pgsql: Relax lock level for setting PGPROC->statusFlags

Relax lock level for setting PGPROC->statusFlags

We don't actually need a lock to set PGPROC->statusFlags itself; what we
do need is a shared lock on either XidGenLock or ProcArrayLock in order to
ensure MyProc->pgxactoff keeps still while we modify the mirror array in
ProcGlobal->statusFlags.  Some places were using an exclusive lock for
that, which is excessive.  Relax those to use shared lock only.

procarray.c has a couple of places with somewhat brittle assumptions
about PGPROC changes: ProcArrayEndTransaction uses only shared lock, so
it's permissible to change MyProc only.  On the other hand,
ProcArrayEndTransactionInternal also changes other procs, so it must
hold exclusive lock.  Add asserts to ensure those assumptions continue
to hold.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/20201117155501.GA13805@alvherre.pgsql

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/27838981be9de35d54ffcdc6cc06b5d5ea9e0cee

Modified Files
--------------
src/backend/commands/vacuum.c             | 2 +-
src/backend/replication/logical/logical.c | 2 +-
src/backend/replication/slot.c            | 2 +-
src/backend/storage/ipc/procarray.c       | 8 +++++++-
src/backend/storage/lmgr/deadlock.c       | 2 +-
src/include/storage/proc.h                | 5 +++++
6 files changed, 16 insertions(+), 5 deletions(-)