Patch is good, but while we're at it, is there any reason why this multi-allocation design was even used? It PyMem_Mallocs a buffer, makes a C-style string in it, then uses PyUnicode_FromString to convert C-style string to Python str.

Seems like the correct approach would be to just use PyUnicode_New to preallocate the final string buffer up front, then pull out the internal buffer with PyUnicode_1BYTE_DATA and populate that directly, saving a pointless allocation/deallocation, which also means the failure case means no cleanup needed at all, while barely changing the code (aside from removing the need to explicitly NUL terminate).

Only reason I can see to avoid this would be if the codec names could contain arbitrary Unicode encoded as UTF-8 (and therefore strlen wouldn't tell you the final length in Unicode ordinals), but I'm pretty sure that's not the case (if it is, we're not normalizing properly, since we only lower case ASCII). If Unicode codec names need to be handled, there are other options, though the easy savings go away.