Function exec_sync_output 

async fn exec_sync_output(
    io: &Arc<ContainerIO>,
    token: &CancellationToken,
    exit_rx: Receiver<ExitChannelData>,
    kill: impl FnOnce(),
) -> Result<ExecSyncOutput>

Read the output of an exec while waiting for its exit, and take the result from the watcher alone. The watcher cancels the token once it reaped the process or killed it at its timeout, the drain bounds the reading after that. A deadline of the reading on its own would race the one of the watcher, and report a timeout with the exit code of a process which exited right at it, without its last output. A watcher which got lost never cancels the token, nothing ends the process and its output otherwise, like for a streaming exec, so it gets killed with kill. Cancelling the token then stops the output, see StopOutput.