SKIP LOCKED DATA (work in progress)

Hi

A couple of years ago I posted an outline of a plan [1] and an
initial patch [2] for implementing SKIP LOCKED DATA. I have
recently come back to this idea, rebased the patch and added a
simple isolation test -- please see attached.

However, heap_lock_tuple is clearly not for the faint hearted,
and I freely admit that I don't understand half of the things
going on in there yet. My general approach has been to follow
the example set by NOWAIT, generalising that flag into a 3-way
policy... but of course NOWAIT doesn't have to worry about
cleaning anything up, because it uses ereport, hence my TODOs.

As always, I would be grateful for any feedback.

Thanks!

Thomas Munro

[1]
http://www.postgresql.org/message-id/CADLWmXV7inA-HS=bJ-s6+Ai8DsVGx8zMohr0Ht38EWmXNeqPWw@mail.gmail.com
[2]
http://www.postgresql.org/message-id/CADLWmXUUjmrPU-+9ss7BCATxM-hr6__9mB6Wv7ry3-r+KXGgBw@mail.gmail.com