pgpool: Convert pcp_worker die handler to flag-only.

Convert pcp_worker die handler to flag-only.

die() (SIGTERM/SIGINT/SIGQUIT handler in the PCP worker child) directly
called ereport() and proceeded to exit logic from signal context. These
operations are not async-signal-safe, and ereport() in particular can
allocate memory or acquire locks that the main loop may already be
holding, risking corruption or a hang if the signal arrives at the
wrong point.

Convert die() to the canonical pgpool flag-only pattern used elsewhere
in this file. die() now only saves errno, records the signal number
into pcp_worker_shutdown_signal, sets pcp_worker_shutdown_request, and
restores errno before returning. The actual handling, logging the
shutdown request and performing the SIGTERM "smart shutdown" or
SIGINT/SIGQUIT immediate exit, is moved into the new
process_pcp_worker_shutdown_request(), which is called from the main
loop at safe points (before and after each PCP packet read).

Reported-by: Emond Papegaaij
Reported-by: Claude code
Author: Taiki Koshino <koshino(at)sraoss(dot)co(dot)jp>
Discussion: https://www.postgresql.org/message-id/OS9P286MB6486805F1222F8F50767639094AE2%40OS9P286MB6486.JPNP286.PROD.OUTLOOK.COM
Backpatch-through: v4.3

Branch
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V4_3_STABLE

Details
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https://git.postgresql.org/gitweb?p=pgpool2.git;a=commitdiff;h=8964c11f5691d1237d4ec4bd32ec0a295be4f438

Modified Files
--------------
src/pcp_con/pcp_worker.c | 53 ++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 44 insertions(+), 9 deletions(-)