From 0d21f919eb86cd3baa267844d111c6a5af480696 Mon Sep 17 00:00:00 2001
From: Alvaro Herrera
Date: Wed, 16 Oct 2019 14:51:34 +0200
Subject: Fix crash when reporting CREATE INDEX progress

A race condition can make us try to dereference a NULL pointer to the
PGPROC struct of a process that's already finished.  That results in
crashes during REINDEX CONCURRENTLY and CREATE INDEX CONCURRENTLY.

This was introduced in ab0dfc961b6a, so backpatch to pg12.

Reported by: Justin Pryzby
Reviewed-by: Michaël Paquier
Discussion: https://postgr.es/m/20191012004446.GT10470@telsasoft.com
---
 src/backend/commands/indexcmds.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

(limited to 'src/backend/commands')

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 70f9b6729a7..589b8816a4d 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -384,12 +384,14 @@ WaitForOlderSnapshots(TransactionId limitXmin, bool progress)
 
 		if (VirtualTransactionIdIsValid(old_snapshots[i]))
 		{
+			/* If requested, publish who we're going to wait for. */
 			if (progress)
 			{
 				PGPROC	   *holder = BackendIdGetProc(old_snapshots[i].backendId);
 
-				pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
-											 holder->pid);
+				if (holder)
+					pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
+												 holder->pid);
 			}
 			VirtualXactLock(old_snapshots[i], true);
 		}
-- 
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