pg_locks display of speculative locks is bogus

Hi,

I noticed this when tightening up some races for [1] I noticed that the
way speculative locks are displayed in pg_locks is completely bogus. As
pg_locks has no branch specific to speculative locks, the object etc
path is used:
            case LOCKTAG_OBJECT:
            case LOCKTAG_USERLOCK:
            case LOCKTAG_ADVISORY:
            default:            /* treat unknown locktags like OBJECT */
                values[1] = ObjectIdGetDatum(instance->locktag.locktag_field1);
                values[7] = ObjectIdGetDatum(instance->locktag.locktag_field2);
                values[8] = ObjectIdGetDatum(instance->locktag.locktag_field3);
                values[9] = Int16GetDatum(instance->locktag.locktag_field4);
                nulls[2] = true;
                nulls[3] = true;
                nulls[4] = true;
                nulls[5] = true;
                nulls[6] = true;
                break;

but speculative locks are defined like:

/*
 * ID info for a speculative insert is TRANSACTION info +
 * its speculative insert counter.
 */
#define SET_LOCKTAG_SPECULATIVE_INSERTION(locktag,xid,token) \
    ((locktag).locktag_field1 = (xid), \
     (locktag).locktag_field2 = (token),        \
     (locktag).locktag_field3 = 0, \
     (locktag).locktag_field4 = 0, \
     (locktag).locktag_type = LOCKTAG_SPECULATIVE_TOKEN, \
     (locktag).locktag_lockmethodid = DEFAULT_LOCKMETHOD)

which means that currently a speculative lock's xid is displayed as the
database, the token as the classid, and that objid and objsubid are 0
instead of NULL.

Doesn't seem great.

It's trivial to put the xid in the correct place, but it's less obvious
what to do with the token? For master we should probably add a column,
but what about the back branches? Ignore it? Put it in classid or such?

Greetings,

Andres Freund

[1] https://www.postgresql.org/message-id/CAAKRu_ZRmxy_OEryfY3G8Zp01ouhgw59_-_Cm8n7LzRH5BAvng%40mail.gmail.com