Re: DSM segment handle generation in background workers

On Tue, Oct 9, 2018 at 1:53 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Thomas Munro <thomas.munro@enterprisedb.com> writes:
> > On Mon, Oct 8, 2018 at 1:17 AM Thomas Munro
> > <thomas.munro@enterprisedb.com> wrote:
> >> That's because the bgworker startup path doesn't contain a call to
> >> srandom(...distinguishing stuff...), unlike BackendRun().  I suppose
> >> do_start_bgworker() could gain a similar call... or perhaps that call
> >> should be moved into InitPostmasterChild().  If we put it in there
> >> right after MyStartTime is assigned a new value, we could use the same
> >> incantation that PostmasterMain() uses.
>
> > Maybe something like this?
>
> I think the bit with
>
> #ifndef HAVE_STRONG_RANDOM
>         random_seed = 0;
>         random_start_time.tv_usec = 0;
> #endif
>
> should also be done in every postmaster child, no?  If we want to hide the
> postmaster's state from child processes, we ought to hide it from all.

Ok, here is a sketch patch with a new function InitRandomSeeds() to do
that.  It is called from PostmasterMain(), InitPostmasterChild() and
InitStandaloneProcess() for consistency.

It seems a bit strange to me that internal infrastructure shares a
random number generator with SQL-callable functions random() and
setseed(), though I'm not saying it's harmful.

While wondering if something like this should be back-patched, I
noticed that SQL random() is declared as parallel-restricted, which is
good: it means we aren't exposing a random() function that generates
the same values in every parallel worker.  So I think this is probably
just a minor nuisance and should probably only be pushed to master, or
at most to 11 (since Parallel Hash likes to create DSM segments in
workers), unless someone can think of a more serious way this can hurt
you.

(Tangentially:  I suppose it might be useful to have a different SQL
random number function that is parallel safe, that isn't associated
with a user-controllable seed, and whose seed is different in each
backend.)

-- 
Thomas Munro
http://www.enterprisedb.com