Allow interrupting GetMultiXactIdMembers

This function has a loop which can lead to uninterruptible process
"stalls" (actually infinite loops) when some bugs are triggered.  Avoid
that unpleasant situation by adding a check for interrupts in a place
that shouldn't degrade performance in the normal case.

Backpatch to 9.3.  Older branches have an identical loop here, but the
aforementioned bugs are only a problem starting in 9.3 so there doesn't
seem to be any point in backpatching any further.

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index 9f259bb54ebb22ec6d941a4105d3c7fceb855aee..ddb61f1ffb284dbb52759bec5cb26fb045d85eaf 100644 (file)
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1258,6 +1258,7 @@ retry:
        {
            /* Corner case 2: next multixact is still being filled in */
            LWLockRelease(MultiXactOffsetControlLock);
+           CHECK_FOR_INTERRUPTS();
            pg_usleep(1000L);
            goto retry;
        }