gh-161098: Unlink the new ShareableList block if packing a value fails (GH-161099)

ShareableList(sequence) created its shared memory block before packing the
values, so an unpackable value (an int outside int64, a lone surrogate)
left the block behind with no way for the caller to release it.

Author: gpshead

Lib/multiprocessing/shared_memory.py

@@ -349,31 +349,40 @@ def __init__(self, sequence=None, *, name=None):
 
         if sequence is not None:
             _enc = _encoding
-            struct.pack_into(
-                "q" + self._format_size_metainfo,
-                self.shm.buf,
-                0,
-                self._list_len,
-                *(self._allocated_offsets)
-            )
-            struct.pack_into(
-                "".join(_formats),
-                self.shm.buf,
-                self._offset_data_start,
-                *(self._encode_value(v) for v in sequence)
-            )
-            struct.pack_into(
-                self._format_packing_metainfo,
-                self.shm.buf,
-                self._offset_packing_formats,
-                *(v.encode(_enc) for v in _formats)
-            )
-            struct.pack_into(
-                self._format_back_transform_codes,
-                self.shm.buf,
-                self._offset_back_transform_codes,
-                *(_recreation_codes)
-            )
+            try:
+                struct.pack_into(
+                    "q" + self._format_size_metainfo,
+                    self.shm.buf,
+                    0,
+                    self._list_len,
+                    *(self._allocated_offsets)
+                )
+                struct.pack_into(
+                    "".join(_formats),
+                    self.shm.buf,
+                    self._offset_data_start,
+                    *(self._encode_value(v) for v in sequence)
+                )
+                struct.pack_into(
+                    self._format_packing_metainfo,
+                    self.shm.buf,
+                    self._offset_packing_formats,
+                    *(v.encode(_enc) for v in _formats)
+                )
+                struct.pack_into(
+                    self._format_back_transform_codes,
+                    self.shm.buf,
+                    self._offset_back_transform_codes,
+                    *(_recreation_codes)
+                )
+            except BaseException:
+                # A value of a supported type may still be unpackable, such as
+                # an int outside the signed 64-bit range.  The block was created
+                # above and the caller gets no object to release it with, so
+                # destroy it here.
+                self.shm.close()
+                self.shm.unlink()
+                raise
 
         else:
             self._list_len = len(self)  # Obtains size from offset 0 in buffer.

Lib/test/_test_multiprocessing.py

@@ -5218,6 +5218,23 @@ def test_shared_memory_ShareableList_basics(self):
         finally:
             empty_sl.shm.unlink()
 
+    def test_shared_memory_ShareableList_unpackable_value(self):
+        # Values of a supported type that cannot be packed must not leave
+        # the newly created shared memory block behind.
+        name = self._new_shm_name('test03_unpackable')
+        for value, exc in (
+            (2**63, struct.error),
+            (-2**63 - 1, struct.error),
+        ):
+            with self.subTest(value=value):
+                with self.assertRaises(exc):
+                    shared_memory.ShareableList(['abc', value], name=name)
+                if sys.platform != "win32":
+                    with self.assertRaises(FileNotFoundError):
+                        # Should not be there to be attached to.
+                        leftover = shared_memory.SharedMemory(name)
+                        leftover.unlink()  # Error should occur on prior line.
+
     def test_shared_memory_ShareableList_pickling(self):
         for proto in range(pickle.HIGHEST_PROTOCOL + 1):
             with self.subTest(proto=proto):

Misc/NEWS.d/next/Library/2026-10-22-05-20-27.gh-issue-161098.voMVnd.rst

@@ -0,0 +1,3 @@
+Fix :class:`multiprocessing.shared_memory.ShareableList` leaving its newly
+created shared memory block behind when a value cannot be stored, such as an
+integer outside the signed 64-bit range.