[GithubService] Add username lookup

Adds a lookup for usernames to check to see if they are valid.

- If a valid username is provided, it returns the ID of the user.
- If username is invalid, it sets the key to `nil`

Only valid users will take up an API call, hence the initial
`Net::HTTP.get_response` that is made first.  This might slow down the
call to this method overall, but it saves API calls in case someone is
providing large commit messages with many code examples included (what
this will be used for in a future commit)

lib/github_service.rb

@@ -113,6 +113,21 @@ def refresh_assignees(fq_name)
       assignees_cache.delete(fq_name)
     end
 
+    def username_lookup(username)
+      if username_lookup_cache.key?(username)
+        username_lookup_cache[username]
+      else
+        username_lookup_cache[username] ||= begin
+          case Net::HTTP.new("github.com", 443).tap { |h| h.use_ssl = true }.request_head("/#{username}")
+          when Net::HTTPNotFound then nil # invalid username
+          when Net::HTTPOK       then service.user(username)[:id]
+          else
+            raise "Error on GitHub with username lookup"
+          end
+        end
+      end
+    end
+
     private
 
     def service
@@ -151,6 +166,10 @@ def assignees_cache
       @assignees_cache ||= {}
     end
 
+    def username_lookup_cache
+      @username_lookup_cache ||= {}
+    end
+
     def respond_to_missing?(method_name, include_private = false)
       service.respond_to?(method_name, include_private)
     end

spec/lib/github_service_spec.rb

@@ -0,0 +1,63 @@
+describe GithubService do
+  describe "#username_lookup" do
+    let(:lookup_username) { "NickLaMuro" }
+    let(:lookup_status)   { 200 }
+
+    before do
+      # HTTP lookup
+      stub_request(:head, "https://github.com/#{lookup_username}")
+        .with(:headers => {'Accept' => '*/*', 'User-Agent' => 'Ruby'})
+        .to_return(:status => lookup_status, :body => "", :headers => {})
+    end
+
+    after do
+      lookup_cache.delete(lookup_username)
+    end
+
+    def lookup_cache
+      described_class.send(:username_lookup_cache)
+    end
+
+    context "for a valid user" do
+      before do
+        github_service_add_stub :url           => "/users/#{lookup_username}",
+                                :response_body => {'id' => 123}.to_json
+      end
+
+      it "looks up a user and stores the user's ID in the cache" do
+        expect(described_class.username_lookup(lookup_username)).to eq(123)
+        expect(lookup_cache).to eq("NickLaMuro" => 123)
+      end
+    end
+
+    context "for a user that is not found" do
+      let(:lookup_status) { 404 }
+
+      it "looks up a user and stores the user's ID in the cache" do
+        expect(described_class.username_lookup(lookup_username)).to eq(nil)
+        expect(lookup_cache).to eq("NickLaMuro" => nil)
+      end
+
+      it "does a lookup call only once" do
+        http_instance  = Net::HTTP.new("github.com", 443)
+        fake_not_found = Net::HTTPNotFound.new(nil, nil, nil)
+        expect(Net::HTTP).to     receive(:new).and_return(http_instance)
+        expect(http_instance).to receive(:request_head).once.and_return(fake_not_found)
+
+        expect(described_class.username_lookup(lookup_username)).to eq(nil)
+        expect(described_class.username_lookup(lookup_username)).to eq(nil)
+      end
+    end
+
+    context "when GitHub is having a bad time..." do
+      let(:lookup_status) { 500 }
+
+      it "looks up a user and does not stores the username in the cache" do
+        expect do
+          described_class.username_lookup(lookup_username)
+        end.to raise_error(RuntimeError, "Error on GitHub with username lookup")
+        expect(lookup_cache).to eq({})
+      end
+    end
+  end
+end