If a user starts an exec session with a shell and leaves it idle long
enough, they will eventually hit the Kubelet's HTTP server's read/write
timeout of 5 minutes. At this time, the StartExec call to Docker exits,
but if the user requested a TTY, the exec'd process does not exit.
After StartExec finishes, we try to determine the exit code of the
exec'd process, but in this case, we'll never get it. This change exits
the loop after 5 tries if the process is still running.
Add support for pluggable Docker exec handlers. The default handler is
now Docker's native exec API call. The previous default, nsenter, can be
selected by passing --docker-exec-handler=nsenter when starting the
kubelet.
