Connection timeout does not cause origin to be marked down

[vuori]

As far as I understand from the documentation, hosts that are unresponsive are supposed to be marked as "down" in hostdb and retried according to the connect_attempts_max_retries_down_server setting. However with the following steps this does not seem to be happening:

1. Attempt to make a forward-proxied request to a host that is not responding at all (error is ETIMEDOUT).
2. Note in trace log that connect_attempts_max_retries attempts are made.
3. Try the same request again.
4. connect_attempts_max_retries attempts are still made, the count never goes down to connect_attempts_max_retries_down_server.

In particular if the latter is set to 0, ATS still always tries to connect and the trace message Not connecting to the server because it is marked down from HttpSM.cc never appears.

Is this a bug or am I misunderstanding how hostdb is supposed to work?