Debugging build crashes in some circumstances in str.decode() with error handler which produces replacement string with length larger than malformed data. For example the backslashreplace error handler produces 4-character string for every illegal byte. All other standard error handlers produce no longer than 1 character for every illegal unit. Here is a patch which fixes this issue. I'll commit it without review because buildbots are broken without it. This issue is open for reference and post-commit review.
> Debugging build crashes in some circumstances in str.decode() (...) buildbots are broken without it Is it a regression? Would it be possible to identify the changeset responsible of the regression?
I think the changeset which made decoders to use _PyUnicodeWriter () is responsible of the regression. For example consider b'\x80abc'.decode('utf-8', 'backslashreplace'). The writer reserves string buffer with size 4 (every byte produces at most 1 character). First byte is incorrect and replaced by 4-character string '\\x80'. The writer increases min_length but doesn't resize the buffer because its size is enough to write replacement string. But following writes of ASCII characters cause buffer overflow.
This issue has been migrated to GitHub: https://github.com/python/cpython/issues/67510 
