TS_MILESTONE_CACHE_OPEN_READ_END is unavailable in TS_HTTP_CACHE_LOOKUP_COMPLETE_HOOK after 9.1.x

[midchildan]

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. 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? ​

[github-actions[bot]]

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