Avoid memory leakage when a series of subtransactions invoke AFTER triggers

that are fired at end-of-statement (as is the normal case for foreign keys,
for example).  In this situation the per-subxact deferred trigger context
is always empty when subtransaction exit is reached; so we could free it,
but were not doing so, leading to an intratransaction leak of 8K or more
per subtransaction.  Per off-list example from Viatcheslav Kalinin
subsequent to bug #3418 (his original bug report omitted a foreign key
constraint needed to cause this leak).

Back-patch to 8.2; prior versions were not using per-subxact contexts
for deferred triggers, so did not have this leak.

diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 21570fb8859eace8404bc7654498bd9a72df339c..2ccc75f8ff299a7909e8b8945738d63072cc4870 100644 (file)
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -2766,6 +2766,24 @@ AfterTriggerEndSubXact(bool isCommit)
                afterTriggers->state_stack[my_level] = NULL;
                Assert(afterTriggers->query_depth ==
                           afterTriggers->depth_stack[my_level]);
+               /*
+                * It's entirely possible that the subxact created an event_cxt but
+                * there is not anything left in it (because all the triggers were
+                * fired at end-of-statement).  If so, we should release the context
+                * to prevent memory leakage in a long sequence of subtransactions.
+                * We can detect whether there's anything of use in the context by
+                * seeing if anything was added to the global events list since
+                * subxact start.  (This test doesn't catch every case where the
+                * context is deletable; for instance maybe the only additions were
+                * from a sub-sub-xact.  But it handles the common case.)
+                */
+               if (afterTriggers->cxt_stack[my_level] &&
+                       afterTriggers->events.tail == afterTriggers->events_stack[my_level].tail)
+               {
+                       MemoryContextDelete(afterTriggers->cxt_stack[my_level]);
+                       /* avoid double delete if abort later */
+                       afterTriggers->cxt_stack[my_level] = NULL;
+               }
        }
        else
        {