diff --git a/Doc/library/multiprocessing.shared_memory.rst b/Doc/library/multiprocessing.shared_memory.rst index f453e6403d932d..671130d9b29fc0 100644 --- a/Doc/library/multiprocessing.shared_memory.rst +++ b/Doc/library/multiprocessing.shared_memory.rst @@ -36,7 +36,7 @@ or other communications requiring the serialization/deserialization and copying of data. -.. class:: SharedMemory(name=None, create=False, size=0) +.. class:: SharedMemory(name=None, create=False, size=0, *, track=True) Creates a new shared memory block or attaches to an existing shared memory block. Each shared memory block is assigned a unique name. @@ -64,26 +64,45 @@ copying of data. memory block may be larger or equal to the size requested. When attaching to an existing shared memory block, the ``size`` parameter is ignored. + *track*, when enabled, registers the shared memory block with a resource + tracker process on platforms where the OS does not do this automatically. + The resource tracker ensures proper cleanup of the shared memory even + if all other processes with access to the memory exit without doing so. + Python processes created from a common ancestor using :mod:`multiprocessing` + facilities share a single resource tracker process, and the lifetime of + shared memory segments is handled automatically among these processes. + Python processes created in any other way will receive their own + resource tracker when accessing shared memory with *track* enabled. + This will cause the shared memory to be deleted by the resource tracker + of the first process that terminates. + To avoid this issue, users of :mod:`subprocess` or standalone Python + processes should set *track* to ``False`` when there is already another + process in place that does the bookkeeping. + *track* is ignored on Windows, which has its own tracking and + automatically deletes shared memory when all handles to it have been closed. + + .. versionchanged:: 3.13 Added *track* parameter. + .. method:: close() - Closes access to the shared memory from this instance. In order to - ensure proper cleanup of resources, all instances should call - ``close()`` once the instance is no longer needed. Note that calling - ``close()`` does not cause the shared memory block itself to be - destroyed. + Closes the file descriptor/handle to the shared memory from this + instance. :meth:`close()` should be called once access to the shared + memory block from this instance is no longer needed. Depending + on operating system, the underlying memory may or may not be freed + even if all handles to it have been closed. To ensure proper cleanup, + use the :meth:`unlink()` method. .. method:: unlink() - Requests that the underlying shared memory block be destroyed. In - order to ensure proper cleanup of resources, ``unlink()`` should be - called once (and only once) across all processes which have need - for the shared memory block. After requesting its destruction, a - shared memory block may or may not be immediately destroyed and - this behavior may differ across platforms. Attempts to access data - inside the shared memory block after ``unlink()`` has been called may - result in memory access errors. Note: the last process relinquishing - its hold on a shared memory block may call ``unlink()`` and - :meth:`close()` in either order. + Deletes the underlying shared memory block. This should be called only + once per shared memory block regardless of the number of handles to it, + even in other processes. + :meth:`unlink()` and :meth:`close()` can be called in any order, but + trying to access data inside a shared memory block after :meth:`unlink()` + may result in memory access errors, depending on platform. + + This method has no effect on Windows, where the only way to delete a + shared memory block is to close all handles. .. attribute:: buf diff --git a/Lib/multiprocessing/shared_memory.py b/Lib/multiprocessing/shared_memory.py index 9a1e5aa17b87a2..67e70fdc27cf31 100644 --- a/Lib/multiprocessing/shared_memory.py +++ b/Lib/multiprocessing/shared_memory.py @@ -71,8 +71,9 @@ class SharedMemory: _flags = os.O_RDWR _mode = 0o600 _prepend_leading_slash = True if _USE_POSIX else False + _track = True - def __init__(self, name=None, create=False, size=0): + def __init__(self, name=None, create=False, size=0, *, track=True): if not size >= 0: raise ValueError("'size' must be a positive integer") if create: @@ -82,6 +83,7 @@ def __init__(self, name=None, create=False, size=0): if name is None and not self._flags & os.O_EXCL: raise ValueError("'name' can only be None if create=True") + self._track = track if _USE_POSIX: # POSIX Shared Memory @@ -116,8 +118,8 @@ def __init__(self, name=None, create=False, size=0): except OSError: self.unlink() raise - - resource_tracker.register(self._name, "shared_memory") + if self._track: + resource_tracker.register(self._name, "shared_memory") else: @@ -236,12 +238,20 @@ def close(self): def unlink(self): """Requests that the underlying shared memory block be destroyed. - In order to ensure proper cleanup of resources, unlink should be - called once (and only once) across all processes which have access - to the shared memory block.""" + Unlink should be called once (and only once) across all handles + which have access to the shared memory block, even if these + handles belong to different processes. Closing and unlinking may + happen in any order, but trying to access data inside a shared + memory block after unlinking may result in memory errors, + depending on platform. + + This method has no effect on Windows, where the only way to + delete a shared memory block is to close all handles.""" + if _USE_POSIX and self._name: _posixshmem.shm_unlink(self._name) - resource_tracker.unregister(self._name, "shared_memory") + if self._track: + resource_tracker.unregister(self._name, "shared_memory") _encoding = "utf8" diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py index ec003d8dc4314d..a94eb6c0ae4b8e 100644 --- a/Lib/test/_test_multiprocessing.py +++ b/Lib/test/_test_multiprocessing.py @@ -4455,6 +4455,59 @@ def test_shared_memory_cleaned_after_process_termination(self): "resource_tracker: There appear to be 1 leaked " "shared_memory objects to clean up at shutdown", err) + @unittest.skipIf(os.name != "posix", "resource_tracker is posix only") + def test_shared_memory_untracking(self): + # gh-82300: When a separate Python process accesses shared memory + # with track=False, it must not cause the memory to be deleted + # when terminating. + cmd = '''if 1: + import sys + from multiprocessing.shared_memory import SharedMemory + mem = SharedMemory(create=False, name=sys.argv[1], track=False) + mem.close() + ''' + mem = shared_memory.SharedMemory(create=True, size=10) + # The resource tracker shares pipes with the subprocess, and so + # err existing means that the tracker process has terminated now. + try: + rc, out, err = script_helper.assert_python_ok("-c", cmd, mem.name) + self.assertNotIn(b"resource_tracker", err) + self.assertEqual(rc, 0) + mem2 = shared_memory.SharedMemory(create=False, name=mem.name) + mem2.close() + finally: + try: + mem.unlink() + except OSError: + pass + mem.close() + + @unittest.skipIf(os.name != "posix", "resource_tracker is posix only") + def test_shared_memory_tracking(self): + # gh-82300: When a separate Python process accesses shared memory + # with track=True, it must cause the memory to be deleted when + # terminating. + cmd = '''if 1: + import sys + from multiprocessing.shared_memory import SharedMemory + mem = SharedMemory(create=False, name=sys.argv[1], track=True) + mem.close() + ''' + mem = shared_memory.SharedMemory(create=True, size=10) + try: + rc, out, err = script_helper.assert_python_ok("-c", cmd, mem.name) + self.assertEqual(rc, 0) + self.assertIn( + b"resource_tracker: There appear to be 1 leaked " + b"shared_memory objects to clean up at shutdown", err) + finally: + try: + mem.unlink() + except OSError: + pass + resource_tracker.unregister(mem._name, "shared_memory") + mem.close() + # # Test to verify that `Finalize` works. # diff --git a/Misc/NEWS.d/next/Library/2023-10-12-18-19-47.gh-issue-82300.P8-O38.rst b/Misc/NEWS.d/next/Library/2023-10-12-18-19-47.gh-issue-82300.P8-O38.rst new file mode 100644 index 00000000000000..d7e6b225489b99 --- /dev/null +++ b/Misc/NEWS.d/next/Library/2023-10-12-18-19-47.gh-issue-82300.P8-O38.rst @@ -0,0 +1 @@ +Add ``track`` parameter to :class:`multiprocessing.shared_memory.SharedMemory` that allows using shared memory blocks without having to register with the POSIX resource tracker that automatically releases them upon process exit.