Wait for copy to complete before propagating exit-status.

[russjones]

Purpose

Occasionally Teleport session recordings don't contain the end of a session. This happens because of a race condition between when everything has been copied from the PTY and when exit-status has been received by the server. To fix this problem, the PTY is not closed until everything has been copied from the PTY.

Implementation

• New sessions create two goroutines, one that performs a io.Copy from the PTY to the writer (which is the session recorder and a direct connection to the remote client) and another goroutine that waits until either exit-status has been received (for the forwarding node) or exec.Cmd is complete (regular Teleport).
• Use a doneCh to notify the goroutine that is waiting for the execution of the shell to complete to also wait for the io.Copy to finish before broadcasting the exit-status to the client. This has to be done because broadcasting of the exit-status causes the PTY to be closed.
• Timeout after 5 seconds if everything has not been copied and log an error that end of the session may be missing.

Related Issues

Fixes #1646