Add ability to get a single subscription

fixes PLAT-2185

Test plan:
* Configure canvas and the subscription service to use the same signing
  secret and encryption key
* With the subscription service running, open up a rails console and run
  the following commands
  ToolProxy = Struct.new("ToolProxy", :guid, :product_family)
  Family = Struct.new("Family", :developer_key)
  f = Family.new(10000000000002)
  tp = ToolProxy.new('hahahah', f)
  res = Services::LiveEventsSubscriptionService.tool_proxy_subscription(tp, "<id for a subscription>")
* Ensure that you get the subscription back successfully

Change-Id: If65ac06c6174cd16195c3bd218f7521b389ed0aa
Reviewed-on: https://gerrit.instructure.com/101425
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nathan Mills <nathanm@instructure.com>
Product-Review: Andrew Butterfield <abutterfield@instructure.com>

lib/services/live_events_subscription_service.rb

@@ -5,25 +5,19 @@ module Services
         settings.present?
       end
 
+      def tool_proxy_subscription(tool_proxy, subscription_id)
+        options = { headers: headers(tool_proxy_jwt_body(tool_proxy)) }
+        request(:get, "/api/subscriptions/#{subscription_id}", options)
+      end
+
       def tool_proxy_subscriptions(tool_proxy)
-        options = {
-          headers: headers({
-            sub: "ltiToolProxy:#{tool_proxy.guid}",
-            developerKey: tool_proxy.product_family.developer_key.to_s
-          })
-        }
+        options = { headers: headers(tool_proxy_jwt_body(tool_proxy)) }
         request(:get, '/api/subscriptions', options)
       end
 
       def create_tool_proxy_subscription(tool_proxy, subscription)
         options = {
-          headers: headers({
-            sub: "ltiToolProxy:#{tool_proxy.guid}",
-            developerKey: tool_proxy.product_family.developer_key.to_s
-          },
-          {
-            'Content-Type' => 'application/json'
-          }),
+          headers: headers(tool_proxy_jwt_body(tool_proxy), { 'Content-Type' => 'application/json' }),
           body: subscription.to_json
         }
         request(:post, '/api/subscriptions', options)
@@ -51,6 +45,13 @@ module Services
         Canvas::Errors.capture_exception(:live_events_subscription, e)
         nil
       end
+
+      def tool_proxy_jwt_body(tool_proxy, options = {})
+        options.merge({
+          sub: "ltiToolProxy:#{tool_proxy.guid}",
+          developerKey: tool_proxy.product_family.developer_key.to_s
+        })
+      end
     end
   end
 end

spec/lib/services/live_events_subscription_service_spec.rb

@@ -56,6 +56,19 @@ module Services
         end
       end
 
+      describe '.tool_proxy_subscription' do
+        it 'makes the expected request' do
+          HTTParty.expects(:send).with do |method, endpoint, options|
+            expect(method).to eq(:get)
+            expect(endpoint).to eq('http://example.com/api/subscriptions/subscription_id')
+            jwt = Canvas::Security::ServicesJwt.new(options[:headers]['Authorization'].gsub('Bearer ',''), false).original_token
+            expect(jwt["developerKey"]).to eq('10000000000003')
+            expect(jwt["sub"]).to eq('ltiToolProxy:151b52cd-d670-49fb-bf65-6a327e3aaca0')
+          end
+          LiveEventsSubscriptionService.tool_proxy_subscription(tool_proxy, 'subscription_id')
+        end
+      end
+
       describe '.tool_proxy_subscriptions' do
         it 'makes the expected request' do
           HTTParty.expects(:send).with do |method, endpoint, options|