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Connlib's routing logic and networking code is entirely platform agnostic. The only platform-specific bit is how we interact with the TUN device. From connlib's perspective though, all it needs is an interface for reading and writing. How the device gets initialised and updated is client-business. For the most part, this is the same on all platforms: We call callbacks and the client updates the state accordingly. The only annoying bit here is that Android recreates the TUN interface on every update and thus our old file descriptor is invalid. The current design works around this by returning the new file descriptor on Android. This is a problematic design for several reasons: - It forces the callback handler to finish synchronously, and halting connlib until this is complete. - The synchronous nature also means we cannot replace the callbacks with events as events don't have a return value. To fix this, we introduce a new `set_tun` method on `Tunnel`. This moves the business of how the `Tun` device is created up to the client. The clients are already platform-specific so this makes sense. In a future iteration, we can move all the various `Tun` implementations all the way up to the client-specific crates, thus co-locating the platform-specific code. Initialising `Tun` from the outside surfaces another issue: The routes are still set via the `Tun` handle on Windows. To fix this, we introduce a `make_tun` function on `TunDeviceManager` in order for it to remember the interface index on Windows and being able to move the setting of routes to `TunDeviceManager`. This simplifies several of connlib's APIs which are now infallible. Resolves: #4473. --------- Co-authored-by: Reactor Scram <ReactorScram@users.noreply.github.com> Co-authored-by: conectado <gabrielalejandro7@gmail.com>