Re: SKIP LOCKED DATA (work in progress)

On 24 August 2014 22:04, Thomas Munro <munro(at)ip9(dot)org> wrote:
> On 22 August 2014 23:02, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>> Did you consider heap_lock_updated_tuple? A rationale for saying it
>> doesn't need to pay attention to the wait policy is: if you're trying to
>> lock-skip-locked an updated tuple, then you either skip it because its
>> updater is running, or you return it because it's no longer running; and
>> if you return it, it is not possible for the updater to be locking the
>> updated version. However, what if there's a third transaction that
>> locked the updated version? It might be difficult to hit this case or
>> construct an isolationtester spec file though, since there's a narrow
>> window you need to race to.
>
> Hmm. I will look into this, thanks.

While trying to produce the heap_lock_updated_tuple_rec case you
describe (so far unsuccessfully), I discovered I could make SELECT ...
FOR UPDATE NOWAIT block indefinitely on unpatched 9.3 in a different
code path after heap_lock_tuple returns: in another session, UPDATE,
COMMIT, then UPDATE, all after the first session has taken its
snapshot but before it tries to lock a given row. The code in
EvalPlanQualFetch (reached from the HeapTupleUpdated case in
ExecLockRow) finishes up waiting for the uncommitted transaction.

I think I see how to teach EvalPlanQualFetch how to handle wait
policies: for NOWAIT it should ereport (fixing a pre-existing bug
(?)), and I guess it should handle SKIP LOCKED by returning NULL,
similarly to the way it handles deleted rows, and of course in all
cases passing the wait policy forward to heap_lock_tuple, which it
eventually calls.

Still looking at heap_lock_updated_tuple.

The difficulty of course will be testing all these racy cases reproducibly...

Best regards,
Thomas Munro