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This issue is a regression introduced by r72208 to fix the issue #3672.
Attached patch fixes PyUnicode_EncodeUTF8() if unicode_encode_call_errorhandler() returns an unicode string (eg. backslackreplace error handler). I don't know unicodeobject.c code (very well), and my patch should be far from being perfect.
I suppose that the maximum length of an escaped characters is 8 bytes (xmlcharrefreplace error error for U+DFFFF). When the first lone surrogate is found, reallocate the buffer to size*8 bytes. The escaped character have to be an ASCII character or an UnicodeEncodeError is raised.
Note: unicode_encode_ucs1() doesn't have hardcoded for the maximum length ot escaped string. Its code might be reused in PyUnicode_EncodeUTF8() to remove the hardcoded limits.