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

Comienza leyendo PEP 293 y siguiendo el comportamiento de los encoder/decoder error-callbacks descrito en el issue. Investiga cómo se gestionan los valores newpos devueltos cuando son iguales o inferiores a start y, después, añade cobertura para los casos de bucle y de posición no válida indicados. Se considerará terminado cuando estos callbacks ya no provoquen un procesamiento que no avance ni un almacenamiento en búfer ilimitado.