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[email protected]

diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 70f9b6729a7d0165927eab706352b1bf57e76d73..589b8816a4d6293f5a4bd403fa91f304a018350e 100644 (file)
--- 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);
                }

diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 4682438114655ae3104ad4ad96943a227bf5b870..1dd0e5e957687d644c71a1a187e73d876691df4f 100644 (file)
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -900,16 +900,14 @@ WaitForLockersMultiple(List *locktags, LOCKMODE lockmode, bool progress)
 
                while (VirtualTransactionIdIsValid(*lockholders))
                {
-                       /*
-                        * If requested, publish who we're going to wait for.  This is not
-                        * 100% accurate if they're already gone, but we don't care.
-                        */
+                       /* If requested, publish who we're going to wait for. */
                        if (progress)
                        {
                                PGPROC     *holder = BackendIdGetProc(lockholders->backendId);
 
-                               pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
-                                                                                        holder->pid);
+                               if (holder)
+                                       pgstat_progress_update_param(PROGRESS_WAITFOR_CURRENT_PID,
+                                                                                                holder->pid);
                        }
                        VirtualXactLock(*lockholders, true);
                        lockholders++;