ssh/tailssh: terminate sessions on stdout copy failures

Currently, killing a SCP copy with a Ctrl+C leaves the session hanging
even though the stdout copy goroutine fails with an io.EOF. Taking a
step back, when we are unable to send any more data back to the client
we should just terminate the session as the client will stop getting any
response from the server anyways.

Updates #3802

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/tailssh.go

@@ -946,15 +946,17 @@ func (ss *sshSession) run() {
 		_, err := io.Copy(rec.writer("i", ss.stdin), ss)
 		if err != nil {
 			// TODO: don't log in the success case.
-			logf("ssh: stdin copy: %v", err)
+			logf("stdin copy: %v", err)
 		}
 		ss.stdin.Close()
 	}()
 	go func() {
 		_, err := io.Copy(rec.writer("o", ss), ss.stdout)
 		if err != nil {
-			// TODO: don't log in the success case.
-			logf("ssh: stdout copy: %v", err)
+			logf("stdout copy: %v", err)
+			// If we got an error here, it's probably because the client has
+			// disconnected.
+			ss.ctx.CloseWithError(err)
 		}
 	}()
 	// stderr is nil for ptys.
@@ -962,8 +964,7 @@ func (ss *sshSession) run() {
 		go func() {
 			_, err := io.Copy(ss.Stderr(), ss.stderr)
 			if err != nil {
-				// TODO: don't log in the success case.
-				logf("ssh: stderr copy: %v", err)
+				logf("stderr copy: %v", err)
 			}
 		}()
 	}