gh-92408: cleanup during error handling for POSIX shared memory

`multiprocessing.shared_memory.SharedMemory` currently tries to unlink a POSIX
shared memory object if it fails to `mmap` it after opening/creating it.

However, since it only catches `OSError`, it fails to do so when opening an
existing zero-length shared memory object, as `mmap.mmap` raises a `ValueError`
when given a 0 size.

Fix this by catching all exceptions with a bare `except`, so the cleanup is run
for all errors.

Author: duaneg

Lib/multiprocessing/shared_memory.py

@@ -115,7 +115,7 @@ def __init__(self, name=None, create=False, size=0, *, track=True):
                 stats = os.fstat(self._fd)
                 size = stats.st_size
                 self._mmap = mmap.mmap(self._fd, size)
-            except OSError:
+            except:
                 self.unlink()
                 raise
             if self._track:

Lib/test/_test_multiprocessing.py

@@ -4851,6 +4851,23 @@ def test_shared_memory_tracking(self):
             resource_tracker.unregister(mem._name, "shared_memory")
             mem.close()
 
+    @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)
+
+        # ...however it should delete the shared memory as part of its cleanup
+        with self.assertRaises(FileNotFoundError):
+            _posixshmem.shm_open(name, os.O_RDWR)
+
 #
 # Test to verify that `Finalize` works.
 #