Re: Prepared transaction releasing locks before deregistering its GID

Hi,

Michael Paquier <michael@paquier.xyz> wrote:

> On Wed, Feb 20, 2019 at 11:50:42AM +0100, Oleksii Kliukin wrote:
>> RecoverPreparedTransactions calls ProcessRecords with
>> twophase_recover_callbacks right after releasing the TwoPhaseStateLock, so I
>> think lock_held should be false here (matching the similar call of
>> TwoPhaseGetDummyProc at lock_twophase_recover).
>
> Indeed.  That would be a bad idea.  We don't actually stress this
> routine in 009_twophase.pl or the assertion I added would have blown
> up immediately.  So I propose to close the gap, and the updated patch
> attached adds dedicated tests causing the problem you spotted to be
> triggered (soft and hard restarts with 2PC transactions including
> DDLs).  The previous version of the patch and those tests cause the
> assertion to blow up, failing the tests.

Thank you for updating the patch and sorry for the delay, it looks good to
me, the tests pass on my system.

>> Since the patch touches TwoPhaseGetDummyBackendId, let’s fix the comment
>> header to mention it instead of TwoPhaseGetDummyProc
>
> Not sure I understand the issue you are pointing out here.  The patch
> adds an extra argument to both TwoPhaseGetDummyProc() and
> TwoPhaseGetDummyBackendId(), and the headers of both functions
> document the new argument.

Just this:

@@ -844,17 +851,18 @@ TwoPhaseGetGXact(TransactionId xid)
 }

 /*
- * TwoPhaseGetDummyProc
+ * TwoPhaseGetDummyBackendId
  *      Get the dummy backend ID for prepared transaction specified by XID

>
> One extra trick I have been using for testing this patch is the
> following:
> --- a/src/backend/access/transam/twophase.c
> +++ b/src/backend/access/transam/twophase.c
> @@ -816,13 +816,6 @@ TwoPhaseGetGXact(TransactionId xid, bool lock_held)
>
>    Assert(!lock_held || LWLockHeldByMe(TwoPhaseStateLock));
>
> -   /*
> -    * During a recovery, COMMIT PREPARED, or ABORT PREPARED, we'll be called
> -    * repeatedly for the same XID.  We can save work with a simple cache.
> -    */
> -   if (xid == cached_xid)
> -       return cached_gxact;
>
> This has the advantage to check more aggressively for lock conflicts,
> causing the tests to deadlock if the flag is not correctly set in the
> worst case scenario.

Nice, thank you. I ran all my tests with it and found no further issues.

Regards,
Oleksii Kliukin