Improve filtering for paid RPC methods in payment proxy (#16)

* Check for Content-Type and request fields for paid methods

* Include the RPC method in response handler log

paymentproxy/proxy.go

@@ -28,7 +28,8 @@ const (
 	CHANNEL_ID_VOUCHER_PARAM = "channelId"
 	SIGNATURE_VOUCHER_PARAM  = "signature"
 
-	VOUCHER_CONTEXT_ARG contextKey = "voucher"
+	VOUCHER_CONTEXT_ARG    contextKey = "voucher"
+	RPC_METHOD_CONTEXT_ARG contextKey = "rpcMethod"
 
 	ErrPayment = types.ConstError("payment error")
 )
@@ -113,36 +114,10 @@ func (p *PaymentProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	queryParams := r.URL.Query()
 	requiresPayment := true
+	var rpcMethod string
 
 	if p.enablePaidRpcMethods {
-
-		var ReqBody struct {
-			Method string `json:"method"`
-		}
-
-		bodyBytes, _ := io.ReadAll(r.Body)
-		// TODO: Check for content type
-		err := json.Unmarshal(bodyBytes, &ReqBody)
-		if err != nil {
-			p.handleError(w, r, createPaymentError(fmt.Errorf("could not unmarshall request body: %w", err)))
-			return
-		}
-
-		slog.Debug("Serving RPC request", "method", ReqBody.Method)
-
-		// Reassign request body as io.ReadAll consumes it
-		r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
-
-		rpcMethod := ReqBody.Method
-		requiresPayment = false
-
-		// Check if payment is required for RPC method
-		for _, paidRPCMethod := range paidRPCMethods {
-			if paidRPCMethod == rpcMethod {
-				requiresPayment = true
-				break
-			}
-		}
+		requiresPayment, rpcMethod = isPaymentRequired(r)
 	}
 
 	if requiresPayment {
@@ -154,8 +129,9 @@ func (p *PaymentProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 		removeVoucher(r)
 
-		// We add the voucher to the request context so we can access it in the response handler
+		// We add the voucher and rpcMethod to the request context so we can access them in the response handler
 		r = r.WithContext(context.WithValue(r.Context(), VOUCHER_CONTEXT_ARG, v))
+		r = r.WithContext(context.WithValue(r.Context(), RPC_METHOD_CONTEXT_ARG, rpcMethod))
 	}
 
 	p.reverseProxy.ServeHTTP(w, r)
@@ -192,7 +168,12 @@ func (p *PaymentProxy) handleDestinationResponse(r *http.Response) error {
 	}
 	cost := p.costPerByte * contentLength
 
-	slog.Debug("Request cost", "cost-per-byte", p.costPerByte, "response-length", contentLength, "cost", cost)
+	rpcMethod, ok := r.Request.Context().Value(RPC_METHOD_CONTEXT_ARG).(string)
+	if ok {
+		slog.Debug("Request cost", "cost-per-byte", p.costPerByte, "response-length", contentLength, "cost", cost, "method", rpcMethod)
+	} else {
+		slog.Debug("Request cost", "cost-per-byte", p.costPerByte, "response-length", contentLength, "cost", cost)
+	}
 
 	s, err := p.nitroClient.ReceiveVoucher(v)
 	if err != nil {
@@ -252,6 +233,43 @@ func (p *PaymentProxy) Stop() error {
 	return p.nitroClient.Close()
 }
 
+// Helper method to parse request and determine whether it qualifies for a payment
+// Payment is required for a request if:
+//   - "Content-Type" header is set to "application/json"
+//   - Request body has non-empty "jsonrpc" and "method" fields
+func isPaymentRequired(r *http.Request) (bool, string) {
+	if r.Header.Get("Content-Type") != "application/json" {
+		return false, ""
+	}
+
+	var ReqBody struct {
+		JsonRpc string `json:"jsonrpc"`
+		Method  string `json:"method"`
+	}
+	bodyBytes, _ := io.ReadAll(r.Body)
+
+	err := json.Unmarshal(bodyBytes, &ReqBody)
+	if err != nil || ReqBody.JsonRpc == "" || ReqBody.Method == "" {
+		return false, ""
+	}
+
+	slog.Debug("Serving RPC request", "method", ReqBody.Method)
+
+	// Reassign request body as io.ReadAll consumes it
+	r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
+
+	rpcMethod := ReqBody.Method
+
+	// Check if payment is required for RPC method
+	for _, paidRPCMethod := range paidRPCMethods {
+		if paidRPCMethod == rpcMethod {
+			return true, rpcMethod
+		}
+	}
+
+	return false, ""
+}
+
 // parseVoucher takes in an a collection of query params and parses out a voucher.
 func parseVoucher(params url.Values) (payments.Voucher, error) {
 	rawChId := params.Get(CHANNEL_ID_VOUCHER_PARAM)