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

src/backend/access/transam/xlogarchive.c

@@ -159,20 +159,27 @@ RestoreArchivedFile(char *path, const char *xlogfname,
 			(errmsg_internal("executing restore command \"%s\"",
 							 xlogRestoreCmd)));
 
+	fflush(NULL);
+	pgstat_report_wait_start(WAIT_EVENT_RESTORE_COMMAND);
+
 	/*
-	 * Check signals before restore command and reset afterwards.
+	 * PreRestoreCommand() informs the SIGTERM handler for the startup process
+	 * that it should proc_exit() right away.  This is done for the duration
+	 * of the system() call because there isn't a good way to break out while
+	 * it is executing.  Since we might call proc_exit() in a signal handler,
+	 * it is best to put any additional logic before or after the
+	 * PreRestoreCommand()/PostRestoreCommand() section.
 	 */
 	PreRestoreCommand();
 
 	/*
 	 * Copy xlog from archival storage to XLOGDIR
 	 */
-	fflush(NULL);
-	pgstat_report_wait_start(WAIT_EVENT_RESTORE_COMMAND);
 	rc = system(xlogRestoreCmd);
-	pgstat_report_wait_end();
 
 	PostRestoreCommand();
+
+	pgstat_report_wait_end();
 	pfree(xlogRestoreCmd);
 
 	if (rc == 0)