bpo-21423: Add an initializer argument to {Process,Thread}PoolExecutor (

#4241)

* bpo-21423: Add an initializer argument to {Process,Thread}PoolExecutor

* Fix docstring

Doc/library/concurrent.futures.rst

@@ -124,11 +124,17 @@ And::
    executor.submit(wait_on_future)
 
 
-.. class:: ThreadPoolExecutor(max_workers=None, thread_name_prefix='')
+.. class:: ThreadPoolExecutor(max_workers=None, thread_name_prefix='', initializer=None, initargs=())
 
    An :class:`Executor` subclass that uses a pool of at most *max_workers*
    threads to execute calls asynchronously.
 
+   *initializer* is an optional callable that is called at the start of
+   each worker thread; *initargs* is a tuple of arguments passed to the
+   initializer.  Should *initializer* raise an exception, all currently
+   pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
+   as well any attempt to submit more jobs to the pool.
+
    .. versionchanged:: 3.5
       If *max_workers* is ``None`` or
       not given, it will default to the number of processors on the machine,
@@ -142,6 +148,10 @@ And::
       control the threading.Thread names for worker threads created by
       the pool for easier debugging.
 
+   .. versionchanged:: 3.7
+      Added the *initializer* and *initargs* arguments.
+
+
 .. _threadpoolexecutor-example:
 
 ThreadPoolExecutor Example
@@ -191,7 +201,7 @@ that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.
 Calling :class:`Executor` or :class:`Future` methods from a callable submitted
 to a :class:`ProcessPoolExecutor` will result in deadlock.
 
-.. class:: ProcessPoolExecutor(max_workers=None, mp_context=None)
+.. class:: ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=())
 
    An :class:`Executor` subclass that executes calls asynchronously using a pool
    of at most *max_workers* processes.  If *max_workers* is ``None`` or not
@@ -202,6 +212,12 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
    launch the workers. If *mp_context* is ``None`` or not given, the default
    multiprocessing context is used.
 
+   *initializer* is an optional callable that is called at the start of
+   each worker process; *initargs* is a tuple of arguments passed to the
+   initializer.  Should *initializer* raise an exception, all currently
+   pending jobs will raise a :exc:`~concurrent.futures.thread.BrokenThreadPool`,
+   as well any attempt to submit more jobs to the pool.
+
    .. versionchanged:: 3.3
       When one of the worker processes terminates abruptly, a
       :exc:`BrokenProcessPool` error is now raised.  Previously, behaviour
@@ -212,6 +228,8 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
       The *mp_context* argument was added to allow users to control the
       start_method for worker processes created by the pool.
 
+      Added the *initializer* and *initargs* arguments.
+
 
 .. _processpoolexecutor-example:
 
@@ -432,13 +450,31 @@ Exception classes
 
    Raised when a future operation exceeds the given timeout.
 
+.. exception:: BrokenExecutor
+
+   Derived from :exc:`RuntimeError`, this exception class is raised
+   when an executor is broken for some reason, and cannot be used
+   to submit or execute new tasks.
+
+   .. versionadded:: 3.7
+
+.. currentmodule:: concurrent.futures.thread
+
+.. exception:: BrokenThreadPool
+
+   Derived from :exc:`~concurrent.futures.BrokenExecutor`, this exception
+   class is raised when one of the workers of a :class:`ThreadPoolExecutor`
+   has failed initializing.
+
+   .. versionadded:: 3.7
+
 .. currentmodule:: concurrent.futures.process
 
 .. exception:: BrokenProcessPool
 
-   Derived from :exc:`RuntimeError`, this exception class is raised when
-   one of the workers of a :class:`ProcessPoolExecutor` has terminated
-   in a non-clean fashion (for example, if it was killed from the outside).
+   Derived from :exc:`~concurrent.futures.BrokenExecutor` (formerly
+   :exc:`RuntimeError`), this exception class is raised when one of the
+   workers of a :class:`ProcessPoolExecutor` has terminated in a non-clean
+   fashion (for example, if it was killed from the outside).
 
    .. versionadded:: 3.3
-

Lib/concurrent/futures/__init__.py

@@ -10,6 +10,7 @@
                                       ALL_COMPLETED,
                                       CancelledError,
                                       TimeoutError,
+                                      BrokenExecutor,
                                       Future,
                                       Executor,
                                       wait,

Lib/concurrent/futures/_base.py

@@ -610,3 +610,9 @@ def __enter__(self):
     def __exit__(self, exc_type, exc_val, exc_tb):
         self.shutdown(wait=True)
         return False
+
+
+class BrokenExecutor(RuntimeError):
+    """
+    Raised when a executor has become non-functional after a severe failure.
+    """

Lib/concurrent/futures/process.py

@@ -131,6 +131,7 @@ def __init__(self, work_id, fn, args, kwargs):
         self.args = args
         self.kwargs = kwargs
 
+
 def _get_chunks(*iterables, chunksize):
     """ Iterates over zip()ed iterables in chunks. """
     it = zip(*iterables)
@@ -151,7 +152,7 @@ def _process_chunk(fn, chunk):
     """
     return [fn(*args) for args in chunk]
 
-def _process_worker(call_queue, result_queue):
+def _process_worker(call_queue, result_queue, initializer, initargs):
     """Evaluates calls from call_queue and places the results in result_queue.
 
     This worker is run in a separate process.
@@ -161,7 +162,17 @@ def _process_worker(call_queue, result_queue):
             evaluated by the worker.
         result_queue: A ctx.Queue of _ResultItems that will written
             to by the worker.
+        initializer: A callable initializer, or None
+        initargs: A tuple of args for the initializer
     """
+    if initializer is not None:
+        try:
+            initializer(*initargs)
+        except BaseException:
+            _base.LOGGER.critical('Exception in initializer:', exc_info=True)
+            # The parent will notice that the process stopped and
+            # mark the pool broken
+            return
     while True:
         call_item = call_queue.get(block=True)
         if call_item is None:
@@ -277,7 +288,9 @@ def shutdown_worker():
             # Mark the process pool broken so that submits fail right now.
             executor = executor_reference()
             if executor is not None:
-                executor._broken = True
+                executor._broken = ('A child process terminated '
+                                    'abruptly, the process pool is not '
+                                    'usable anymore')
                 executor._shutdown_thread = True
                 executor = None
             # All futures in flight must be marked failed
@@ -372,15 +385,16 @@ def _chain_from_iterable_of_lists(iterable):
             yield element.pop()
 
 
-class BrokenProcessPool(RuntimeError):
+class BrokenProcessPool(_base.BrokenExecutor):
     """
     Raised when a process in a ProcessPoolExecutor terminated abruptly
     while a future was in the running state.
     """
 
 
 class ProcessPoolExecutor(_base.Executor):
-    def __init__(self, max_workers=None, mp_context=None):
+    def __init__(self, max_workers=None, mp_context=None,
+                 initializer=None, initargs=()):
         """Initializes a new ProcessPoolExecutor instance.
 
         Args:
@@ -389,6 +403,8 @@ def __init__(self, max_workers=None, mp_context=None):
                 worker processes will be created as the machine has processors.
             mp_context: A multiprocessing context to launch the workers. This
                 object should provide SimpleQueue, Queue and Process.
+            initializer: An callable used to initialize worker processes.
+            initargs: A tuple of arguments to pass to the initializer.
         """
         _check_system_limits()
 
@@ -403,6 +419,11 @@ def __init__(self, max_workers=None, mp_context=None):
             mp_context = mp.get_context()
         self._mp_context = mp_context
 
+        if initializer is not None and not callable(initializer):
+            raise TypeError("initializer must be a callable")
+        self._initializer = initializer
+        self._initargs = initargs
+
         # Make the call queue slightly larger than the number of processes to
         # prevent the worker processes from idling. But don't make it too big
         # because futures in the call queue cannot be cancelled.
@@ -450,15 +471,16 @@ def _adjust_process_count(self):
             p = self._mp_context.Process(
                 target=_process_worker,
                 args=(self._call_queue,
-                      self._result_queue))
+                      self._result_queue,
+                      self._initializer,
+                      self._initargs))
             p.start()
             self._processes[p.pid] = p
 
     def submit(self, fn, *args, **kwargs):
         with self._shutdown_lock:
             if self._broken:
-                raise BrokenProcessPool('A child process terminated '
-                    'abruptly, the process pool is not usable anymore')
+                raise BrokenProcessPool(self._broken)
             if self._shutdown_thread:
                 raise RuntimeError('cannot schedule new futures after shutdown')
 

Lib/concurrent/futures/thread.py

@@ -41,6 +41,7 @@ def _python_exit():
 
 atexit.register(_python_exit)
 
+
 class _WorkItem(object):
     def __init__(self, future, fn, args, kwargs):
         self.future = future
@@ -61,7 +62,17 @@ def run(self):
         else:
             self.future.set_result(result)
 
-def _worker(executor_reference, work_queue):
+
+def _worker(executor_reference, work_queue, initializer, initargs):
+    if initializer is not None:
+        try:
+            initializer(*initargs)
+        except BaseException:
+            _base.LOGGER.critical('Exception in initializer:', exc_info=True)
+            executor = executor_reference()
+            if executor is not None:
+                executor._initializer_failed()
+            return
     try:
         while True:
             work_item = work_queue.get(block=True)
@@ -83,18 +94,28 @@ def _worker(executor_reference, work_queue):
     except BaseException:
         _base.LOGGER.critical('Exception in worker', exc_info=True)
 
+
+class BrokenThreadPool(_base.BrokenExecutor):
+    """
+    Raised when a worker thread in a ThreadPoolExecutor failed initializing.
+    """
+
+
 class ThreadPoolExecutor(_base.Executor):
 
     # Used to assign unique thread names when thread_name_prefix is not supplied.
     _counter = itertools.count().__next__
 
-    def __init__(self, max_workers=None, thread_name_prefix=''):
+    def __init__(self, max_workers=None, thread_name_prefix='',
+                 initializer=None, initargs=()):
         """Initializes a new ThreadPoolExecutor instance.
 
         Args:
             max_workers: The maximum number of threads that can be used to
                 execute the given calls.
             thread_name_prefix: An optional name prefix to give our threads.
+            initializer: An callable used to initialize worker threads.
+            initargs: A tuple of arguments to pass to the initializer.
         """
         if max_workers is None:
             # Use this number because ThreadPoolExecutor is often
@@ -103,16 +124,25 @@ def __init__(self, max_workers=None, thread_name_prefix=''):
         if max_workers <= 0:
             raise ValueError("max_workers must be greater than 0")
 
+        if initializer is not None and not callable(initializer):
+            raise TypeError("initializer must be a callable")
+
         self._max_workers = max_workers
         self._work_queue = queue.Queue()
         self._threads = set()
+        self._broken = False
         self._shutdown = False
         self._shutdown_lock = threading.Lock()
         self._thread_name_prefix = (thread_name_prefix or
                                     ("ThreadPoolExecutor-%d" % self._counter()))
+        self._initializer = initializer
+        self._initargs = initargs
 
     def submit(self, fn, *args, **kwargs):
         with self._shutdown_lock:
+            if self._broken:
+                raise BrokenThreadPool(self._broken)
+
             if self._shutdown:
                 raise RuntimeError('cannot schedule new futures after shutdown')
 
@@ -137,12 +167,27 @@ def weakref_cb(_, q=self._work_queue):
                                      num_threads)
             t = threading.Thread(name=thread_name, target=_worker,
                                  args=(weakref.ref(self, weakref_cb),
-                                       self._work_queue))
+                                       self._work_queue,
+                                       self._initializer,
+                                       self._initargs))
             t.daemon = True
             t.start()
             self._threads.add(t)
             _threads_queues[t] = self._work_queue
 
+    def _initializer_failed(self):
+        with self._shutdown_lock:
+            self._broken = ('A thread initializer failed, the thread pool '
+                            'is not usable anymore')
+            # Drain work queue and mark pending futures failed
+            while True:
+                try:
+                    work_item = self._work_queue.get_nowait()
+                except queue.Empty:
+                    break
+                if work_item is not None:
+                    work_item.future.set_exception(BrokenThreadPool(self._broken))
+
     def shutdown(self, wait=True):
         with self._shutdown_lock:
             self._shutdown = True