1 files changed, 183 insertions, 0 deletions
diff --git a/monzo_ynab/tokens.go b/monzo_ynab/tokens.go
new file mode 100644
index 000000000000..47e991e56132
--- /dev/null
+++ b/monzo_ynab/tokens.go
@@ -0,0 +1,183 @@
+// Creating a Tokens server to manage my access and refresh tokens. Keeping this
+// as a separate server allows me to develop and use the access tokens without
+// going through client authorization.
+package main
+
+////////////////////////////////////////////////////////////////////////////////
+// Dependencies
+////////////////////////////////////////////////////////////////////////////////
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"time"
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Types
+////////////////////////////////////////////////////////////////////////////////
+
+// This is the response from Monzo's API after we request an access token
+// refresh.
+type refreshTokenResponse struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+	ClientId     string `json:"client_id"`
+	ExpiresIn    int    `json:"expires_in"`
+}
+
+// This is the shape of the request from clients wishing to set state of the
+// server.
+type setTokensRequest struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+	ExpiresIn    int    `json:"expires_in"`
+}
+
+// This is our application state.
+type state struct {
+	accessToken  string `json:"access_token"`
+	refreshToken string `json:"refresh_token"`
+}
+
+type readMsg struct {
+	sender chan state
+}
+
+type writeMsg struct {
+	state state
+}
+
+type channels struct {
+	reads  chan readMsg
+	writes chan writeMsg
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Top-level Definitions
+////////////////////////////////////////////////////////////////////////////////
+
+var chans = &channels{
+	reads:  make(chan readMsg),
+	writes: make(chan writeMsg),
+}
+
+var (
+	monzoClientId      = os.Getenv("monzo_client_id")
+	monzoClientSecret  = os.Getenv("monzo_client_secret")
+	cachedAccessToken  = os.Getenv("monzo_cached_access_token")
+	cachedRefreshToken = os.Getenv("monzo_cached_access_token")
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Utils
+////////////////////////////////////////////////////////////////////////////////
+
+// Schedule a token refresh for `expiresIn` seconds using the provided
+// `refreshToken`. This will update the application state with the access token
+// and schedule an additional token refresh for the newly acquired tokens.
+func scheduleTokenRefresh(expiresIn int, refreshToken string) {
+	duration := time.Second * time.Duration(expiresIn)
+	timestamp := time.Now().Local().Add(duration)
+	log.Printf("Scheduling token refresh for %v\n", timestamp)
+	time.Sleep(duration)
+	log.Println("Refreshing tokens now...")
+	access, refresh := refreshTokens(refreshToken)
+	log.Println("Successfully refreshed tokens.")
+	chans.writes <- writeMsg{state{access, refresh}}
+}
+
+// Exchange existing credentials for a new access token and `refreshToken`. Also
+// schedule the next refresh. This function returns the newly acquired access
+// token and refresh token.
+func refreshTokens(refreshToken string) (string, string) {
+	// TODO(wpcarro): Support retries with exponential backoff.
+	res, err := http.PostForm("https://api.monzo.com/oauth2/token", url.Values{
+		"grant_type":    {"refresh_token"},
+		"client_id":     {monzoClientId},
+		"client_secret": {monzoClientSecret},
+		"refresh_token": {refreshToken},
+	})
+	if err != nil {
+		log.Println(res)
+		log.Fatal("The request to Monzo to refresh our access token failed.", err)
+	}
+	defer res.Body.Close()
+	payload := &refreshTokenResponse{}
+	err = json.NewDecoder(res.Body).Decode(payload)
+	if err != nil {
+		log.Println(res)
+		log.Fatal("Could not decode the JSON response from Monzo.", err)
+	}
+	go scheduleTokenRefresh(payload.ExpiresIn, payload.RefreshToken)
+
+	return payload.AccessToken, payload.RefreshToken
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Main
+////////////////////////////////////////////////////////////////////////////////
+
+func main() {
+	// Manage application state.
+	go func() {
+		state := &state{cachedAccessToken, cachedRefreshToken}
+		for {
+			select {
+			case msg := <-chans.reads:
+				log.Printf("Reading from state.")
+				log.Printf("Access Token: %s\n", state.accessToken)
+				log.Printf("Refresh Token: %s\n", state.refreshToken)
+				msg.sender <- *state
+			case msg := <-chans.writes:
+				fmt.Printf("Writing new state: %v\n", msg.state)
+				*state = msg.state
+			}
+		}
+	}()
+
+	// Listen to inbound requests.
+	fmt.Println("Listening on http://localhost:4242 ...")
+	log.Fatal(http.ListenAndServe(":4242", http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+		if req.URL.Path == "/refresh-tokens" && req.Method == "POST" {
+			msg := readMsg{make(chan state)}
+			chans.reads <- msg
+			state := <-msg.sender
+			go scheduleTokenRefresh(0, state.refreshToken)
+			fmt.Fprintf(w, "Done.")
+
+		} else if req.URL.Path == "/set-tokens" && req.Method == "POST" {
+			// Parse
+			payload := &setTokensRequest{}
+			err := json.NewDecoder(req.Body).Decode(payload)
+			if err != nil {
+				log.Fatal("Could not decode the user's JSON request.", err)
+			}
+
+			// Update application state
+			msg := writeMsg{state{payload.AccessToken, payload.RefreshToken}}
+			chans.writes <- msg
+
+			// Refresh tokens
+			go scheduleTokenRefresh(payload.ExpiresIn, payload.RefreshToken)
+
+			// Ack
+			fmt.Fprintf(w, "Done.")
+		} else if req.URL.Path == "/state" && req.Method == "GET" {
+			// TODO(wpcarro): Ensure that this returns serialized state.
+			w.Header().Set("Content-type", "application/json")
+			msg := readMsg{make(chan state)}
+			chans.reads <- msg
+			state := <-msg.sender
+			payload, _ := json.Marshal(state)
+			fmt.Fprintf(w, "Application state: %s\n", bytes.NewBuffer(payload))
+		} else {
+			log.Printf("Unhandled request: %v\n", *req)
+		}
+	})))
+}