worker: rewrite error handling so os._exit covers all cases Previously the worker error handling is like: pid = os.fork() --+ if pid == 0: | .... | problematic .... --+ try: --+ .... | worker error handling --+ If a signal arrives when Python is executing the "problematic" lines, an external error handling (dispatch.py) will take over the control flow and it's no longer guaranteed "os._exit" is called (see 86cd09bc13ba for why it is necessary). This patch rewrites the error handling so it covers all possible code paths for a worker even during fork. Note: "os.getpid() == parentpid" is used to test if the process is parent or not intentionally, instead of checking "pid", because "pid = os.fork()" may be not atomic - it's possible that that a signal hits the worker before the assignment completes [1]. The newly added test replaces "os.fork" to exercise that extreme case. [1]: CPython compiles "pid = os.fork()" to 2 byte codes: "CALL_FUNCTION" and "STORE_FAST", so it's probably not atomic: def f(): pid = os.fork() dis.dis(f) 2 0 LOAD_GLOBAL 0 (os) 3 LOAD_ATTR 1 (fork) 6 CALL_FUNCTION 0 9 STORE_FAST 0 (pid) 12 LOAD_CONST 0 (None) 15 RETURN_VALUE