Re: SKIP LOCKED DATA (work in progress)

heap_lock_tuple() has the following comment on top:

* In the failure cases, the routine fills *hufd with the tuple's t_ctid,
* t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
* (the last only for HeapTupleSelfUpdated, since we
* cannot obtain cmax from a combocid generated by another transaction).
* See comments for struct HeapUpdateFailureData for additional info.

With the patch as submitted, this struct is not filled in the
HeapTupleWouldBlock case. I'm not sure this is okay, though I admit the
only caller that passes LockWaitSkip doesn't care, so maybe we could
just ignore the issue (after properly modifying the comment). It seems
easy to add a LockBuffer() and "goto failed" rather than a return; that
would make that failure case conform to HeapTupleUpdated and
HeapTupleSelfUpdated. OTOH it might be pointless to worry about what
would be essentially dead code currently ...

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.

I also pgindented.

--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services