libpq: Reset singlerow flag correctly in pipeline mode

When a query whose results were requested in single-row mode is the last
in the queue by the time those results are being read, the single-row
flag was not being reset, because we were returning early from
pqPipelineProcessQueue.  Move that stanza up so that the flag is always
reset at the end of sending that query's results.

Add a test for the situation.

Backpatch to 14.

Author: Denis Laxalde <denis.laxalde@dalibo.com>
Discussion: https://postgr.es/m/01af18c5-dacc-a8c8-07ee-aecc7650c3e8@dalibo.com

src/interfaces/libpq/fe-exec.c

@@ -3088,6 +3088,12 @@ pqPipelineProcessQueue(PGconn *conn)
 			break;
 	}
 
+	/*
+	 * Reset single-row processing mode.  (Client has to set it up for each
+	 * query, if desired.)
+	 */
+	conn->singleRowMode = false;
+
 	/*
 	 * If there are no further commands to process in the queue, get us in
 	 * "real idle" mode now.
@@ -3101,12 +3107,6 @@ pqPipelineProcessQueue(PGconn *conn)
 	/* Initialize async result-accumulation state */
 	pqClearAsyncResult(conn);
 
-	/*
-	 * Reset single-row processing mode.  (Client has to set it up for each
-	 * query, if desired.)
-	 */
-	conn->singleRowMode = false;
-
 	if (conn->pipelineStatus == PQ_PIPELINE_ABORTED &&
 		conn->cmd_queue_head->queryclass != PGQUERY_SYNC)
 	{

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -1143,11 +1143,11 @@ test_singlerowmode(PGconn *conn)
 	int			i;
 	bool		pipeline_ended = false;
 
-	/* 1 pipeline, 3 queries in it */
 	if (PQenterPipelineMode(conn) != 1)
 		pg_fatal("failed to enter pipeline mode: %s",
 				 PQerrorMessage(conn));
 
+	/* One series of three commands, using single-row mode for the first two. */
 	for (i = 0; i < 3; i++)
 	{
 		char	   *param[1];
@@ -1239,6 +1239,49 @@ test_singlerowmode(PGconn *conn)
 			pg_fatal("didn't get expected terminating TUPLES_OK");
 	}
 
+	/*
+	 * Now issue one command, get its results in with single-row mode, then
+	 * issue another command, and get its results in normal mode; make sure
+	 * the single-row mode flag is reset as expected.
+	 */
+	if (PQsendQueryParams(conn, "SELECT generate_series(0, 0)",
+						  0, NULL, NULL, NULL, NULL, 0) != 1)
+		pg_fatal("failed to send query: %s",
+				 PQerrorMessage(conn));
+	if (PQsendFlushRequest(conn) != 1)
+		pg_fatal("failed to send flush request");
+	if (PQsetSingleRowMode(conn) != 1)
+		pg_fatal("PQsetSingleRowMode() failed");
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_SINGLE_TUPLE)
+		pg_fatal("Expected PGRES_SINGLE_TUPLE, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		pg_fatal("Expected PGRES_TUPLES_OK, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	if (PQgetResult(conn) != NULL)
+		pg_fatal("expected NULL result");
+
+	if (PQsendQueryParams(conn, "SELECT 1",
+						  0, NULL, NULL, NULL, NULL, 0) != 1)
+		pg_fatal("failed to send query: %s",
+				 PQerrorMessage(conn));
+	if (PQsendFlushRequest(conn) != 1)
+		pg_fatal("failed to send flush request");
+	res = PQgetResult(conn);
+	if (res == NULL)
+		pg_fatal("unexpected NULL");
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+		pg_fatal("Expected PGRES_TUPLES_OK, got %s",
+				 PQresStatus(PQresultStatus(res)));
+	if (PQgetResult(conn) != NULL)
+		pg_fatal("expected NULL result");
+
 	if (PQexitPipelineMode(conn) != 1)
 		pg_fatal("failed to end pipeline mode: %s", PQerrorMessage(conn));
 

src/test/modules/libpq_pipeline/traces/singlerow.trace

@@ -36,4 +36,24 @@ B	12	DataRow	 1 2 '45'
 B	12	DataRow	 1 2 '46'
 B	13	CommandComplete	 "SELECT 5"
 B	5	ReadyForQuery	 I
+F	36	Parse	 "" "SELECT generate_series(0, 0)" 0
+F	14	Bind	 "" "" 0 0 1 0
+F	6	Describe	 P ""
+F	9	Execute	 "" 0
+F	4	Flush
+B	4	ParseComplete
+B	4	BindComplete
+B	40	RowDescription	 1 "generate_series" NNNN 0 NNNN 4 -1 0
+B	11	DataRow	 1 1 '0'
+B	13	CommandComplete	 "SELECT 1"
+F	16	Parse	 "" "SELECT 1" 0
+F	14	Bind	 "" "" 0 0 1 0
+F	6	Describe	 P ""
+F	9	Execute	 "" 0
+F	4	Flush
+B	4	ParseComplete
+B	4	BindComplete
+B	33	RowDescription	 1 "?column?" NNNN 0 NNNN 4 -1 0
+B	11	DataRow	 1 1 '1'
+B	13	CommandComplete	 "SELECT 1"
 F	4	Terminate