[quicwg/base-drafts] Separate HTTP/3 stream errors from connection errors. (#2911)

Daan De Meyer <notifications@github.com> Sat, 20 July 2019 14:46 UTC

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Subject: [quicwg/base-drafts] Separate HTTP/3 stream errors from connection errors. (#2911)
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With the recent changes in HTTP/3 errors, it becomes possible to completely divide errors into stream errors and connection errors, with each error being either a stream error or connection error. As a result, we can also remove the possibility to treat a stream error as a connection error which allows implementations to determine whether an error is a stream error or a connection error based solely on its type.

As far as I can determine from the draft, only HTTP_STREAM_CREATION_ERROR is used as both a stream error and a connection error. It is only used as a stream error when a peer opens a unidirectional stream of an unknown type. If a new error HTTP_UKNOWN_STREAM_TYPE is split of as a stream error from HTTP_STREAM_CREATION_ERROR, HTTP_STREAM_CREATION_ERROR can be classed as solely a connection error.

[This analysis](https://github.com/quicwg/base-drafts/pull/2662#issuecomment-507529717) from @kazuho further lists HTTP_NO_ERROR and HTTP_GENERAL_PROTOCOL_ERROR as errors that can be both connection and stream errors. I think these can be safely classed as solely connection errors. 

HTTP_NO_ERROR is only defined in the draft for usage in CONNECTION_CLOSE, I'm not sure when it would be used as a stream error (we have more specific error codes to indicate intent when closing a stream).

HTTP_GENERAL_PROTOCOL_ERROR is only used as a stream error in the section discussing malformed requests/responses. I'd suggest using a more descriptive error for this case (HTTP_MALFORMED_MESSAGE  for example) and treating HTTP_GENERAL_PROTOCOL_ERROR as solely a connection error.

An error being either a connection error or a stream error simplifies implementation as they don't have to check on every error if it was communicated as a stream error or a connection error. Upgrading a stream error to a connection error is not a sensible approach for the existing stream errors (for example, closing the connection because of `REQUEST_CANCELLED` does not sound like a reasonable approach). If an implementation chooses to treat a stream error as a connection error, perhaps the INTERNAL_ERROR code should be used instead as not being able to handle a stream error without closing the connection is an implementation issue and should be treated as a bug.

There's probably some things I'm overlooking. Any feedback is welcome.

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