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10ba02e341
We appear to have caused a pretty big performance regression (~40%) in037a2e64b6(identified through `git-bisect`). Specifically, the regression appears to have been caused by [`aef411a` (#7605)](aef411abf5). Weirdly enough, undoing just that on top of `main` doesn't fix the regression. My hypothesis is that using the same file descriptor for read AND write interests on the same runtime causes issues because those interests are occasionally cleared (i.e. on false-positive wake-ups). In this PR, we spawn a dedicated thread each for the sending and receiving operations of the TUN device. On unix-based systems, a TUN device is just a file descriptor and can therefore simply be copied and read & written to from different threads. Most importantly, we only construct the `AsyncFd` _within_ the newly spawned thread and runtime because constructing an `AsyncFd` implicitly registers with the runtime active on the current thread. As a nice benefit, this allows us to get rid of a `future::select`. Those are always kind of nasty because they cancel the future that wasn't ready. My original intuition was that we drop packets due to cancelled futures there but that could not be confirmed in experiments.