Based on some discussions here at the summit, we would like to add the ability for the core to report certain error conditions to plugins. We have at least one use case where it is critical to be able to distinguish different causes for a 502 error (origin server down, cache write lock missed, etc.).
The basic concept is the core adds a header to the transaction which reports the error. This lets plugins do special handling without embedding that (or related configuration values) in the core and potentially to log those conditions. There are two approaches. The first is to have a single header, @ats-internal, which is used for reporting. This would be a comma list in the standard HTTP header way with each condition code required to be unique. For instance, a cache write lock miss would be "CACHE_WL_MISS" yielding '@ats-internal: CACHE_WL_MISS". This is easier to log but a bit harder to process for different errors (since you have to search the values). The second is to designate another character as "do not print this header", such as '%', and reserve that for the ATS core. In this case we can use the header namespace without concern for name collisions. The previous case would then be "%cache-doc-error: WRITE_LOCK_FAILED". This is harder to log but easier to detect a specific condition you want to handle. Any comments are welcome.
