Re: Fix orphaned 2pc file which may casue instance restart failed

On Sun, Sep 08, 2024 at 01:01:37PM +0800, 清浅 wrote:
> Hi all,  I found that there is a race condition
> between two global transaction, which may cause instance restart
> failed with error 'could not access status of transaction
> xxx","Could not open file ""pg_xact/xxx"": No such file or
> directory'.
> 
> 
>   The scenery to reproduce the problem is:
>     1. gxact1 is doing `FinishPreparedTransaction` and checkpoint
>       is issued, so gxact1 will generate a 2pc file.
>     2. then gxact1 was removed from `TwoPhaseState->prepXacts` and
>       its state memory was returned to freelist.
>     3. but just before gxact1 remove its 2pc file, gxact2 is issued,
>       gxact2 will reuse the same state memory of gxact1 and will
>       reset `gxact->ondisk` to false.
>     4. gxact1 continue and found that `gxact->ondisk` is false, it won't
>       remove its 2pc file. This file is orphaned.
> 
>   If gxact1's local xid is not frozen, the startup process will remove
> the orphaned 2pc file. However, if the xid's corresponding clog file is
> truncated by `vacuum`, the startup process will raise error 'could not
> access status of transaction xxx', due to it could not found the
> transaction's status file in dir `pg_xact`.

Hmm.  I've not seen that in the field.  Let me check that..
--
Michael