refact(pool): explicitly terminate workers on interrupt

Instead of juggling with signal handlers and hoping that
`ShutDownRequired` will be fired in the appropriate place in
`multiprocessing.BasePool`, on exceptional termination, we signal
workers via `SIGTERM`.

As a side-effect this makes it possible to run `process_file` in
non-main thread.

unblob/pool.py

@@ -1,18 +1,24 @@
 import abc
+import contextlib
 import multiprocessing as mp
 import os
 import queue
+import signal
 import sys
 import threading
 from multiprocessing.queues import JoinableQueue
-from typing import Any, Callable, Union
+from typing import Any, Callable, Set, Union
 
 from .logging import multiprocessing_breakpoint
 
 mp.set_start_method("fork")
 
 
 class PoolBase(abc.ABC):
+    def __init__(self):
+        with pools_lock:
+            pools.add(self)
+
     @abc.abstractmethod
     def submit(self, args):
         pass
@@ -24,15 +30,20 @@ def process_until_done(self):
     def start(self):
         pass
 
-    def close(self):
-        pass
+    def close(self, *, immediate=False):  # noqa: ARG002
+        with pools_lock:
+            pools.remove(self)
 
     def __enter__(self):
         self.start()
         return self
 
-    def __exit__(self, *args):
-        self.close()
+    def __exit__(self, exc_type, _exc_value, _tb):
+        self.close(immediate=exc_type is not None)
+
+
+pools_lock = threading.Lock()
+pools: Set[PoolBase] = set()
 
 
 class Queue(JoinableQueue):
@@ -53,9 +64,15 @@ class _Sentinel:
 
 
 def _worker_process(handler, input_, output):
-    # Creates a new process group, making sure no signals are propagated from the main process to the worker processes.
+    # Creates a new process group, making sure no signals are
+    # propagated from the main process to the worker processes.
     os.setpgrp()
 
+    # Restore default signal handlers, otherwise workers would inherit
+    # them from main process
+    signal.signal(signal.SIGTERM, signal.SIG_DFL)
+    signal.signal(signal.SIGINT, signal.SIG_DFL)
+
     sys.breakpointhook = multiprocessing_breakpoint
     while (args := input_.get()) is not _SENTINEL:
         result = handler(args)
@@ -71,11 +88,14 @@ def __init__(
         *,
         result_callback: Callable[["MultiPool", Any], Any],
     ):
+        super().__init__()
         if process_num <= 0:
             raise ValueError("At process_num must be greater than 0")
 
+        self._running = False
         self._result_callback = result_callback
         self._input = Queue(ctx=mp.get_context())
+        self._input.cancel_join_thread()
         self._output = mp.SimpleQueue()
         self._procs = [
             mp.Process(
@@ -87,14 +107,32 @@ def __init__(
         self._tid = threading.get_native_id()
 
     def start(self):
+        self._running = True
         for p in self._procs:
             p.start()
 
-    def close(self):
-        self._clear_input_queue()
-        self._request_workers_to_quit()
-        self._clear_output_queue()
+    def close(self, *, immediate=False):
+        if not self._running:
+            return
+        self._running = False
+
+        if immediate:
+            self._terminate_workers()
+        else:
+            self._clear_input_queue()
+            self._request_workers_to_quit()
+            self._clear_output_queue()
+
         self._wait_for_workers_to_quit()
+        super().close(immediate=immediate)
+
+    def _terminate_workers(self):
+        for proc in self._procs:
+            proc.terminate()
+
+        self._input.close()
+        if sys.version_info >= (3, 9):
+            self._output.close()
 
     def _clear_input_queue(self):
         try:
@@ -129,14 +167,16 @@ def submit(self, args):
         self._input.put(args)
 
     def process_until_done(self):
-        while not self._input.is_empty():
-            result = self._output.get()
-            self._result_callback(self, result)
-            self._input.task_done()
+        with contextlib.suppress(EOFError):
+            while not self._input.is_empty():
+                result = self._output.get()
+                self._result_callback(self, result)
+                self._input.task_done()
 
 
 class SinglePool(PoolBase):
     def __init__(self, handler, *, result_callback):
+        super().__init__()
         self._handler = handler
         self._result_callback = result_callback
 
@@ -157,3 +197,20 @@ def make_pool(process_num, handler, result_callback) -> Union[SinglePool, MultiP
         handler=handler,
         result_callback=result_callback,
     )
+
+
+orig_signal_handlers = {}
+
+
+def _on_terminate(signum, frame):
+    with contextlib.suppress(StopIteration):
+        while True:
+            pool = next(iter(pools))
+            pool.close(immediate=True)
+
+    if callable(orig_signal_handlers[signum]):
+        orig_signal_handlers[signum](signum, frame)
+
+
+orig_signal_handlers[signal.SIGTERM] = signal.signal(signal.SIGTERM, _on_terminate)
+orig_signal_handlers[signal.SIGINT] = signal.signal(signal.SIGINT, _on_terminate)

unblob/processing.py

@@ -45,7 +45,6 @@
     StatReport,
     UnknownError,
 )
-from .signals import terminate_gracefully
 from .ui import NullProgressReporter, ProgressReporter
 
 logger = get_logger()