Larry Douzie <cs4482003@yahoo.com> writes:
>> A row lock is represented by storing the locking transaction's ID in
>> xmax and setting the HEAP_MARKED_FOR_UPDATE infomask bit. The bit is
>> needed to distinguish this from the case where the transaction is
>> deleting the tuple.
> Is there a similar bit modified if the row in question is being waited
> upon by some transaction?
No. We handle that case by waiting for the transaction that's locked
the row to commit or abort. (Waiting for a transaction is done by
having every transaction take out exclusive lock on its xact ID when it
starts; then would-be waiters try to take share lock on the xact ID,
causing them to block till the exclusive lock is released.)
In general, the Postgres lock management code is not designed for
transparency :-( ... it does the jobs it's supposed to do, but it's not
always easy to inspect the visible state to find out what's happening.
Perhaps someday someone will get motivated to rewrite this stuff.
regards, tom lane