pgsql: Allow a no-wait lock acquisition to succeed in more cases.

Allow a no-wait lock acquisition to succeed in more cases.

We don't determine the position at which a process waiting for a lock
should insert itself into the wait queue until we reach ProcSleep(),
and we may at that point discover that we must insert ourselves ahead
of everyone who wants a conflicting lock, in which case we obtain the
lock immediately. Up until now, a no-wait lock acquisition would fail
in such cases, erroneously claiming that the lock couldn't be obtained
immediately. Fix that by trying ProcSleep even in the no-wait case.

No back-patch for now, because I'm treating this as an improvement to
the existing no-wait feature. It could instead be argued that it's a
bug fix, on the theory that there should never be any case whatsoever
where no-wait fails to obtain a lock that would have been obtained
immediately without no-wait, but I'm reluctant to interpret the
semantics of no-wait that strictly.

Robert Haas and Jingxian Li

Discussion: http://postgr.es/m/CA+TgmobCH-kMXGVpb0BB-iNMdtcNkTvcZ4JBxDJows3kYM+GDg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2346df6fc373df9c5ab944eebecf7d3036d727de

Modified Files
--------------
src/backend/storage/lmgr/lock.c | 119 ++++++++++++++++------------
src/backend/storage/lmgr/proc.c | 20 ++++-
src/include/storage/proc.h | 4 +-
src/test/isolation/expected/lock-nowait.out | 9 +++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/lock-nowait.spec | 28 +++++++
6 files changed, 128 insertions(+), 53 deletions(-)