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Reading and writing to the TUN device within `connlib` happens in a separate thread. The task running within these threads is connected to the rest of `connlib` via channels. When the application shuts down, these threads also need to exit. Currently, we attempt to detect this from within the task when these channels close. It appears that there is a race condition here because we first attempt to read from the TUN device before reading from the channels. We treat read & write errors on the TUN device as non-fatal so we loop around and attempt to read from it again, causing an infinite-loop and log spam. To fix this, we swap the order in which we evaluate the two concurrent tasks: The first task to be polled is now the channel for outbound packets and only if that one is empty, we attempt to read new packets from the TUN device. This is also better from a backpressure point of view: We should attempt to flush out our local buffers of already processed packets before taking on "new work". As a defense-in-depth strategy, we also attempt to detect the particular error from the tokio runtime when it is being shut down and exit the task. Resolves: #7601. Related: https://github.com/tokio-rs/tokio/issues/7056.