Re: Is RecoveryConflictInterrupt() entirely safe in a signal handler? - Mailing list pgsql-hackers

On Sat, Dec 31, 2022 at 10:06:53AM +1300, Thomas Munro wrote:
> On Thu, Dec 29, 2022 at 9:40 PM Noah Misch <noah@leadboat.com> wrote:
> > On Tue, Jul 26, 2022 at 07:22:52PM -0400, Tom Lane wrote:
> > > Thomas Munro <thomas.munro@gmail.com> writes:
> > > > ... The regular expression machinery is capable of
> > > > consuming a lot of CPU, and does CANCEL_REQUESTED(nfa->v->re)
> > > > frequently to avoid getting stuck.  With the patch as it stands, that
> > > > would never be true.
> > >
> > > Surely that can't be too hard to fix.  We might have to refactor
> > > the code around QueryCancelPending a little bit so that callers
> > > can ask "do we need a query cancel now?" without actually triggering
> > > a longjmp ... but why would that be problematic?
> >
> > It could work.  The problems are like those of making code safe to run in a
> > signal handler.  You can use e.g. snprintf in rcancelrequested(), but you
> > still can't use palloc() or ereport().  I see at least these strategies:
> >
> > 1. Accept that recovery conflict checks run after a regex call completes.
> > 2. Have rcancelrequested() return true unconditionally if we need a conflict
> >    check.  If there's no actual conflict, restart the regex.
> > 3. Have rcancelrequested() run the conflict check, including elog-using
> >    PostgreSQL code.  On longjmp(), accept the leak of regex mallocs.
> > 4. Have rcancelrequested() run the conflict check, including elog-using
> >    PostgreSQL code.  On longjmp(), escalate to FATAL.
> > 5. Write the conflict check code to dutifully avoid longjmp().
> > 6. Convert src/backend/regex to use palloc, so longjmp() is fine.
> 
> Thanks!  I appreciate the help getting unstuck here.  I'd thought
> about some of these but not all.  I also considered a couple more:
> 
> 7.  Do a CFI() in a try/catch if INTERRUPTS_PENDING_CONDITION() is
> true, and copy the error somewhere to be re-thrown later after the
> regexp code exits with REG_CANCEL.
> 8.  Do a CFI() in a try/catch if INTERRUPTS_PENDING_CONDITION() is
> true, and call a new regexp function that will free everything before
> re-throwing.
> 
> After Tom's response I spent some time trying to figure out how to
> make a SOFT_CHECK_FOR_INTERRUPTS(), which would return a value to
> indicate that it would like to throw.  I think it would need to re-arm
> various flags and introduce a programming rule for all interrupt
> processing routines that if they fired once under a soft check they
> must fire again later under a non-soft check.  That all seems a bit
> complicated, and a general mechanism like that seemed like overkill
> for a single user, which led me to idea #7.
> 
> Idea #8 is a realisation that twisting oneself into a pretzel to avoid
> having to change the regexp code or its REG_CANCEL control flow may be
> a bit silly.  If the only thing it really needs to do is free some
> memory, maybe the regexp module should provide a function that frees
> everything that is safe to call from our rcancelrequested callback, so
> we can do so before we longjmp back to Kansas.  Then the REG_CANCEL
> code paths would be effectively unreachable in PostgreSQL.  I don't
> know if it's better or worse than your idea #6, "use palloc instead,
> it already has garbage collection, duh", but it's a different take on
> the same realisation that this is just about free().

PG_TRY() isn't free, so it's nice that (6) doesn't add one.  If (6) fails in
some not-yet-revealed way, (8) could get more relevant.

> I guess idea #6 must be pretty easy to try: just point that MALLOC()
> macro to palloc(), and do a plain old CFI() in rcancelrequested().
> Why do you suggest #3 as an interim measure?

No strong reason.  I think I suggested it because it's a strict subset of (6),
but I didn't think through in detail.  (I've never modified src/backend/regex
and have barely read its code, for whatever that's worth.)

> Do we fear that palloc() might hurt regexp performance?

Nah.  I don't recall any place in PostgreSQL where performance is an argument
for raw malloc() calls.

> > Incidentally, the affected test
> > contains comment "# DROP TABLE containing block which standby has in a pinned
> > buffer".  The standby holds no pin at that moment; the LOCK TABLE pins system
> > catalog pages, but it drops every pin it acquires.
> 
> Oh, I guess the comment is just wrong?  There are earlier sections
> concerned with buffer pins, but the section "RECOVERY CONFLICT 3" is
> about locks.

Yes.