[GitHub] [trafficserver] midchildan opened a new issue, #9687: TS_MILESTONE_CACHE_OPEN_READ_END is unavailable in TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK after 9.1.x

["midchildan (via GitHub)" <gi...@apache.org>]

midchildan opened a new issue, #9687:
URL: https://github.com/apache/trafficserver/issues/9687

   Prior to 9.1.x, HTTP transactions reached `TS_MILESTONE_CACHE_OPEN_READ_END` before `TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK`. However, after 9.1.x, the order was reversed. This has affected one of our plugins because it was failing to obtain the correct timestamp of the completion of cache reads in a timely manner.
   ​
   The exact change that caused this was made in #8168. Previously, `TS_MILESTONE_CACHE_OPEN_READ_END` was placed at the beginning of `HttpSM::state_cache_open_read`. After the change, it was moved to the end of the same function. The problem is that `state_cache_open_read` now calls into the HTTP state machine before it reaches the milestone [1] [2]. Once the state machine is invoked, the HTTP transaction will progress further and won't return until it triggers an asynchronous operation along the way.
   ​
   The new timing for `TS_MILESTONE_CACHE_OPEN_READ_END` varies depending on the result of the cache lookup. When it misses, the milestone will be reached as ATS attempts to connect to the origin server. When it's a hit, the milestone will be delayed even further, past the point where it prepares to send a response back to the client in `TS_HTTTP_SEND_RESPONSE_HDR_HOOK`.
   ​
   Is it okay to revert to the previous behavior by moving the milestone back to its original location? Or were there any problems with it?
   ​
   [1]: https://github.com/apache/trafficserver/blob/4cf91f8acd7290e960edefc6b5f87d8a50d2271f/proxy/http/HttpSM.cc#L2746
   [2]: https://github.com/apache/trafficserver/blob/4cf91f8acd7290e960edefc6b5f87d8a50d2271f/proxy/http/HttpSM.cc#L2766


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