pgsql: Allow callback functions to deregister themselves during a call.

Allow callback functions to deregister themselves during a call.

Fetch the next-item pointer before the call not after, so that
we aren't dereferencing a dangling pointer if the callback
deregistered itself during the call. The risky coding pattern
appears in CallXactCallbacks, CallSubXactCallbacks, and
ResourceOwnerReleaseInternal. (There are some other places that
might be at hazard if they offered deregistration functionality,
but they don't.)

I (tgl) considered back-patching this, but desisted because it
wouldn't be very safe for extensions to rely on this working in
pre-v16 branches.

Hao Wu

Discussion: https://postgr.es/m/CAH+9SWXTiERkmhRke+QCcc+jRH8d5fFHTxh8ZK0-Yn4BSpyaAg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d2a844242dcfb34e05dd0d880b1a283a514b16b

Modified Files
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src/backend/access/transam/xact.c | 14 ++++++++++++--
src/backend/utils/resowner/resowner.c | 7 ++++++-
2 files changed, 18 insertions(+), 3 deletions(-)