Allow zero-length shared memory objects

These are allowed by POSIX and it seems more consistent/cleaner to support them
than forcing the user to treat them as a special case.

Windows does not support creating a zero-length mapping, and neither does
`mmap` on any platform, so skip those operations and assign a zero-length
buffer instead.

TODO: The way the POSIX/Windows code is interleaved inside if statements makes
for difficult to follow and deeply nested logic. It might be better to refactor
this code and keep the platform-specific logic in separate functions or
possibly even implementation classes.

Author: duaneg

Lib/multiprocessing/shared_memory.py

@@ -74,12 +74,10 @@ class SharedMemory:
     _track = True
 
     def __init__(self, name=None, create=False, size=0, *, track=True):
-        if not size >= 0:
-            raise ValueError("'size' must be a positive integer")
+        if size < 0:
+            raise ValueError("'size' must be a non-negative integer")
         if create:
             self._flags = _O_CREX | os.O_RDWR
-            if size == 0:
-                raise ValueError("'size' must be a positive number different from zero")
         if name is None and not self._flags & os.O_EXCL:
             raise ValueError("'name' can only be None if create=True")
 
@@ -114,7 +112,8 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
                     os.ftruncate(self._fd, size)
                 stats = os.fstat(self._fd)
                 size = stats.st_size
-                self._mmap = mmap.mmap(self._fd, size)
+                if size > 0:
+                    self._mmap = mmap.mmap(self._fd, size)
             except:
                 if create:
                     _posixshmem.shm_unlink(self._name)
@@ -126,7 +125,7 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
 
             # Windows Named Shared Memory
 
-            if create:
+            if create and size > 0:
                 while True:
                     temp_name = _make_filename() if name is None else name
                     # Create and reserve shared memory block with this name
@@ -156,7 +155,9 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
                         _winapi.CloseHandle(h_map)
                     self._name = temp_name
                     break
-
+            elif create and size == 0:
+                # TODO: Leave as None?
+                self._name = _make_filename() if name is None else name
             else:
                 self._name = name
                 # Dynamically determine the existing named shared memory
@@ -180,10 +181,14 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
                     size = _winapi.VirtualQuerySize(p_buf)
                 finally:
                     _winapi.UnmapViewOfFile(p_buf)
-                self._mmap = mmap.mmap(-1, size, tagname=name)
+                if size > 0:
+                    self._mmap = mmap.mmap(-1, size, tagname=name)
 
         self._size = size
-        self._buf = memoryview(self._mmap)
+        if size > 0:
+            self._buf = memoryview(self._mmap)
+        else:
+            self._buf = memoryview(b'')
 
     def __del__(self):
         try:

Lib/test/_test_multiprocessing.py

@@ -4447,14 +4447,6 @@ def test_invalid_shared_memory_creation(self):
         with self.assertRaises(ValueError):
             sms_invalid = shared_memory.SharedMemory(create=True, size=-1)
 
-        # Test creating a shared memory segment with size 0
-        with self.assertRaises(ValueError):
-            sms_invalid = shared_memory.SharedMemory(create=True, size=0)
-
-        # Test creating a shared memory segment without size argument
-        with self.assertRaises(ValueError):
-            sms_invalid = shared_memory.SharedMemory(create=True)
-
     def test_shared_memory_pickle_unpickle(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             with self.subTest(proto=proto):
@@ -4871,21 +4863,12 @@ def test_existing_shm_not_unlinked_on_error(self):
             with self.assertRaises(OSError):
                 shared_memory.SharedMemory(name, create=False)
 
-    @unittest.skipIf(os.name != "posix", "posix-only test w/ empty file")
-    def test_cleanup_zero_length_shared_memory(self):
-        import _posixshmem
-
-        name = self._new_shm_name("test_init_cleanup")
-        mem = _posixshmem.shm_open(name, os.O_RDWR | os.O_CREAT | os.O_EXCL, 0o600)
-        os.close(mem)
-
-        # First time the mmap.mmap fails with a ValueError, as the shared
-        # memory is zero-length and hence the mmap fails...
-        with self.assertRaises(ValueError):
-            shared_memory.SharedMemory(name, create=False)
-
-        # ...but it should NOT delete the shared memory as part of its cleanup
-        _posixshmem.shm_unlink(name)
+    def test_zero_length_shared_memory(self):
+        name = self._new_shm_name("test_zero_length_shared_memory")
+        mem = shared_memory.SharedMemory(name, create=True, size=0)
+        self.addCleanup(mem.unlink)
+        self.assertEqual(mem.size, 0)
+        self.assertEqual(len(mem.buf), 0)
 
 #
 # Test to verify that `Finalize` works.