,6 +229,16 @@ parse_output_parameters(List *options, PGOutputData *data)
 
 			data->binary = defGetBoolean(defel);
 		}
+		else if (strcmp(defel->defname, "messages") == 0)
+		{
+			if (messages_option_given)
+				ereport(ERROR,
+						(errcode(ERRCODE_SYNTAX_ERROR),
+						 errmsg("conflicting or redundant options")));
+			messages_option_given = true;
+
+			data->messages = defGetBoolean(defel);
+		}
 		else if (strcmp(defel->defname, "streaming") == 0)
 		{
 			if (streaming_given)
@@ -689,6 +707,35 @@ pgoutput_truncate(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	MemoryContextReset(data->context);
 }
 
+static void
+pgoutput_message(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
+				 XLogRecPtr message_lsn, bool transactional, const char *prefix, Size sz,
+				 const char *message)
+{
+	PGOutputData *data = (PGOutputData *) ctx->output_plugin_private;
+	TransactionId xid = InvalidTransactionId;
+
+	if (!data->messages)
+		return;
+
+	/*
+	 * Remember the xid for the message in streaming mode. See
+	 * pgoutput_change.
+	 */
+	if (in_streaming)
+		xid = txn->xid;
+
+	OutputPluginPrepareWrite(ctx, true);
+	logicalrep_write_message(ctx->out,
+							 xid,
+							 message_lsn,
+							 transactional,
+							 prefix,
+							 sz,
+							 message);
+	OutputPluginWrite(ctx, true);
+}
+
 /*
  * Currently we always forward.
  */

src/include/replication/logicalproto.h

@@ -54,6 +54,7 @@ typedef enum LogicalRepMsgType
 	LOGICAL_REP_MSG_TRUNCATE = 'T',
 	LOGICAL_REP_MSG_RELATION = 'R',
 	LOGICAL_REP_MSG_TYPE = 'Y',
+	LOGICAL_REP_MSG_MESSAGE = 'M',
 	LOGICAL_REP_MSG_STREAM_START = 'S',
 	LOGICAL_REP_MSG_STREAM_END = 'E',
 	LOGICAL_REP_MSG_STREAM_COMMIT = 'c',
@@ -151,6 +152,8 @@ extern void logicalrep_write_truncate(StringInfo out, TransactionId xid,
 									  bool cascade, bool restart_seqs);
 extern List *logicalrep_read_truncate(StringInfo in,
 									  bool *cascade, bool *restart_seqs);
+extern void logicalrep_write_message(StringInfo out, TransactionId xid, XLogRecPtr lsn,
+									 bool transactional, const char *prefix, Size sz, const char *message);
 extern void logicalrep_write_rel(StringInfo out, TransactionId xid,
 								 Relation rel);
 extern LogicalRepRelation *logicalrep_read_rel(StringInfo in);

src/include/replication/pgoutput.h

@@ -26,6 +26,7 @@ typedef struct PGOutputData
 	List	   *publications;
 	bool		binary;
 	bool		streaming;
+	bool		messages;
 } PGOutputData;
 
 #endif							/* PGOUTPUT_H */

src/test/subscription/t/020_messages.pl

+# Tests that logical decoding messages
+use strict;
+use warnings;
+use PostgresNode;
+use TestLib;
+use Test::More tests => 5;
+
+# Create publisher node
+my $node_publisher = get_new_node('publisher');
+$node_publisher->init(allows_streaming => 'logical');
+$node_publisher->start;
+
+# Create subscriber node
+my $node_subscriber = get_new_node('subscriber');
+$node_subscriber->init(allows_streaming => 'logical');
+$node_subscriber->start;
+
+# Create some preexisting content on publisher
+$node_publisher->safe_psql('postgres',
+	"CREATE TABLE tab_test (a int primary key)");
+
+# Setup structure on subscriber
+$node_subscriber->safe_psql('postgres',
+	"CREATE TABLE tab_test (a int primary key)");
+
+# Setup logical replication
+my $publisher_connstr = $node_publisher->connstr . ' dbname=postgres';
+$node_publisher->safe_psql('postgres', "CREATE PUBLICATION tap_pub FOR TABLE tab_test");
+
+$node_subscriber->safe_psql('postgres',
+	"CREATE SUBSCRIPTION tap_sub CONNECTION '$publisher_connstr' PUBLICATION tap_pub"
+);
+
+# Ensure a transactional logical decoding message shows up on the slot
+$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
+
+$node_publisher->safe_psql('postgres',
+	"SELECT pg_logical_emit_message(true, 'pgoutput', 'a transactional message')"
+);
+
+my $result = $node_publisher->safe_psql(
+	'postgres', qq(
+		SELECT get_byte(data, 0)
+		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+			'proto_version', '1',
+			'publication_names', 'tap_pub',
+			'messages', 'true')
+));
+
+# 66 77 67 == B M C == BEGIN MESSAGE COMMIT
+is($result, qq(66
+77
+67),
+	'messages on slot are B M C with message option');
+
+$result = $node_publisher->safe_psql(
+	'postgres', qq(
+		SELECT get_byte(data, 1), encode(substr(data, 11, 8), 'escape')
+		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+			'proto_version', '1',
+			'publication_names', 'tap_pub',
+			'messages', 'true')
+		OFFSET 1 LIMIT 1
+));
+
+is($result, qq(1|pgoutput),
+	"flag transactional is set to 1 and prefix is pgoutput");
+
+$result = $node_publisher->safe_psql(
+	'postgres', qq(
+		SELECT get_byte(data, 0)
+		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+			'proto_version', '1',
+			'publication_names', 'tap_pub')
+));
+
+# 66 67 == B C == BEGIN COMMIT
+is($result, qq(66
+67),
+	'option messages defaults to false so message (M) is not available on slot');
+
+$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub ENABLE");
+$node_publisher->wait_for_catchup('tap_sub');
+
+# ensure a non-transactional logical decoding message shows up on the slot
+$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
+
+$node_publisher->safe_psql('postgres', "INSERT INTO tab_test VALUES (1)");
+
+my $message_lsn = $node_publisher->safe_psql('postgres',
+	"SELECT pg_logical_emit_message(false, 'pgoutput', 'a non-transactional message')");
+
+$node_publisher->safe_psql('postgres', "INSERT INTO tab_test VALUES (2)");
+
+$result = $node_publisher->safe_psql(
+	'postgres', qq(
+		SELECT get_byte(data, 0), get_byte(data, 1)
+		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+			'proto_version', '1',
+			'publication_names', 'tap_pub',
+			'messages', 'true')
+		WHERE lsn = '$message_lsn' AND xid = 0
+));
+
+is($result, qq(77|0), 'non-transactional message on slot is M');
+
+$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub ENABLE");
+$node_publisher->wait_for_catchup('tap_sub');
+
+$node_subscriber->safe_psql('postgres', "ALTER SUBSCRIPTION tap_sub DISABLE");
+
+# wait for the replication connection to drop from the publisher
+$node_publisher->poll_query_until('postgres',
+	'SELECT COUNT(*) FROM pg_catalog.pg_stat_replication', 0);
+
+# Ensure a non-transactional logical decoding message shows up on the slot when
+# it is emitted within an aborted transaction. The message won't emit until
+# something advances the LSN, hence, we intentionally forces the server to
+# switch to a new WAL file.
+$node_publisher->safe_psql(
+	'postgres', qq(
+		BEGIN;
+		SELECT pg_logical_emit_message(false, 'pgoutput',
+			'a non-transactional message is available even if the transaction is aborted 1');
+		INSERT INTO tab_test VALUES (3);
+		SELECT pg_logical_emit_message(true, 'pgoutput',
+			'a transactional message is not available if the transaction is aborted');
+		SELECT pg_logical_emit_message(false, 'pgoutput',
+			'a non-transactional message is available even if the transaction is aborted 2');
+		ROLLBACK;
+		SELECT pg_switch_wal();
+));
+
+$result = $node_publisher->safe_psql(
+	'postgres', qq(
+		SELECT get_byte(data, 0), get_byte(data, 1)
+		FROM pg_logical_slot_peek_binary_changes('tap_sub', NULL, NULL,
+			'proto_version', '1',
+			'publication_names', 'tap_pub',
+			'messages', 'true')
+));
+
+is($result, qq(77|0
+77|0),
+	'non-transactional message on slot from aborted transaction is M');
+
+$node_subscriber->stop('fast');
+$node_publisher->stop('fast');