Remove temporary files after backend crash

After a crash of a backend using temporary files, the files used to be
left behind, on the basis that it might be useful for debugging. But we
don't have any reports of anyone actually doing that, and it means the
disk usage may grow over time due to repeated backend failures (possibly
even hitting ENOSPC). So this behavior is a bit unfortunate, and fixing
it required either manual cleanup (deleting files, which is error-prone)
or restart of the instance (i.e. service disruption).

This implements automatic cleanup of temporary files, controled by a new
GUC remove_temp_files_after_crash. By default the files are removed, but
it can be disabled to restore the old behavior if needed.

Author: Euler Taveira
Reviewed-by: Tomas Vondra, Michael Paquier, Anastasia Lubennikova, Thomas Munro
Discussion: https://postgr.es/m/CAH503wDKdYzyq7U-QJqGn%3DGm6XmoK%2B6_6xTJ-Yn5WSvoHLY1Ww%40mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -242,6 +242,7 @@ bool		Db_user_namespace = false;
 bool		enable_bonjour = false;
 char	   *bonjour_name;
 bool		restart_after_crash = true;
+bool		remove_temp_files_after_crash = true;
 
 /* PIDs of special child processes; 0 when not running */
 static pid_t StartupPID = 0,
@@ -3976,6 +3977,10 @@ PostmasterStateMachine(void)
 		ereport(LOG,
 				(errmsg("all server processes terminated; reinitializing")));
 
+		/* remove leftover temporary files after a crash */
+		if (remove_temp_files_after_crash)
+			RemovePgTempFiles();
+
 		/* allow background workers to immediately restart */
 		ResetBackgroundWorkerCrashTimes();