Add PQsendFlushRequest to libpq

This new libpq function allows the application to send an 'H' message,
which instructs the server to flush its outgoing buffer.

This hasn't been needed so far because the Sync message already requests
a buffer; and I failed to realize that this was needed in pipeline mode
because PQpipelineSync also causes the buffer to be flushed.  However,
sometimes it is useful to request a flush without establishing a
synchronization point.

Backpatch to 14, where pipeline mode was introduced in libpq.

Reported-by: Boris Kolpackov <boris@codesynthesis.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/202106252350.t76x73nt643j@alvherre.pgsql

src/interfaces/libpq/exports.txt

@@ -184,4 +184,5 @@ PQexitPipelineMode        181
 PQpipelineSync            182
 PQpipelineStatus          183
 PQsetTraceFlags           184
-PQmblenBounded              185
+PQmblenBounded            185
+PQsendFlushRequest        186

src/interfaces/libpq/fe-exec.c

@@ -3099,6 +3099,42 @@ sendFailed:
 	return 0;
 }
 
+/*
+ * PQsendFlushRequest
+ *		Send request for server to flush its buffer.  Useful in pipeline
+ *		mode when a sync point is not desired.
+ */
+int
+PQsendFlushRequest(PGconn *conn)
+{
+	if (!conn)
+		return 0;
+
+	/* Don't try to send if we know there's no live connection. */
+	if (conn->status != CONNECTION_OK)
+	{
+		appendPQExpBufferStr(&conn->errorMessage,
+							 libpq_gettext("no connection to the server\n"));
+		return 0;
+	}
+
+	/* Can't send while already busy, either, unless enqueuing for later */
+	if (conn->asyncStatus != PGASYNC_IDLE &&
+		conn->pipelineStatus == PQ_PIPELINE_OFF)
+	{
+		appendPQExpBufferStr(&conn->errorMessage,
+							 libpq_gettext("another command is already in progress\n"));
+		return false;
+	}
+
+	if (pqPutMsgStart('H', conn) < 0 ||
+		pqPutMsgEnd(conn) < 0)
+	{
+		return 0;
+	}
+
+	return 1;
+}
 
 /* ====== accessor funcs for PGresult ======== */
 

src/interfaces/libpq/libpq-fe.h

@@ -470,6 +470,7 @@ extern int	PQconsumeInput(PGconn *conn);
 extern int	PQenterPipelineMode(PGconn *conn);
 extern int	PQexitPipelineMode(PGconn *conn);
 extern int	PQpipelineSync(PGconn *conn);
+extern int	PQsendFlushRequest(PGconn *conn);
 
 /* LISTEN/NOTIFY support */
 extern PGnotify *PQnotifies(PGconn *conn);