Read Response Retry Regression

[moonchen]

We noticed that some requests were generating HTTP 502 errors. The cause is the origin server closed an HTTP/1 keep-alive connection. It happens as follows.

1. An origin connection is released from the session pool.
2. ATS buffers a request.
3. Immediately after, ATS enters state_read_server_response_header
4. Somewhere between steps 1 and 5, the server decides to close the connection, probably due to a keep alive time out, before the request arrives.
5. ATS reads from the socket, and gets an EOS.6. The EOS is handled as follows

   trafficserver/src/proxy/http/HttpSM.cc

   Line 1942 in 2e244e5

   t_state.current.retry_attempts.maximize(t_state.configured_connect_attempts_max_retries());

Note that EOS falls through, and retries are disabled from this point onwards. This behavior was introduced in PR #9366 Http2 to origin. This issue can be mitigated by reducing the keep alive timeout in ATS so it's lower than the origin's keep alive timeout.

I think ATS should provide an option to retry a request if an invalid response has been received.

[bryancall]

This was fixed by PR #12446 "Retry connect if server closes before receiving response", merged on 2025-08-19 and first released in 10.1.1.

The regression was that the VC_EVENT_EOS case in state_read_server_response_header fell through into the READ_READY/READ_COMPLETE case, which unconditionally maximized retry_attempts and disabled retries even when zero response bytes had been received. PR #12446 gave VC_EVENT_EOS its own branch and now only disables retries when some response bytes have already arrived (if (server_response_hdr_bytes > 0)), restoring the 9.2 behavior of retrying when the origin closes a keep-alive connection before any response is read. You can see the current behavior on master here: https://github.com/apache/trafficserver/blob/master/src/proxy/http/HttpSM.cc

This addresses the exact 502 case described, so I am closing it. If a configurable knob for the bytes-already-received case is still wanted, that can be tracked as a separate enhancement.