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A failure of PyUnicode_AsUTF8AndSize() in various fromisoformat() functions in Modules/_datetimemodule.c leads to NULL dereference due to the missing check, e.g.:
>>> from datetime import date
>>> date.fromisoformat('\ud800')
Segmentation fault (core dumped)
This is similar to msg123474. The missing NULL check was reported by Svace static analyzer.
While preparing tests for this issue, I've discovered a deeper problem. The C datetime implementation uses PyUnicode_AsUTF8AndSize() in several places, making some functions reject strings containing surrogate code points (0xD800 - 0xDFFF) since they can't be encoded in UTF-8. On the other hand, the pure-Python datetime implementation doesn't have this restriction. For example:
>>> import sys
>>> sys.modules['_datetime'] = None # block C implementation
>>> from datetime import time
>>> time().strftime('\ud800')
'\ud800'
>>> del sys.modules['datetime']
>>> del sys.modules['_datetime']
>>> from datetime import time
>>> time().strftime('\ud800')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'utf-8' codec can't encode character '\ud800' in position 0: surrogates not allowed
My PR (coming soon) doesn't address this difference but focuses on fixing the immediate problem instead. Suggestions are appreciated.