How to expose session vs txn lock info in pg_locks view?

Presently there doesn't seem to be a way to tell whether a lock is
session-level or transaction-level in the pg_locks view.

I was expecting this to be a quick patch, but the comment on the definition
of PROCLOCKTAG in lock.h notes that shmem state for heavyweight locks does
not track whether the lock is session-level or txn-level. That explains why
it's not already exposed in pg_locks.

AFAICS it'd be necessary to expand PROCLOG to expose this in shmem.
Probably by adding a small bitfield where bit 0 is set if there's a txn
level lock and bit 1 is set if there's a session level lock. But I'm not
convinced that expanding PROCLOCK is justifiable for this. sizeof(PROCLOCK)
is 64 on a typical x64 machine. Adding anything to it increases it to 72
bytes.

(gdb) ptype /o struct PROCLOCK
/* offset | size */ type = struct PROCLOCK {
/* 0 | 16 */ PROCLOCKTAG tag;
/* 16 | 8 */ PGPROC *groupLeader;
/* 24 | 4 */ LOCKMASK holdMask;
/* 28 | 4 */ LOCKMASK releaseMask;
/* 32 | 16 */ SHM_QUEUE lockLink;
/* 48 | 16 */ SHM_QUEUE procLink;
/* 64 | 1 */ unsigned char locktypes;
/* XXX 7-byte padding */

/* total size (bytes): 72 */
}

Going over 64 sets off possible alarm bells about cache line sizing to me,
but maybe it's not that critical? It'd also require (8 * max_locks_per_xact
* (MaxBackends+max_prepared_xacts)) extra shmem space; that could land up
being 128k on a default setup and a couple of megabytes on a big system.
Not huge, but not insignificant if it's hot data.

It's frustrating to be unable to tell the difference between session-level
and txn-level locks in diagnostic output. And the deadlock detector has no
way to tell the difference when selecting a victim for a deadlock abort -
it'd probably make sense to prefer to send a deadlock abort for txn-only
lockers. But I'm not sure I see a sensible way to add the info - PROCLOCK
is already free of any padding, and I wouldn't want to use hacks like
pointer-tagging.

Thoughts anyone?