Add 'Portal Close' message to pipelined PQsendQuery()

Commit acb7e4eb added a new implementation for PQsendQuery so that
it works in pipeline mode (by using extended query protocol), but it
behaves differently from the 'Q' message (in simple query protocol) used
by regular implementation: the new one doesn't close the unnamed portal.
Change the new code to have identical behavior to the old.
Reported-by: Yura Sokolov <y.sokolov@postgrespro.ru>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/202106072107.d4i55hdscxqj@alvherre.pgsql

src/interfaces/libpq/fe-exec.c

@@ -1329,7 +1329,8 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 	{
 		/*
 		 * In pipeline mode we cannot use the simple protocol, so we send
-		 * Parse, Bind, Describe Portal, Execute.
+		 * Parse, Bind, Describe Portal, Execute, Close Portal (with the
+		 * unnamed portal).
 		 */
 		if (pqPutMsgStart('P', conn) < 0 ||
 			pqPuts("", conn) < 0 ||
@@ -1355,6 +1356,11 @@ PQsendQueryInternal(PGconn *conn, const char *query, bool newQuery)
 			pqPutInt(0, 4, conn) < 0 ||
 			pqPutMsgEnd(conn) < 0)
 			goto sendFailed;
+		if (pqPutMsgStart('C', conn) < 0 ||
+			pqPutc('P', conn) < 0 ||
+			pqPuts("", conn) < 0 ||
+			pqPutMsgEnd(conn) < 0)
+			goto sendFailed;
 
 		entry->queryclass = PGQUERY_EXTENDED;
 		entry->query = strdup(query);

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

@@ -38,6 +38,7 @@ F	26	Parse	 "" "SELECT 1; SELECT 2" 0
 F	12	Bind	 "" "" 0 0 0
 F	6	Describe	 P ""
 F	9	Execute	 "" 0
+F	6	Close	 P ""
 F	4	Sync
 B	NN	ErrorResponse	 S "ERROR" V "ERROR" C "42601" M "cannot insert multiple commands into a prepared statement" F "SSSS" L "SSSS" R "SSSS" \x00
 B	5	ReadyForQuery	 I
@@ -45,6 +46,7 @@ F	54	Parse	 "" "SELECT 1.0/g FROM generate_series(3, -1, -1) g" 0
 F	12	Bind	 "" "" 0 0 0
 F	6	Describe	 P ""
 F	9	Execute	 "" 0
+F	6	Close	 P ""
 F	4	Sync
 B	4	ParseComplete
 B	4	BindComplete