HttpParser and multiple concatenated HTTP responses

Hi!

If my understanding is correct HttpParser.execute returns the number of bytes consumed, hence it is possible to parse multiple concatenated HTTP responses by checking is_message_complete() and starting a new parser from the same very position the last one stopped.

However the observed behavior is very different: execute() happily consumes the whole string, the subsequent responses text ends up in the message body and _clen_rest is negative.

首先复现围绕 HttpParser.execute() 和 is_message_complete() 的连续响应场景，然后检查响应结束时 _clen_rest 是如何更新的。完成的标准是，解析器报告第一个响应的边界，而不会将后续响应文本放入 body，这样另一个解析器就可以从返回的位置开始解析。

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