From 6599c9ac3340b6cd3d86a0a7f866b80a009fecab Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Tue, 11 Apr 2017 13:03:44 -0400
Subject: [PATCH] Add an Assert() to max_parallel_workers enforcement.

To prevent future bugs along the lines of the one corrected by commit
8ff518699f19dd0a5076f5090bac8400b8233f7f, or find any that remain
in the current code, add an Assert() that the difference between
parallel_register_count and parallel_terminate_count is in a sane
range.

Kuntal Ghosh, with considerable tidying-up by me, per a suggestion
from Neha Khatri.  Reviewed by Tomas Vondra.

Discussion: http://postgr.es/m/CAFO0U+-E8yzchwVnvn5BeRDPgX2z9vZUxQ8dxx9c0XFGBC7N1Q@mail.gmail.com
---
 src/backend/postmaster/bgworker.c           | 3 +++
 src/backend/utils/misc/guc.c                | 6 +++---
 src/include/postmaster/bgworker_internals.h | 7 +++++++
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/backend/postmaster/bgworker.c b/src/backend/postmaster/bgworker.c
index 6b2385c4711..9ad3e915db3 100644
--- a/src/backend/postmaster/bgworker.c
+++ b/src/backend/postmaster/bgworker.c
@@ -971,6 +971,9 @@ RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
 					 BackgroundWorkerData->parallel_terminate_count) >=
 		max_parallel_workers)
 	{
+		Assert(BackgroundWorkerData->parallel_register_count -
+			   BackgroundWorkerData->parallel_terminate_count <=
+			   MAX_PARALLEL_WORKER_LIMIT);
 		LWLockRelease(BackgroundWorkerLock);
 		return false;
 	}
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 6e39a676094..9ad8361a9bd 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -57,7 +57,7 @@
 #include "parser/scansup.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
-#include "postmaster/bgworker.h"
+#include "postmaster/bgworker_internals.h"
 #include "postmaster/bgwriter.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
@@ -2713,7 +2713,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_workers_per_gather,
-		2, 0, 1024,
+		2, 0, MAX_PARALLEL_WORKER_LIMIT,
 		NULL, NULL, NULL
 	},
 
@@ -2723,7 +2723,7 @@ static struct config_int ConfigureNamesInt[] =
 			NULL
 		},
 		&max_parallel_workers,
-		8, 0, 1024,
+		8, 0, MAX_PARALLEL_WORKER_LIMIT,
 		NULL, NULL, NULL
 	},
 
diff --git a/src/include/postmaster/bgworker_internals.h b/src/include/postmaster/bgworker_internals.h
index 9a2de4f4d04..9e0b0621cf4 100644
--- a/src/include/postmaster/bgworker_internals.h
+++ b/src/include/postmaster/bgworker_internals.h
@@ -16,6 +16,13 @@
 #include "lib/ilist.h"
 #include "postmaster/bgworker.h"
 
+/* GUC options */
+
+/*
+ * Maximum possible value of parallel workers.
+ */
+#define MAX_PARALLEL_WORKER_LIMIT 1024
+
 /*
  * List of background workers, private to postmaster.
  *