Round up the allocated size instead of rejecting the exact fit

Allocating one extra alignment block for a value whose encoded size is
a multiple of the alignment made a copy of the list wider than the
original.  Round the size up instead, so that it is the same for the
same value, and keep allowing a value which fills the slot exactly:
rejecting it broke setting an 8-byte string in a slot of an integer.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/multiprocessing/shared_memory.py

@@ -319,7 +319,7 @@ def __init__(self, sequence=None, *, name=None):
                 self._types_mapping[type(item)]
                     if not isinstance(item, (str, bytes))
                     else self._types_mapping[type(item)] % (
-                        self._alignment * (len(self._encode_value(item)) // self._alignment + 1),
+                        self._alignment * ((len(self._encode_value(item)) - 1) // self._alignment + 1),
                     )
                 for item in sequence
             ]
@@ -471,7 +471,7 @@ def __setitem__(self, position, value):
             allocated_length = self._allocated_offsets[position + 1] - item_offset
 
             encoded_value = self._encode_value(value)
-            if len(encoded_value) >= allocated_length:
+            if len(encoded_value) > allocated_length:
                 raise ValueError("bytes/str item exceeds available storage")
             if current_format[-1] == "s":
                 new_format = current_format

Lib/test/_test_multiprocessing.py

@@ -5018,11 +5018,12 @@ def test_shared_memory_ShareableList_basics(self):
                                     "exceeds available storage"):
             sl[4] = 'far too many'
         self.assertEqual(sl[4], 'some')
-        sl[0] = 'encodé'
-        self.assertEqual(sl[0], 'encodé')  # no spillage
+        sl[0] = 'éncodé'  # Exactly 8 bytes of UTF-8 data
+        self.assertEqual(sl[0], 'éncodé')
+        self.assertEqual(sl[1], b'HoWdY')  # no spillage
         with self.assertRaisesRegex(ValueError,
                                     "exceeds available storage"):
-            sl[0] = 'encodés'  # Exactly 8 bytes of UTF-8 data
+            sl[0] = 'éncodés'  # Exactly 9 bytes of UTF-8 data
         self.assertEqual(sl[1], b'HoWdY')
         with self.assertRaisesRegex(ValueError,
                                     "exceeds available storage"):

Misc/NEWS.d/next/Library/2021-05-24-14-41-29.bpo-44170._v28mv.rst

@@ -1 +1 @@
-Fix UnicodeDecodeError with multibyte utf8 characters in ShareableList.
+Fix UnicodeDecodeError with multibyte utf8 characters in ShareableList.