Crash in `Objects/unicodeobject.c::_copy_characters` when there is nothing to copy (DEBUG build only)

[shadchin]

Crash report

What happened?

Reproduce on Python 3.12.8+ or 3.13.1+, main work fine, but there is a problem there too:

./configure --with-pydebug
make
./python --version
Python 3.13.1+
./python -c 'import datetime as dt; dt.datetime(2013, 11, 10, 14, 20, 59).strftime("%z")'
Segmentation fault

No need to check to if we don't write there

Fix in #127876

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.13.1+ (heads/3.13:d51c1444e3, Dec 12 2024, 19:36:26) [GCC 9.4.0]

Linked PRs

• GH-127903: Fix a crash on DEBUG builds when calling _copy_characters #127876
• [3.13] gh-127903: Fix a crash on debug builds when calling Objects/unicodeobject::_copy_characters (GH-127876) #128458
• [3.12] gh-127903: Fix a crash on debug builds when calling Objects/unicodeobject::_copy_characters (GH-127876) #128459

[mgorny]

Just hit this in the wild with pikepdf.