Re: BUG #17158: Distinct ROW fails with Postgres 14

Peter Eisentraut <peter.eisentraut@enterprisedb.com> writes:
> On 02.09.21 10:15, Peter Eisentraut wrote:
>> On 31.08.21 22:43, Tom Lane wrote:
>>> I find variant 1 a bit cleaner, and safer.  I'd rather default to
>>> assuming that RECORD doesn't hash, when we don't have enough info
>>> to be sure.

>> Ok, here is a more polished patch for that.

> committed

I apologize for not having found the time to review this before
it went in ... but what you did in hash_array is pretty awful:

        /*
         * The type cache doesn't believe that record is hashable (see
         * cache_record_field_properties()), but since we're here, we're
         * committed to hashing, so we can assume it does.  Worst case, if any
         * components of the record don't support hashing, we will fail at
         * execution.
         */
        if (element_type == RECORDOID)
        {
            MemoryContext oldcontext;
            TypeCacheEntry *record_typentry;

            oldcontext = MemoryContextSwitchTo(CacheMemoryContext);

            /*
             * Make fake type cache entry structure.  Note that we can't just
             * modify typentry, since that points directly into the type cache.
             */
            record_typentry = palloc(sizeof(*record_typentry));

            /* fill in what we need below */
            record_typentry->typlen = typentry->typlen;
            record_typentry->typbyval = typentry->typbyval;
            record_typentry->typalign = typentry->typalign;
            fmgr_info(F_HASH_RECORD, &record_typentry->hash_proc_finfo);

            MemoryContextSwitchTo(oldcontext);

            typentry = record_typentry;
        }

        fcinfo->flinfo->fn_extra = (void *) typentry;
    }

The reason skink has been falling over since this went in is that
this kluge didn't bother to fill record_typentry->type_id, which
results in the next call seeing an undefined value in

    if (typentry == NULL ||
        typentry->type_id != element_type)

which most likely will cause it to allocate another dummy typcache
entry; lather, rinse, repeat for each call.  But even with that
fixed, I do not think this is even a little bit acceptable, because
it will permanently leak a TypeCacheEntry plus subsidiary FmgrInfo data
for each query that uses hash_array.

Perhaps it'd work to put the phony entry into fcinfo->flinfo->fn_mcxt
instead of CacheMemoryContext.

BTW, skink's failure can be reproduced pretty quickly by running the
attached under valgrind.

            regards, tom lane


create temp table graph0( f int, t int, label text );

insert into graph0 values
    (1, 2, 'arc 1 -> 2'),
    (1, 3, 'arc 1 -> 3'),
    (2, 3, 'arc 2 -> 3'),
    (1, 4, 'arc 1 -> 4'),
    (4, 5, 'arc 4 -> 5');

with recursive search_graph(f, t, label) as (
    select * from graph0 g
    union distinct
    select g.*
    from graph0 g, search_graph sg
    where g.f = sg.t
) search depth first by f, t set seq
select * from search_graph order by seq;