pgsql: Move extra code out of the Pre/PostRestoreCommand() section.

Move extra code out of the Pre/PostRestoreCommand() section.

If SIGTERM is received within this section, the startup process
will immediately proc_exit() in the signal handler, so it is
inadvisable to include any more code than is required there (as
such code is unlikely to be compatible with doing proc_exit() in a
signal handler). This commit moves the code recently added to this
section (see 1b06d7bac9 and 7fed801135) to outside of the section.
This ensures that the startup process only calls proc_exit() in its
SIGTERM handler for the duration of the system() call, which is how
this code worked from v8.4 to v14.

Reported-by: Michael Paquier, Thomas Munro
Analyzed-by: Andres Freund
Suggested-by: Tom Lane
Reviewed-by: Michael Paquier, Robert Haas, Thomas Munro, Andres Freund
Discussion: https://postgr.es/m/Y9nGDSgIm83FHcad%40paquier.xyz
Discussion: https://postgr.es/m/20230223231503.GA743455%40nathanxps13
Backpatch-through: 15

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d1c56ad37b9676df23cb12bdabfffce2a5fc01a7

Modified Files
--------------
src/backend/access/transam/xlogarchive.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)