hi,
> YAMAMOTO Takashi <y...@mwd.biglobe.ne.jp> wrote:
>
>> it seems that PredicateLockTupleRowVersionLink sometimes create
>> a loop of targets (it founds an existing 'newtarget' whose
>> nextVersionOfRow chain points to the 'oldtarget') and it later
>> causes CheckTargetForConflictsIn loop forever.
>
> Is this a hypothetical risk based on looking at the code, or have
> you seen this actually happen? Either way, could you provide more
> details? (A reproducible test case would be ideal.)
i have seen this actually happen. i've confirmed the creation of the loop
with the attached patch. it's easily reproducable with my application.
i can provide the full source code of my application if you want.
(but it isn't easy to run unless you are familiar with the recent
version of NetBSD)
i haven't found a smaller reproducible test case yet.
YAMAMOTO Takashi
>
> This being the newest part of the code, I'll grant that it is the
> most likely to have an unidentified bug; but given that the pointers
> are from one predicate lock target structure identified by a tuple
> ID to one identified by the tuple ID of the next version of the row,
> it isn't obvious to me how a cycle could develop.
>
> -Kevin
diff --git a/src/backend/storage/lmgr/predicate.c
b/src/backend/storage/lmgr/predicate.c
index 722d0f8..3e1a3e2 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -2350,7 +2350,25 @@ PredicateLockTupleRowVersionLink(const Relation relation,
newtarget->nextVersionOfRow = NULL;
}
else
+ {
Assert(newtarget->priorVersionOfRow == NULL);
+#if 0
+ Assert(newtarget->nextVersionOfRow == NULL);
+#endif
+ if (newtarget->nextVersionOfRow != NULL) {
+ PREDICATELOCKTARGET *t;
+
+ elog(WARNING, "new %p has next %p\n",
+ newtarget, newtarget->nextVersionOfRow);
+ for (t = newtarget->nextVersionOfRow; t != NULL;
+ t = t->nextVersionOfRow) {
+ if (oldtarget != t) {
+ elog(WARNING, "creating a loop
new=%p old=%p\n",
+ newtarget, oldtarget);
+ }
+ }
+ }
+ }
newtarget->priorVersionOfRow = oldtarget;
oldtarget->nextVersionOfRow = newtarget;
--
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