pgsql: Unify SIGHUP handling between normal and walsender backends.

Unify SIGHUP handling between normal and walsender backends.

Because walsender and normal backends share the same main loop it's
problematic to have two different flag variables, set in signal
handlers, indicating a pending configuration reload. Only certain
walsender commands reach code paths checking for the
variable (START_[LOGICAL_]REPLICATION, CREATE_REPLICATION_SLOT
... LOGICAL, notably not base backups).

This is a bug present since the introduction of walsender, but has
gotten worse in releases since then which allow walsender to do more.

A later patch, not slated for v10, will similarly unify SIGHUP
handling in other types of processes as well.

Author: Petr Jelinek, Andres Freund
Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/20170423235941.qosiuoyqprq4nu7v@alap3.anarazel.de
Backpatch: 9.2-, bug is present since 9.0

Branch
------
REL9_2_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/133b1920c5b8647c67cf2e017b50266ba71c788c

Modified Files
--------------
src/backend/replication/walsender.c | 26 ++++++--------------------
src/backend/tcop/postgres.c | 30 ++++++++++++++----------------
src/backend/utils/init/globals.c | 1 +
src/include/miscadmin.h | 5 +++++
4 files changed, 26 insertions(+), 36 deletions(-)