cmd/tailscaled,*: add start of configuration file support

Updates #1412

Co-authored-by: Maisem Ali <maisem@tailscale.com>
Change-Id: I38d559c1784d09fc804f521986c9b4b548718f7d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/apitype/apitype.go

@@ -40,3 +40,12 @@ type SetPushDeviceTokenRequest struct {
 	// PushDeviceToken is the iOS/macOS APNs device token (and any future Android equivalent).
 	PushDeviceToken string
 }
+
+// ReloadConfigResponse is the response to a LocalAPI reload-config request.
+//
+// There are three possible outcomes: (false, "") if no config mode in use,
+// (true, "") on success, or (false, "error message") on failure.
+type ReloadConfigResponse struct {
+	Reloaded bool   // whether the config was reloaded
+	Err      string // any error message
+}

client/tailscale/localclient.go

@@ -1244,6 +1244,25 @@ func (lc *LocalClient) ProfileStatus(ctx context.Context) (current ipn.LoginProf
 	return current, all, err
 }
 
+// ReloadConfig reloads the config file, if possible.
+func (lc *LocalClient) ReloadConfig(ctx context.Context) (ok bool, err error) {
+	body, err := lc.send(ctx, "POST", "/localapi/v0/reload-config", 200, nil)
+	if err != nil {
+		return
+	}
+	res, err := decodeJSON[apitype.ReloadConfigResponse](body)
+	if err != nil {
+		return
+	}
+	if err != nil {
+		return false, err
+	}
+	if res.Err != "" {
+		return false, errors.New(res.Err)
+	}
+	return res.Reloaded, nil
+}
+
 // SwitchToEmptyProfile creates and switches to a new unnamed profile. The new
 // profile is not assigned an ID until it is persisted after a successful login.
 // In order to login to the new profile, the user must call LoginInteractive.