Add exception handling

A PaymentError is raised if something goes wrong.
if:
the invoice amount is too high
the invoice is expired,
the payment takes too long (default 5 seconds)

lib/faraday_ln_paywall/middleware.rb

@@ -4,12 +4,15 @@ require 'lightning/invoice'
 ENV['GRPC_SSL_CIPHER_SUITES'] = "HIGH+ECDSA"
 
 module FaradayLnPaywall
+  class PaymentError < StandardError; end
+
   class Middleware <  Faraday::Middleware
 
     def initialize(app, options = {})
       super(app)
       @options = options
       @options[:address] ||= 'localhost:10009'
+      @options[:timeout] ||= 5
       @options[:credentials] ||= File.read(File.expand_path(@options[:credentials_path] || "~/.lnd/tls.cert"))
       macaroon_binary ||= File.read(File.expand_path(@options[:macaroon_path] || "~/.lnd/data/chain/bitcoin/testnet/admin.macaroon"))
       @options[:macaroon] = macaroon_binary.each_byte.map { |b| b.to_s(16).rjust(2,'0') }.join
@@ -20,18 +23,25 @@ module FaradayLnPaywall
       env[:status] == 402 && env[:response_headers]['Content-Type'] == "application/vnd.lightning.bolt11"
     end
 
-    def invoice_valid?(invoice)
-      (@options[:max_amount].nil? || @options[:max_amount] > invoice.amount) &&
-        invoice.expiry.nil? || Time.now.to_i < invoice.timestamp + invoice.expiry.to_i
+    def validate_invoice!(invoice)
+      if !@options[:max_amount].nil? && @options[:max_amount] < invoice.amount
+        raise PaymentError, "invoice amount greater than expected maximum of #{@options[:max_amount]}"
+      end
+      if !invoice.expiry.nil? && Time.now.to_i > invoice.timestamp + invoice.expiry.to_i
+        raise PaymentError, "invoice expired"
+      end
     end
 
     def pay(env)
       invoice = Lightning::Invoice.parse(env.body)
-      return false unless invoice_valid?(invoice)
-      @lnd_client.send_payment_sync(
-        Lnrpc::SendRequest.new(payment_request: env.body),
-        { metadata: { macaroon: @options[:macaroon] }}
-      )
+      validate_invoice!(invoice)
+
+      Timeout::timeout(@options[:timeout], PaymentError, "payment execution expired") do
+        @lnd_client.send_payment_sync(
+          Lnrpc::SendRequest.new(payment_request: env.body),
+          { metadata: { macaroon: @options[:macaroon] }}
+        )
+      end
     end
 
     def call(request_env)
@@ -43,7 +53,7 @@ module FaradayLnPaywall
             request_env[:request_headers].merge!({'X-Preimage' => preimage})
             @app.call(request_env)
           else
-            # payment failed
+            raise PaymentError, payment.payment_error
           end
         end
       end