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

doc/src/sgml/libpq.sgml

@@ -5102,10 +5102,13 @@ int PQflush(PGconn *conn);
      The server executes statements, and returns results, in the order the
      client sends them.  The server will begin executing the commands in the
      pipeline immediately, not waiting for the end of the pipeline.
+     Note that results are buffered on the server side; the server flushes
+     that buffer when a synchronization point is established with
+     <function>PQpipelineSync</function>, or when
+     <function>PQsendFlushRequest</function> is called.
      If any statement encounters an error, the server aborts the current
      transaction and does not execute any subsequent command in the queue
-     until the next synchronization point established by
-     <function>PQpipelineSync</function>;
+     until the next synchronization point;
      a <literal>PGRES_PIPELINE_ABORTED</literal> result is produced for
      each such command.
      (This remains true even if the commands in the pipeline would rollback
@@ -5399,6 +5402,32 @@ int PQpipelineSync(PGconn *conn);
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry id="libpq-PQsendFlushRequest">
+     <term><function>PQsendFlushRequest</function><indexterm><primary>PQsendFlushRequest</primary></indexterm></term>
+
+      <listitem>
+       <para>
+        Sends a request for the server to flush its output buffer.
+<synopsis>
+int PQsendFlushRequest(PGconn *conn);
+</synopsis>
+       </para>
+
+       <para>
+        Returns 1 for success.  Returns 0 on any failure.
+       </para>
+       <para>
+        The server flushes its output buffer automatically as a result of
+        <function>PQpipelineSync</function> being called, or
+        on any request when not in pipeline mode; this function is useful
+        to cause the server to flush its output buffer in pipeline mode
+        without establishing a synchronization point.
+        Note that the request is not itself flushed to the server automatically;
+        use <function>PQflush</function> if necessary.
+       </para>
+      </listitem>
+     </varlistentry>
    </variablelist>
   </sect2>
 

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);