Add new parallel message type to progress reporting.

This commit adds a new type of parallel message 'P' to allow a
parallel worker to poke at a leader to update the progress.

Currently it supports only incremental progress reporting but it's
possible to allow for supporting of other backend progress APIs in the
future.

There are no users of this new message type as of this commit. That
will follow in future commits.

Idea from Andres Freund.

Author: Sami Imseih
Reviewed by: Michael Paquier, Masahiko Sawada
Discussion: https://www.postgresql.org/message-id/flat/5478DFCD-2333-401A-B2F0-0D186AB09228@amazon.com

src/backend/access/transam/parallel.c

@@ -24,6 +24,7 @@
 #include "catalog/pg_enum.h"
 #include "catalog/storage.h"
 #include "commands/async.h"
+#include "commands/progress.h"
 #include "commands/vacuum.h"
 #include "executor/execParallel.h"
 #include "libpq/libpq.h"
@@ -1199,6 +1200,23 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 				break;
 			}
 
+		case 'P':				/* Parallel progress reporting */
+			{
+				/*
+				 * Only incremental progress reporting is currently supported.
+				 * However, it's possible to add more fields to the message to
+				 * allow for handling of other backend progress APIs.
+				 */
+				int			index = pq_getmsgint(msg, 4);
+				int64		incr = pq_getmsgint64(msg);
+
+				pq_getmsgend(msg);
+
+				pgstat_progress_incr_param(index, incr);
+
+				break;
+			}
+
 		case 'X':				/* Terminate, indicating clean exit */
 			{
 				shm_mq_detach(pcxt->worker[i].error_mqh);

src/backend/utils/activity/backend_progress.c

@@ -10,6 +10,8 @@
  */
 #include "postgres.h"
 
+#include "access/parallel.h"
+#include "libpq/pqformat.h"
 #include "port/atomics.h"		/* for memory barriers */
 #include "utils/backend_progress.h"
 #include "utils/backend_status.h"
@@ -79,6 +81,36 @@ pgstat_progress_incr_param(int index, int64 incr)
 	PGSTAT_END_WRITE_ACTIVITY(beentry);
 }
 
+/*-----------
+ * pgstat_progress_parallel_incr_param() -
+ *
+ * A variant of pgstat_progress_incr_param to allow a worker to poke at
+ * a leader to do an incremental progress update.
+ *-----------
+ */
+void
+pgstat_progress_parallel_incr_param(int index, int64 incr)
+{
+	/*
+	 * Parallel workers notify a leader through a 'P' protocol message to
+	 * update progress, passing the progress index and incremented value.
+	 * Leaders can just call pgstat_progress_incr_param directly.
+	 */
+	if (IsParallelWorker())
+	{
+		static StringInfoData progress_message;
+
+		initStringInfo(&progress_message);
+
+		pq_beginmessage(&progress_message, 'P');
+		pq_sendint32(&progress_message, index);
+		pq_sendint64(&progress_message, incr);
+		pq_endmessage(&progress_message);
+	}
+	else
+		pgstat_progress_incr_param(index, incr);
+}
+
 /*-----------
  * pgstat_progress_update_multi_param() -
  *

src/include/utils/backend_progress.h

@@ -37,6 +37,7 @@ extern void pgstat_progress_start_command(ProgressCommandType cmdtype,
 										  Oid relid);
 extern void pgstat_progress_update_param(int index, int64 val);
 extern void pgstat_progress_incr_param(int index, int64 incr);
+extern void pgstat_progress_parallel_incr_param(int index, int64 incr);
 extern void pgstat_progress_update_multi_param(int nparam, const int *index,
 											   const int64 *val);
 extern void pgstat_progress_end_command(void);