ssh/tailssh: better handling of signals and exits

We were not handling errors occurred while copying data between the subprocess and the connection.
This makes it so that we pass the appropriate signals when to the process and the connection.

This also fixes mosh.

Updates #4919

Co-authored-by: James Tucker <raggi@tailscale.com>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Maisem Ali <maisem@tailscale.com>

ssh/tailssh/incubator.go

@@ -310,15 +310,25 @@ func (ss *sshSession) launchProcess() error {
 	if err != nil {
 		return err
 	}
-	go resizeWindow(pty, winCh)
-	ss.stdout = pty // no stderr for a pty
+
+	// We need to be able to close stdin and stdout separately later so make a
+	// dup.
+	ptyDup, err := syscall.Dup(int(pty.Fd()))
+	if err != nil {
+		return err
+	}
+	go resizeWindow(ptyDup /* arbitrary fd */, winCh)
+
 	ss.stdin = pty
+	ss.stdout = os.NewFile(uintptr(ptyDup), pty.Name())
+	ss.stderr = nil // not available for pty
+
 	return nil
 }
 
-func resizeWindow(f *os.File, winCh <-chan ssh.Window) {
+func resizeWindow(fd int, winCh <-chan ssh.Window) {
 	for win := range winCh {
-		unix.IoctlSetWinsize(int(f.Fd()), syscall.TIOCSWINSZ, &unix.Winsize{
+		unix.IoctlSetWinsize(fd, syscall.TIOCSWINSZ, &unix.Winsize{
 			Row: uint16(win.Height),
 			Col: uint16(win.Width),
 		})