Fix handling of persistent SSH connections and improve debug logging #197
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In the case of SSH ControlMaster connections with a PTY, the SSH connection can stay up without an active SSH channel (i.e. after the command execution completes). If this happens the ssh connection context will not be cancelled, as the connection is still up. Wrapping the raw SSH context and deferring the cancel of it before passing it to exec.StreamWithContext() and newTermSizeQueue() ensures that once command execution completes, the context is cancelled and the TSQ goroutine will exit immediately. If the context is not cancelled after command execution completes, the TSQ goroutine will spin trying to write to a closed winch channel and spew EOF errors into stderr.
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In some rare cases the ssh-portal could lose track of the TerminalSizeQueue goroutine, which would cause error messages to be spewed on stderr.
This PR fixes that issue by ensuring the goroutine exits once the exec command completes. It also does some minor refactoring and logging improvements.