Re: Potential ABI breakage in upcoming minor releases

On Thu, Nov 14, 2024 at 09:33:32PM +0100, Alvaro Herrera wrote:
> On 2024-Nov-14, Tom Lane wrote:
> > Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> > > But timescale did crash:
> >
> > So what is timescale doing differently?

> src/ts_catalog/catalog.c-extern TSDLLEXPORT ResultRelInfo *
> src/ts_catalog/catalog.c-ts_catalog_open_indexes(Relation heapRel)
> src/ts_catalog/catalog.c-{
> src/ts_catalog/catalog.c- ResultRelInfo *resultRelInfo;
> src/ts_catalog/catalog.c-
> src/ts_catalog/catalog.c: resultRelInfo = makeNode(ResultRelInfo);
> src/ts_catalog/catalog.c- resultRelInfo->ri_RangeTableIndex = 0; /* dummy */
> src/ts_catalog/catalog.c- resultRelInfo->ri_RelationDesc = heapRel;
> src/ts_catalog/catalog.c- resultRelInfo->ri_TrigDesc = NULL; /* we don't fire triggers */
> src/ts_catalog/catalog.c-
> src/ts_catalog/catalog.c- ExecOpenIndices(resultRelInfo, false);

This is a copy of PostgreSQL's CatalogOpenIndexes(). Assuming timescaledb
uses it like PostgreSQL uses CatalogOpenIndexes(), it's fine. Specifically,
CatalogOpenIndexes() is fine since PostgreSQL does not pass the ResultRelInfo
to ModifyTable.

> Oh, hmm, there's this bit which uses struct assignment into a stack
> element:

Yes, stack allocation of a ResultRelInfo sounds relevant to the crash. It
qualifies as a "very unusual code pattern" in the postgr.es/c/e54a42a sense.

I'm hearing the only confirmed impact on non-assert builds is the need to
recompile timescaledb. (It's unknown whether recompiling will suffice for
timescaledb. For assert builds, six PGXN extensions need recompilation.) I
don't see us issuing another set of back branch releases for the purpose of
making a v16.4-built timescaledb avoid a rebuild. The target audience would
be someone who can get a new PostgreSQL build but can't get a new timescaledb
build. That feels like a small audience. What's missing in that analysis?

Going forward, for struct field additions, I plan to make my own patches more
ABI-breakage-averse than the postgr.es/c/e54a42a standard.