ssh/tailssh: fix race in errors returned when starting recorder

There were two code paths that could fail depending on how fast
the recorder responses. This fixes that by returning the correct
error from both paths.

Fixes #7707

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

ssh/tailssh/tailssh.go

@@ -1065,7 +1065,12 @@ func (ss *sshSession) run() {
 			var err error
 			rec, err = ss.startNewRecording()
 			if err != nil {
-				fmt.Fprintf(ss, "can't start new recording\r\n")
+				var uve userVisibleError
+				if errors.As(err, &uve) {
+					fmt.Fprintf(ss, "%s\r\n", uve)
+				} else {
+					fmt.Fprintf(ss, "can't start new recording\r\n")
+				}
 				ss.logf("startNewRecording: %v", err)
 				ss.Exit(1)
 				return
@@ -1079,7 +1084,7 @@ func (ss *sshSession) run() {
 		logf("start failed: %v", err.Error())
 		if errors.Is(err, context.Canceled) {
 			err := context.Cause(ss.ctx)
-			uve := userVisibleError{}
+			var uve userVisibleError
 			if errors.As(err, &uve) {
 				fmt.Fprintf(ss, "%s\r\n", uve)
 			}
@@ -1500,6 +1505,12 @@ func (ss *sshSession) startNewRecording() (_ *recording, err error) {
 	}
 	j = append(j, '\n')
 	if _, err := pw.Write(j); err != nil {
+		if errors.Is(err, io.ErrClosedPipe) && ss.ctx.Err() != nil {
+			// If we got an io.ErrClosedPipe, it's likely because
+			// the recording server closed the connection on us. Return
+			// the original context error instead.
+			return nil, context.Cause(ss.ctx)
+		}
 		return nil, err
 	}
 	return rec, nil