Re: 9.6, background worker processes, and PL/Java - Mailing list pgsql-hackers

On Wed, Oct 26, 2016 at 4:26 AM, Chapman Flack <chap@anastigmatix.net> wrote:
> Hi,
>
> I have a report of a PL/Java crash in 9.6 where the stack trace
> suggests it was trying to initialize in a background worker
> process (not sure why that even happened, yet), and by my first
> glance, it seems to have crashed dereferencing MyProcPort, which
> I am guessing a BGW might not always have (?).
>
> So, as I try to get up to speed on this PostgreSQL feature, it
> seems to me that I have up to three different cases that I may
> need to make PL/Java detect and respond appropriately to. (If
> you see me veering into any misconceptions, please let me know.)
>
> 1. A worker explicitly created with Register... or RegisterDynamic...
>    that has not called ...InitializeConnection... and so isn't
>    any particular user or connected to any database.
>
> 2. A worker explicitly created that has called ...Initialize...
>    and therefore is connected to some database as some user.
>    (So, is there a MyProcPort in this case?)
>
> 3. A worker implicitly created for a parallel query plan (and therefore
>    associated with a database and a user). Does this have a MyProcPort?
>

No, parallel workers in parallel query doesn't have MyProcPort.

>
> Case 1, I think I at most need to detect and ereport. It is hard to
> imagine how it could even arise, as without a database connection
> there's no pg_extension, pg_language, or pg_proc, but I suppose it
> could happen if someone misguidedly puts libpljava in
> shared_preload_libraries, or some other bgw code inexplicably loads
> it. It's a non-useful case as PL/Java has nothing to do without
> a database connection and sqlj schema.
>
> Case 2 might be worth supporting, but I may need to account for
> anything that differs in this environment from a normal connected
> backend.
>
> Case 3 seems most likely. It should only be possible by invoking
> a declared Java function that somebody marked parallel-safe, right?
> In the parallel-unsafe or -restricted cases, PL/Java can only find
> itself invoked within the leader process?
>
> Such a leader process can only be a normal backend? Or perhaps also
> a case-2 explicitly created BGW that is executing a query?
>
> My main question is, what state do I need to examine at startup
> in order to distinguish these cases? Do I detect I'm in a BGW by
> a non-null MyBgworkerEntry? If it's there, do I detect whether
> I have a database and an identity by checking for a MyProcPort,
> or some other way?
>
> As for declaring functions parallel-unsafe, -restricted, or -safe,
> I assume there should be no problems with PL/Java functions with
> the default designation of unsafe. There should be no essential
> problem if someone declares a function -restricted - provided PL/Java
> itself can be audited to make sure it doesn't do any of the things
> restricted functions can't do - as it will only be running in the
> leader process anyway.
>
> Even should somebody mark a PL/Java function safe, while hard to
> imagine a good case for, shouldn't really break anything; as the
> workers are separate processes, this should be safe. Any imagined
> speed advantage of the parallel query is likely to evaporate while
> the several processes load their own JVMs, but nothing should
> outright break.
>
> That leads me to:
>
> Are BGWs for parallel queries born fresh for each query, or do they
> get pooled and reused?
>

born fresh for each query.



-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com