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

Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)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