Implement pipeline mode in libpq

Pipeline mode in libpq lets an application avoid the Sync messages in
the FE/BE protocol that are implicit in the old libpq API after each
query.  The application can then insert Sync at its leisure with a new
libpq function PQpipelineSync.  This can lead to substantial reductions
in query latency.

Co-authored-by: Craig Ringer <craig.ringer@enterprisedb.com>
Co-authored-by: Matthieu Garrigues <matthieu.garrigues@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Aya Iwata <iwata.aya@jp.fujitsu.com>
Reviewed-by: Daniel Vérité <daniel@manitou-mail.org>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Kirk Jamison <k.jamison@fujitsu.com>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: Nikhil Sontakke <nikhils@2ndquadrant.com>
Reviewed-by: Vaishnavi Prabakaran <VaishnaviP@fast.au.fujitsu.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>

Discussion: https://postgr.es/m/CAMsr+YFUjJytRyV4J-16bEoiZyH=4nj+sQ7JP9ajwz=B4dMMZw@mail.gmail.com
Discussion: https://postgr.es/m/CAJkzx4T5E-2cQe3dtv2R78dYFvz+in8PY7A8MArvLhs_pg75gg@mail.gmail.com

Author: alvherre

doc/src/sgml/libpq.sgml

@@ -130,6 +130,10 @@
     <application>libpq</application> library.
    </para>
 
+   <para>
+    Client applications cannot use these functions while a libpq connection is in pipeline mode.
+   </para>
+
    <sect2 id="lo-create">
     <title>Creating a Large Object</title>
 

doc/src/sgml/lobj.sgml

@@ -1019,6 +1019,12 @@ libpqrcv_exec(WalReceiverConn *conn, const char *query,
 			walres->err = _("empty query");
 			break;
 
+		case PGRES_PIPELINE_SYNC:
+		case PGRES_PIPELINE_ABORTED:
+			walres->status = WALRCV_ERROR;
+			walres->err = _("unexpected pipeline mode");
+			break;
+
 		case PGRES_NONFATAL_ERROR:
 		case PGRES_FATAL_ERROR:
 		case PGRES_BAD_RESPONSE:

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -929,6 +929,8 @@ should_processing_continue(PGresult *res)
 		case PGRES_COPY_IN:
 		case PGRES_COPY_BOTH:
 		case PGRES_SINGLE_TUPLE:
+		case PGRES_PIPELINE_SYNC:
+		case PGRES_PIPELINE_ABORTED:
 			return false;
 	}
 	return true;

src/bin/pg_amcheck/pg_amcheck.c

@@ -179,3 +179,7 @@ PQgetgssctx               176
 PQsetSSLKeyPassHook_OpenSSL         177
 PQgetSSLKeyPassHook_OpenSSL         178
 PQdefaultSSLKeyPassHook_OpenSSL     179
+PQenterPipelineMode       180
+PQexitPipelineMode        181
+PQpipelineSync            182
+PQpipelineStatus          183

src/interfaces/libpq/exports.txt

@@ -522,6 +522,23 @@ pqDropConnection(PGconn *conn, bool flushInput)
 	}
 }
 
+/*
+ * pqFreeCommandQueue
+ * Free all the entries of PGcmdQueueEntry queue passed.
+ */
+static void
+pqFreeCommandQueue(PGcmdQueueEntry *queue)
+{
+	while (queue != NULL)
+	{
+		PGcmdQueueEntry *cur = queue;
+
+		queue = cur->next;
+		if (cur->query)
+			free(cur->query);
+		free(cur);
+	}
+}
 
 /*
  *		pqDropServerData
@@ -553,6 +570,12 @@ pqDropServerData(PGconn *conn)
 	}
 	conn->notifyHead = conn->notifyTail = NULL;
 
+	pqFreeCommandQueue(conn->cmd_queue_head);
+	conn->cmd_queue_head = conn->cmd_queue_tail = NULL;
+
+	pqFreeCommandQueue(conn->cmd_queue_recycle);
+	conn->cmd_queue_recycle = NULL;
+
 	/* Reset ParameterStatus data, as well as variables deduced from it */
 	pstatus = conn->pstatus;
 	while (pstatus != NULL)
@@ -2459,6 +2482,7 @@ PQconnectPoll(PGconn *conn)
 		/* Drop any PGresult we might have, too */
 		conn->asyncStatus = PGASYNC_IDLE;
 		conn->xactStatus = PQTRANS_IDLE;
+		conn->pipelineStatus = PQ_PIPELINE_OFF;
 		pqClearAsyncResult(conn);
 
 		/* Reset conn->status to put the state machine in the right state */
@@ -3917,6 +3941,7 @@ makeEmptyPGconn(void)
 
 	conn->status = CONNECTION_BAD;
 	conn->asyncStatus = PGASYNC_IDLE;
+	conn->pipelineStatus = PQ_PIPELINE_OFF;
 	conn->xactStatus = PQTRANS_IDLE;
 	conn->options_valid = false;
 	conn->nonblocking = false;
@@ -4084,8 +4109,6 @@ freePGconn(PGconn *conn)
 	if (conn->connip)
 		free(conn->connip);
 	/* Note that conn->Pfdebug is not ours to close or free */
-	if (conn->last_query)
-		free(conn->last_query);
 	if (conn->write_err_msg)
 		free(conn->write_err_msg);
 	if (conn->inBuffer)
@@ -4174,6 +4197,7 @@ closePGconn(PGconn *conn)
 	conn->status = CONNECTION_BAD;	/* Well, not really _bad_ - just absent */
 	conn->asyncStatus = PGASYNC_IDLE;
 	conn->xactStatus = PQTRANS_IDLE;
+	conn->pipelineStatus = PQ_PIPELINE_OFF;
 	pqClearAsyncResult(conn);	/* deallocate result */
 	resetPQExpBuffer(&conn->errorMessage);
 	release_conn_addrinfo(conn);
@@ -6726,6 +6750,15 @@ PQbackendPID(const PGconn *conn)
 	return conn->be_pid;
 }
 
+PGpipelineStatus
+PQpipelineStatus(const PGconn *conn)
+{
+	if (!conn)
+		return PQ_PIPELINE_OFF;
+
+	return conn->pipelineStatus;
+}
+
 int
 PQconnectionNeedsPassword(const PGconn *conn)
 {