Re: pg_terminate_backend() issues

Tom Lane wrote:
> I did a quick grep for PG_CATCH uses to see what we have along the lines
> of this bug:
> http://archives.postgresql.org/pgsql-hackers/2007-04/msg00218.php
> 
> In current sources there are three places at risk:
> 
> btbulkdelete, as noted in the above message
> pg_start_backup    needs to reset forcePageWrites = false
> createdb wants to UnlockSharedObject and delete any already-copied files
> 
> In createdb, the Unlock actually doesn't need to get cleaned up, since
> transaction abort would release the lock anyway.  But leaving a possibly
> large mess of orphaned files doesn't seem nice.
> 
> ISTM that there will be more cases like this in future, so we need a
> general solution anyway.  I propose the following sort of code structure
> for these situations:
> 
>     on_shmem_exit(cleanup_routine, arg);
>     PG_TRY();
>     {
>         ... do something ...
>         cancel_shmem_exit(cleanup_routine, arg);
>     }
>     PG_CATCH();
>     {
>         cancel_shmem_exit(cleanup_routine, arg);
>         cleanup_routine(arg);
>         PG_RE_THROW();
>     }
>     PG_END_TRY();
> 
> where cancel_shmem_exit is defined to pop the latest shmem_exit_list
> entry if it matches the passed arguments (I don't see any particular
> need to search further than that in the list).  This structure
> guarantees that cleanup_routine will be called on the way out of
> either a plain ERROR or a FATAL exit.
> 
> Thoughts?  We clearly must do something about this before we can even
> think of calling retail SIGTERM a supported feature ...

Here is a little different idea.  Seems we want something more like:
PG_ON_EXIT();{        cleanup_routine(arg);}
>     PG_TRY();
>     {
>         ... do something ...
>     }
>     PG_CATCH();
>     {
>         PG_RE_THROW();
>     }
>     PG_END_TRY();

PG_ON_EXIT() would place cleanup_routine() into the shmem_exit_list and
it would be called at the end of PG_CATCH and removed from
shmem_exit_list by PG_END_TRY().  Another idea would be to define the
PG_CATCH() first so it would be put in the shmem_exit_list before the
TRY was tried, but I assume you don't want all those functions to run on
exit.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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