Re: SKIP LOCKED DATA (work in progress)

Thomas Munro wrote:

> 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.

Interesting -- perhaps this helps explain the deadlock issue reported in
http://www.postgresql.org/message-id/CAKrjmhdN+GhAjNwqfHsOtGp+7YN27zR79m99RcAJMNazt5NJrA@mail.gmail.com

> 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...

Does this help?
http://www.postgresql.org/message-id/51FB4305.3070600@2ndquadrant.com
The useful trick there is forcing a query to get its snapshot and then
go to sleep before actually doing anything, by way of an advisory lock.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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