libpq: Add PQsendPipelineSync()

This new function is equivalent to PQpipelineSync(), except that it does
not flush anything to the server except if the size threshold of the
output buffer is reached; the user must subsequently call PQflush()
instead.

Its purpose is to reduce the system call overhead of pipeline mode, by
giving to applications more control over the timing of the flushes when
manipulating commands in pipeline mode.

Author: Anton Kirilov
Reviewed-by: Jelte Fennema-Nio, Robert Haas, Álvaro Herrera, Denis
Laxalde, Michael Paquier
Discussion: https://postgr.es/m/CACV6eE5arHFZEA717=iKEa_OewpVFfWJOmsOdGrqqsr8CJVfWQ@mail.gmail.com

src/interfaces/libpq/exports.txt

@@ -192,3 +192,4 @@ PQclosePortal             189
 PQsendClosePrepared       190
 PQsendClosePortal         191
 PQchangePassword          192
+PQsendPipelineSync        193

src/interfaces/libpq/fe-exec.c

@@ -81,6 +81,7 @@ static int	PQsendTypedCommand(PGconn *conn, char command, char type,
 							   const char *target);
 static int	check_field_number(const PGresult *res, int field_num);
 static void pqPipelineProcessQueue(PGconn *conn);
+static int	pqPipelineSyncInternal(PGconn *conn, bool immediate_flush);
 static int	pqPipelineFlush(PGconn *conn);
 
 
@@ -3224,25 +3225,48 @@ pqPipelineProcessQueue(PGconn *conn)
 /*
  * PQpipelineSync
  *		Send a Sync message as part of a pipeline, and flush to server
+ */
+int
+PQpipelineSync(PGconn *conn)
+{
+	return pqPipelineSyncInternal(conn, true);
+}
+
+/*
+ * PQsendPipelineSync
+ *		Send a Sync message as part of a pipeline, without flushing to server
+ */
+int
+PQsendPipelineSync(PGconn *conn)
+{
+	return pqPipelineSyncInternal(conn, false);
+}
+
+/*
+ * Workhorse function for PQpipelineSync and PQsendPipelineSync.
  *
  * It's legal to start submitting more commands in the pipeline immediately,
  * without waiting for the results of the current pipeline. There's no need to
  * end pipeline mode and start it again.
  *
- * If a command in a pipeline fails, every subsequent command up to and including
- * the result to the Sync message sent by PQpipelineSync gets set to
- * PGRES_PIPELINE_ABORTED state. If the whole pipeline is processed without
- * error, a PGresult with PGRES_PIPELINE_SYNC is produced.
+ * If a command in a pipeline fails, every subsequent command up to and
+ * including the result to the Sync message sent by pqPipelineSyncInternal
+ * gets set to PGRES_PIPELINE_ABORTED state. If the whole pipeline is
+ * processed without error, a PGresult with PGRES_PIPELINE_SYNC is produced.
  *
- * Queries can already have been sent before PQpipelineSync is called, but
- * PQpipelineSync needs to be called before retrieving command results.
+ * Queries can already have been sent before pqPipelineSyncInternal is called,
+ * but pqPipelineSyncInternal needs to be called before retrieving command
+ * results.
  *
  * The connection will remain in pipeline mode and unavailable for new
  * synchronous command execution functions until all results from the pipeline
  * are processed by the client.
+ *
+ * immediate_flush controls if the flush happens immediately after sending the
+ * Sync message or not.
  */
-int
-PQpipelineSync(PGconn *conn)
+static int
+pqPipelineSyncInternal(PGconn *conn, bool immediate_flush)
 {
 	PGcmdQueueEntry *entry;
 
@@ -3288,9 +3312,19 @@ PQpipelineSync(PGconn *conn)
 	/*
 	 * Give the data a push.  In nonblock mode, don't complain if we're unable
 	 * to send it all; PQgetResult() will do any additional flushing needed.
+	 * If immediate_flush is disabled, the data is pushed if we are past the
+	 * size threshold.
 	 */
-	if (PQflush(conn) < 0)
-		goto sendFailed;
+	if (immediate_flush)
+	{
+		if (pqFlush(conn) < 0)
+			goto sendFailed;
+	}
+	else
+	{
+		if (pqPipelineFlush(conn) < 0)
+			goto sendFailed;
+	}
 
 	/* OK, it's launched! */
 	pqAppendCmdQueueEntry(conn, entry);

src/interfaces/libpq/libpq-fe.h

@@ -474,6 +474,7 @@ extern int	PQenterPipelineMode(PGconn *conn);
 extern int	PQexitPipelineMode(PGconn *conn);
 extern int	PQpipelineSync(PGconn *conn);
 extern int	PQsendFlushRequest(PGconn *conn);
+extern int	PQsendPipelineSync(PGconn *conn);
 
 /* LISTEN/NOTIFY support */
 extern PGnotify *PQnotifies(PGconn *conn);