[3.8] bpo-40330: Fix utf-8 size check in ShareableList (GH-19606) (GH-19625)

The item size must be checked after encoding to bytes, not before.

Automerge-Triggered-By: @pitrou.
(cherry picked from commit eba9f6155df59c9beed97fb5764c9f01dd941af0)

Co-authored-by: Antoine Pitrou <antoine@python.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/Lib/multiprocessing/shared_memory.py b/Lib/multiprocessing/shared_memory.py
index 184e367..f92eb01 100644
--- a/Lib/multiprocessing/shared_memory.py
+++ b/Lib/multiprocessing/shared_memory.py
@@ -433,9 +433,12 @@ class ShareableList:
 
         if not isinstance(value, (str, bytes)):
             new_format = self._types_mapping[type(value)]
+            encoded_value = value
         else:
-            if len(value) > self._allocated_bytes[position]:
-                raise ValueError("exceeds available storage for existing str")
+            encoded_value = (value.encode(_encoding)
+                             if isinstance(value, str) else value)
+            if len(encoded_value) > self._allocated_bytes[position]:
+                raise ValueError("bytes/str item exceeds available storage")
             if current_format[-1] == "s":
                 new_format = current_format
             else:
@@ -448,8 +451,7 @@ class ShareableList:
             new_format,
             value
         )
-        value = value.encode(_encoding) if isinstance(value, str) else value
-        struct.pack_into(new_format, self.shm.buf, offset, value)
+        struct.pack_into(new_format, self.shm.buf, offset, encoded_value)
 
     def __reduce__(self):
         return partial(self.__class__, name=self.shm.name), ()