ipn/ipnlocal: make FileTargets check IPN state first

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -2425,13 +2425,13 @@ func (b *LocalBackend) FileTargets() ([]*apitype.FileTarget, error) {
 
 	b.mu.Lock()
 	defer b.mu.Unlock()
-	if !b.capFileSharing {
-		return nil, errors.New("file sharing not enabled by Tailscale admin")
-	}
 	nm := b.netMap
 	if b.state != ipn.Running || nm == nil {
 		return nil, errors.New("not connected")
 	}
+	if !b.capFileSharing {
+		return nil, errors.New("file sharing not enabled by Tailscale admin")
+	}
 	for _, p := range nm.Peers {
 		if p.User != nm.User {
 			continue

ipn/ipnlocal/local_test.go

@@ -504,3 +504,33 @@ func TestLazyMachineKeyGeneration(t *testing.T) {
 	// hit panicOnUseTransport).
 	time.Sleep(500 * time.Millisecond)
 }
+
+func TestFileTargets(t *testing.T) {
+	b := new(LocalBackend)
+	_, err := b.FileTargets()
+	if got, want := fmt.Sprint(err), "not connected"; got != want {
+		t.Errorf("before connect: got %q; want %q", got, want)
+	}
+
+	b.netMap = new(netmap.NetworkMap)
+	_, err = b.FileTargets()
+	if got, want := fmt.Sprint(err), "not connected"; got != want {
+		t.Errorf("non-running netmap: got %q; want %q", got, want)
+	}
+
+	b.state = ipn.Running
+	_, err = b.FileTargets()
+	if got, want := fmt.Sprint(err), "file sharing not enabled by Tailscale admin"; got != want {
+		t.Errorf("without cap: got %q; want %q", got, want)
+	}
+
+	b.capFileSharing = true
+	got, err := b.FileTargets()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 0 {
+		t.Fatalf("unexpected %d peers", len(got))
+	}
+	// (other cases handled by TestPeerAPIBase above)
+}