Clear padding in PgStat_HashKey keys

Hi hackers,

While working on a rebase for [0], I noticed some weird behavior on the stats.

The issue is that [0], in conjonction with b14e9ce7d5, does introduce padding in
the PgStat_HashKey:

(gdb) ptype /o struct PgStat_HashKey
/* offset | size */ type = struct PgStat_HashKey {
/* 0 | 4 */ uint32 kind;
/* 4 | 4 */ Oid dboid;
/* 8 | 8 */ uint64 objid;
/* 16 | 4 */ RelFileNumber relfile;
/* XXX 4-byte padding */

/* total size (bytes): 24 */
}

But, the keys are initialized that way:

"
PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid,.relfile = refile};
"

which could lead to random data in the padding bytes.

We are using sizeof(PgStat_HashKey) in pgstat_cmp_hash_key() and we compute the
hash hash key in pgstat_hash_hash_key() using the PgStat_HashKey struct size as
input: this lead to unexpected results if the keys contain random data in the
padding bytes.

Even if currently there is no issues, as without [0] there is no padding:

(gdb) ptype /o struct PgStat_HashKey
/* offset | size */ type = struct PgStat_HashKey {
/* 0 | 4 */ uint32 kind;
/* 4 | 4 */ Oid dboid;
/* 8 | 8 */ uint64 objid;

/* total size (bytes): 16 */
}

I think that we should ensure to $SUBJECT (to prevent unexpected results should
padding be introduced in the future).

For example we currently ensure the same for LOCALLOCKTAG localtag in LockHeldByMe()
while there is no padding:

gdb) ptype /o struct LOCALLOCKTAG
/* offset | size */ type = struct LOCALLOCKTAG {
/* 0 | 16 */ LOCKTAG lock;
/* 16 | 4 */ LOCKMODE mode;

/* total size (bytes): 20 */
}

So, please find attached a patch to $SUBJECT.

[0]: https://www.postgresql.org/message-id/flat/ZlGYokUIlERemvpB%40ip-10-97-1-34.eu-west-3.compute.internal

Looking forward to your feedback,

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com