Codec error handling callback should return newpos > start

According to PEP 293, the codec error handling callback should either raise an exception or return a pair (replacement, newpos). newpos specifies a new position within object, where the encoder/decoder will continue encoding/decoding. Negative values for newpos are treated as being relative to end of object. PEP 293 does not specify the bound of newpos, and this is a problem.

If newpos == start, the codec will not advance. If replacement is empty, it will just hang in an infinite loop, calling the callback again and again. If replacement is not empty, it will additionally add it in an internal buffer, and finally, after long time, you will get a MemoryError. If newpos < start, the behavior may be even more weird.

Before fixing other errors in #81000 it even caused a crash, but now it is only an infinite loop.

I propose to limit the returning value by range [start+1, len(object)].

@malemburg, @doerwalter, @vstinner

まず PEP 293 を読み、issue に記載されている encoder/decoder error-callback の動作を追跡します。返された newpos の値が start と等しいか、start より小さい場合にどのように扱われるかを調査し、その後、報告されたループと無効な位置のケースをカバレッジに追加します。これらの callbacks が処理を進行させない状態や無制限のバッファリングを引き起こさなくなれば完了です。