In successful pg_recvlogical, end PGRES_COPY_OUT cleanly.

pg_recvlogical merely called PQfinish(), so the backend sent messages
after the disconnect.  When that caused EPIPE in internal_flush(),
before a LogicalConfirmReceivedLocation(), the next pg_recvlogical would
repeat already-acknowledged records.  Whether or not the defect causes
EPIPE, post-disconnect messages could contain an ErrorResponse that the
user should see.  One properly ends PGRES_COPY_OUT by repeating
PQgetCopyData() until it returns a negative value.  Augment one of the
tests to cover the case of WAL past --endpos.  Back-patch to v10, where
commit 7c030783 first appeared.  Before
that commit, pg_recvlogical never reached PGRES_COPY_OUT.

Reported by Thomas Munro.

Discussion: https://postgr.es/m/CAEepm=1MzM2Z_xNe4foGwZ1a+MO_2S9oYDq3M5D11=JDU_+0Nw@mail.gmail.com

src/bin/pg_basebackup/pg_recvlogical.c

@@ -580,14 +580,40 @@ StreamLogicalLog(void)
 	res = PQgetResult(conn);
 	if (PQresultStatus(res) == PGRES_COPY_OUT)
 	{
+		PQclear(res);
+
 		/*
 		 * We're doing a client-initiated clean exit and have sent CopyDone to
-		 * the server. We've already sent replay confirmation and fsync'd so
-		 * we can just clean up the connection now.
+		 * the server. Drain any messages, so we don't miss a last-minute
+		 * ErrorResponse. The walsender stops generating XLogData records once
+		 * it sees CopyDone, so expect this to finish quickly. After CopyDone,
+		 * it's too late for sendFeedback(), even if this were to take a long
+		 * time. Hence, use synchronous-mode PQgetCopyData().
 		 */
+		while (1)
+		{
+			int			r;
+
+			if (copybuf != NULL)
+			{
+				PQfreemem(copybuf);
+				copybuf = NULL;
+			}
+			r = PQgetCopyData(conn, &copybuf, 0);
+			if (r == -1)
+				break;
+			if (r == -2)
+			{
+				pg_log_error("could not read COPY data: %s",
+							 PQerrorMessage(conn));
+				time_to_abort = false;	/* unclean exit */
 				goto error;
 			}
-	else if (PQresultStatus(res) != PGRES_COMMAND_OK)
+		}
+
+		res = PQgetResult(conn);
+	}
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
 	{
 		pg_log_error("unexpected termination of replication stream: %s",
 					 PQresultErrorMessage(res));

src/test/recovery/t/006_logical_decoding.pl

@@ -71,6 +71,11 @@ my $endpos = $node_master->safe_psql('postgres',
 );
 print "waiting to replay $endpos\n";
 
+# Insert some rows after $endpos, which we won't read.
+$node_master->safe_psql('postgres',
+	qq[INSERT INTO decoding_test(x,y) SELECT s, s::text FROM generate_series(5,50) s;]
+);
+
 my $stdout_recv = $node_master->pg_recvlogical_upto(
 	'postgres', 'test_slot', $endpos, 180,
 	'include-xids'     => '0',
@@ -89,7 +94,7 @@ $stdout_recv = $node_master->pg_recvlogical_upto(
 	'skip-empty-xacts' => '1');
 chomp($stdout_recv);
 is($stdout_recv, '',
-	'pg_recvlogical acknowledged changes, nothing pending on slot');
+	'pg_recvlogical acknowledged changes');
 
 $node_master->safe_psql('postgres', 'CREATE DATABASE otherdb');