pgsql: Allow lwlocks to be disowned

Allow lwlocks to be disowned

To implement AIO writes, the backend initiating writes needs to transfer the
lock ownership to the AIO subsystem, so the lock held during the write can be
released in another backend.

Other backends need to be able to "complete" an asynchronously started IO to
avoid deadlocks (consider e.g. one backend starting IO for a buffer and then
waiting for a heavyweight lock held by another relation followed by the
current holder of the heavyweight lock waiting for the IO to complete).

To that end, this commit adds LWLockDisown() and LWLockReleaseDisowned(). If
code uses LWLockDisown() it's the code's responsibility to ensure that the
lock is released in case of errors.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Discussion: https://postgr.es/m/1f6b50a7-38ef-4d87-8246-786d39f46ab9@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f8d7f29b3e81db59b95e4b5baaa6943178c89fd8

Modified Files
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src/backend/storage/lmgr/lwlock.c | 75 +++++++++++++++++++++++++++++++++++----
src/include/storage/lwlock.h | 2 ++
2 files changed, 71 insertions(+), 6 deletions(-)