pgsql: bufmgr: Allow conditionally locking of already locked buffer

bufmgr: Allow conditionally locking of already locked buffer

In fcb9c977aa5 I included an assertion in BufferLockConditional() to detect if
a conditional lock acquisition is done on a buffer that we already have
locked. The assertion was added in the course of adding other assertions.
Unfortunately I failed to realize that some of our code relies on such lock
acquisitions to silently fail. E.g. spgist and nbtree may try to conditionally
lock an already locked buffer when acquiring a empty buffer.

LWLockAcquireConditional(), which was previously used to implement
ConditionalLockBuffer(), does not have such an assert.

Instead of just removing the assert, and relying on the lock acquisition to
fail due to the buffer already locked, this commit changes the behaviour of
conditional content lock acquisition to fail if the current backend has any
pre-existing lock on the buffer, even if the lock modes would not
conflict. The reason for that is that we currently do not have space to track
multiple lock acquisitions on a single buffer. Allowing multiple locks on the
same buffer by a backend also seems likely to lead to bugs.

There is only one non-self-exclusive conditional content lock acquisition, in
GetVictimBuffer(), but it only is used if the target buffer is not pinned and
thus can't already be locked by the current backend.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Discussion: https://postgr.es/m/90bd2cbb-49ce-4092-9f61-5ac2ab782c94@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/333f586372aae764b7ad7e2c975b14fd431ce819

Modified Files
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src/backend/storage/buffer/bufmgr.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)