Issue

ShareableList has two issues:

1. It uses len(item) (character count) for string slot allocation instead of len(item.encode('utf-8')) (byte count), causing UnicodeDecodeError with multi-byte UTF-8 characters.
2. It uses rstrip(b'\x00') to recover bytes values, which strips legitimate trailing null bytes.

Reproducer

from multiprocessing.shared_memory import ShareableList

# String corruption
sl = ShareableList(['0\U00010000\U00010000'])
sl[0]  # UnicodeDecodeError

# Bytes corruption
sl = ShareableList([b'\x00'])
sl[0]  # b'' instead of b'\x00'

Fix

Use len(item.encode('utf-8')) for string slot allocation. For bytes, store the actual byte length in the format metadata so retrieval reads exactly the right number of bytes without needing rstrip(b'\x00').

Test updates

Two assertions in test_shared_memory_ShareableList_basics needed adjustments since they were based on the previous behavior:

• Format string assertions updated to reflect actual byte lengths stored for bytes values.
• Removed format comparison between original and copy, as copies may allocate differently with byte-accurate lengths.

• Issue: ShareableList corrupts multi-byte UTF-8 strings and bytes with trailing nulls #145261