Refactor token server initialization

- Move state "gen server" to the top of main/0
- Initialize it as empty
- Ensure that persistTokens/2 is called whenever the state changes
- Support setState/2 (similar in spirit to getState/0)

monzo_ynab/tokens.go

@@ -56,6 +56,7 @@ type readMsg struct {
 
 type writeMsg struct {
 	state  state
+	sender chan bool
 }
 
 type channels struct {
@@ -102,10 +103,10 @@ func scheduleTokenRefresh(expiresIn int, refreshToken string) {
 	log.Printf("Scheduling token refresh for %v\n", timestamp)
 	time.Sleep(duration)
 	log.Println("Refreshing tokens now...")
-	access, refresh := refreshTokens(refreshToken)
+	accessToken, refreshToken := refreshTokens(refreshToken)
 	log.Println("Successfully refreshed tokens.")
-	logTokens(access, refresh)
-	chans.writes <- writeMsg{state{access, refresh}}
+	logTokens(accessToken, refreshToken)
+	setState(accessToken, refreshToken)
 }
 
 // Exchange existing credentials for a new access token and `refreshToken`. Also
@@ -177,6 +178,13 @@ func handleInterrupts() {
 	os.Exit(0)
 }
 
+// Set `accessToken` and `refreshToken` on application state.
+func setState(accessToken string, refreshToken string) {
+	msg := writeMsg{state{accessToken, refreshToken}, make(chan bool)}
+	chans.writes <- msg
+	<-msg.sender
+}
+
 // Return our application state.
 func getState() state {
 	msg := readMsg{make(chan state)}
@@ -189,24 +197,9 @@ func getState() state {
 ////////////////////////////////////////////////////////////////////////////////
 
 func main() {
-	// Retrieve cached tokens from store.
-	accessToken := fmt.Sprintf("%v", kv.Get("monzoAccessToken"))
-	refreshToken := fmt.Sprintf("%v", kv.Get("monzoRefreshToken"))
-
-	log.Println("Attempting to retrieve cached credentials...")
-	logTokens(accessToken, refreshToken)
-
-	if accessToken == "" || refreshToken == "" {
-		log.Println("Cached credentials are absent. Authorizing client...")
-		authCode := auth.GetAuthCode(monzoClientId)
-		tokens := auth.GetTokensFromAuthCode(authCode, monzoClientId, monzoClientSecret)
-		accessToken, refreshToken = tokens.AccessToken, tokens.RefreshToken
-		go persistTokens(accessToken, refreshToken)
-		go scheduleTokenRefresh(tokens.ExpiresIn, refreshToken)
-	}
 	// Manage application state.
 	go func() {
-		state := &state{accessToken, refreshToken}
+		state := &state{}
 		for {
 			select {
 			case msg := <-chans.reads:
@@ -218,18 +211,39 @@ func main() {
 				log.Printf("Old: %s\n", state)
 				*state = msg.state
 				log.Printf("New: %s\n", state)
+				// As an attempt to maintain consistency between application
+				// state and persisted state, everytime we write to the
+				// application state, we will write to the store.
+				persistTokens(state.accessToken, state.refreshToken)
+				msg.sender <- true
 			}
 		}
 	}()
 
-	// Gracefully shutdown.
-	go handleInterrupts()
+	// Retrieve cached tokens from store.
+	accessToken := fmt.Sprintf("%v", kv.Get("monzoAccessToken"))
+	refreshToken := fmt.Sprintf("%v", kv.Get("monzoRefreshToken"))
 
+	log.Println("Attempting to retrieve cached credentials...")
+	logTokens(accessToken, refreshToken)
+
+	if accessToken == "" || refreshToken == "" {
+		log.Println("Cached credentials are absent. Authorizing client...")
+		authCode := auth.GetAuthCode(monzoClientId)
+		tokens := auth.GetTokensFromAuthCode(authCode, monzoClientId, monzoClientSecret)
+		setState(tokens.AccessToken, tokens.RefreshToken)
+		go scheduleTokenRefresh(tokens.ExpiresIn, tokens.RefreshToken)
+	} else {
+		setState(accessToken, refreshToken)
 		// If we have tokens, they may be expiring soon. We don't know because
 		// we aren't storing the expiration timestamp in the state or in the
 		// store. Until we have that information, and to be safe, let's refresh
 		// the tokens.
-	scheduleTokenRefresh(0, refreshToken)
+		go scheduleTokenRefresh(0, refreshToken)
+	}
+
+	// Gracefully handle shutdowns.
+	go handleInterrupts()
 
 	// Listen to inbound requests.
 	fmt.Println("Listening on http://localhost:4242 ...")
@@ -239,7 +253,6 @@ func main() {
 				state := getState()
 				go scheduleTokenRefresh(0, state.refreshToken)
 				fmt.Fprintf(w, "Done.")
-
 			} else if req.URL.Path == "/set-tokens" && req.Method == "POST" {
 				// Parse
 				payload := &setTokensRequest{}
@@ -249,8 +262,7 @@ func main() {
 				}
 
 				// Update application state
-				msg := writeMsg{state{payload.AccessToken, payload.RefreshToken}}
-				chans.writes <- msg
+				setState(payload.AccessToken, payload.RefreshToken)
 
 				// Refresh tokens
 				go scheduleTokenRefresh(payload.ExpiresIn, payload.RefreshToken)