cmd/tailscale/cli: improve error when bg serve config is present (#9961)

We prevent shodow configs when starting a foreground when a background serve config already exists for the serve type and port. This PR improves the messaging to let the user know how to remove the previous config.

Updates #8489
ENG-2314

Signed-off-by: Tyler Smalley <tyler@tailscale.com>

cmd/tailscale/cli/serve_v2.go

@@ -271,7 +271,13 @@ func (e *serveEnv) runServeCombined(subcmd serveMode) execFunc {
 		}
 
 		var watcher *tailscale.IPNBusWatcher
-		if !e.bg && !turnOff {
+		wantFg := !e.bg && !turnOff
+		if wantFg {
+			// validate the config before creating a WatchIPNBus session
+			if err := e.validateConfig(parentSC, srvPort, srvType); err != nil {
+				return err
+			}
+
 			// if foreground mode, create a WatchIPNBus session
 			// and use the nested config for all following operations
 			// TODO(marwan-at-work): nested-config validations should happen here or previous to this point.
@@ -334,6 +340,8 @@ func (e *serveEnv) runServeCombined(subcmd serveMode) execFunc {
 	}
 }
 
+const backgroundExistsMsg = "background configuration already exists, use `tailscale %s --%s=%d off` to remove the existing configuration"
+
 func (e *serveEnv) validateConfig(sc *ipn.ServeConfig, port uint16, wantServe serveType) error {
 	sc, isFg := findConfig(sc, port)
 	if sc == nil {
@@ -343,7 +351,7 @@ func (e *serveEnv) validateConfig(sc *ipn.ServeConfig, port uint16, wantServe se
 		return errors.New("foreground already exists under this port")
 	}
 	if !e.bg {
-		return errors.New("background serve already exists under this port")
+		return fmt.Errorf(backgroundExistsMsg, infoMap[e.subcmd].Name, wantServe.String(), port)
 	}
 	existingServe := serveFromPortHandler(sc.TCP[port])
 	if wantServe != existingServe {

cmd/tailscale/cli/serve_v2_test.go

@@ -792,6 +792,26 @@ func TestServeDevConfigMutations(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "forground_with_bg_conflict",
+			steps: []step{
+				{
+					command: cmd("serve --bg --http=3000  localhost:3000"),
+					want: &ipn.ServeConfig{
+						TCP: map[uint16]*ipn.TCPPortHandler{3000: {HTTP: true}},
+						Web: map[ipn.HostPort]*ipn.WebServerConfig{
+							"foo.test.ts.net:3000": {Handlers: map[string]*ipn.HTTPHandler{
+								"/": {Proxy: "http://127.0.0.1:3000"},
+							}},
+						},
+					},
+				},
+				{
+					command: cmd("serve --http=3000 localhost:3000"),
+					wantErr: exactErrMsg(fmt.Errorf(backgroundExistsMsg, "serve", "http", 3000)),
+				},
+			},
+		},
 	}
 
 	for _, group := range groups {
@@ -1330,6 +1350,6 @@ func exactErrMsg(want error) func(error) string {
 		if got.Error() == want.Error() {
 			return ""
 		}
-		return fmt.Sprintf("got error %v, want %v", got, want)
+		return fmt.Sprintf("\ngot:  %v\nwant: %v\n", got, want)
 	}
 }