pgsql: Fix shared refcount leak in pgstat entry acquisition

Fix shared refcount leak in pgstat entry acquisition

pgstat_acquire_entry_ref() does an entry acquisition with the following
actions:
- Increment of a shared ebtr's refcount.
- Release dshash partition lock.
- Update local entry_ref fields.

With a pending interrupt, the LWLock acquired by dshash_release_lock()
in the second step could fail, leaving around a local entry_ref with
shared_stats set to NULL and the refcount permanently inflated as
pgstat_release_entry_ref() skips the refcount decrement is shared_stats
is not set.

The actions are now reordered so as the lock is released after the local
entry_ref fields are populated.

The consequence of this problem is a minor leak. It can only happen if
an interrupt is processed when a dshash lock is released, which is a
very narrow window (very unlikely), so no backpatch is done.

Author: Niall Newman <nn(at)turacolabs(dot)com>
Discussion: https://postgr.es/m/2FDAA194-9CF3-4FD7-A450-F1A4BEB125F6@turacolabs.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bf80a4c2d238834073b09c231294c4479157f16e

Modified Files
--------------
src/backend/utils/activity/pgstat_shmem.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)