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

首先阅读 PEP 293，并跟踪 issue 中描述的 encoder/decoder error-callback 行为。调查返回的 newpos 值等于或小于 start 时的处理方式，然后为报告的循环和无效位置情况添加覆盖。完成的标准是这些 callbacks 不再导致处理不前进或无限缓冲。