Re: error context for vacuum to include block number

On Fri, Mar 27, 2020 at 09:49:29AM +0530, Amit Kapila wrote:
> On Fri, Mar 27, 2020 at 3:47 AM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> >
> > > Hm, I was just wondering what happens if an error happens *during*
> > > update_vacuum_error_cbarg(). It seems like if we set
> > > errcbarg->phase=VACUUM_INDEX before setting errcbarg->indname=indname, then an
> > > error would cause a crash.
> > >
>
> Can't that be avoided if you check if cbarg->indname is non-null in
> vacuum_error_callback as we are already doing for
> VACUUM_ERRCB_PHASE_TRUNCATE?
>
> > > And if we pfree and set indname before phase, it'd
> > > be a problem when going from an index phase to non-index phase.
>
> How is it possible that we move to the non-index phase without
> clearing indname as we always revert back the old phase information?

The crash scenario I'm trying to avoid would be like statement_timeout or other
asynchronous event occurring between two non-atomic operations.

I said that there's an issue no matter what order we set indname/phase;
If we wrote:
|cbarg->indname = indname;
|cbarg->phase = phase;
..and hit a timeout (or similar) between setting indname=NULL but before
setting phase=VACUUM_INDEX, then we can crash due to null pointer.

But if we write:
|cbarg->phase = phase;
|if (cbarg->indname) {pfree(cbarg->indname);}
|cbarg->indname = indname ? pstrdup(indname) : NULL;
..then we can still crash if we timeout between freeing cbarg->indname and
setting it to null, due to acccessing a pfreed allocation.

> > > So maybe we
> > > have to set errcbarg->phase=VACUUM_ERRCB_PHASE_UNKNOWN while in the function,
> > > and errcbarg->phase=phase last.
>
> I find that a bit ad-hoc, if possible, let's try to avoid it.

I think we can do what you suggesting, if the callback checks if (cbarg->indname!=NULL).

We'd have to write:
// Must set indname *before* updating phase, in case an error occurs before
// phase is set, to avoid crashing if we're going from an index phase to a
// non-index phase (which should not read indname). Must not free indname
// until it's set to null.
char *tmp = cbarg->indname;
cbarg->indname = indname ? pstrdup(indname) : NULL;
cbarg->phase = phase;
if (tmp){pfree(tmp);}

Do you think that's better ?

--
Justin