Generate qlog traces

[Ralith]

qlog is a draft standard for encoding protocol event traces. It might be useful for Quinn to have conditionally compiled support for generating qlog traces to debug complex behavior using qlog-oriented tooling.

[stammw]

Adding an interop tag here because qlog generation is part of the interop listed test under L.

There is an existing qlog implementation that we could integrate.

[djc]

That would make sense, I think!

[Ralith]

Awesome find! That'll make this a lot easier.

[euberdeveloper]

Any news on this?

[Ralith]

Nobody's taken it on yet.

[kixelated]

I'll implement this eventually. qlog was instrumental for debugging and diagnosing congestion control in my (closed source) QUIC implementation.

qlog traces are MASSIVE even with gzip, which also uses a lot of CPU. You would need the ability to enable qlog on a per-connection basis, and it has to be done early so you don't drop those initial events.

I was thinking:

/// Enable logging for events that could not be associated with a connection (ex. unknown packets)
Endpoint::enable_qlog(w: AsyncWrite)

/// Enable logging for all events associated with this connection
Connecting::enable_qlog(w: AsyncWrite)

Most of the events will be emitted inside a Mutex and a a blocking filesystem write would be devastating. AsyncWrite is used mostly so the compiler would catch those.

A filter would also be cool, as most qlog events/fields are optional and some are very verbose. For example, you can emit base64 encoded packet/steam contents with the raw field, but that's extremely situational. It's not as simple as a verbosity level because the most verbose events are often unrelated.

[Ralith]

Note qvis reportedly does not plan to implement qlog 0.4; they're sticking with 0.3 until a 1.0 RC is specced.

You would need the ability to enable qlog on a per-connection basis

I'm not sure about this. I'd imagine usually you're either spinning up an endpoint purely for testing, or don't want qlog at all.

[marcblanchet]

support. very useful for interop and special uses. would love to get