Refactor ResolveRemoteAccountService (#4258)

* Refactor ResolveRemoteAccountService

* Remove trailing whitespace

* Use redis locks around critical ResolveRemoteAccountService code

* Add test for race condition of lock

spec/services/resolve_remote_account_service_spec.rb

@@ -68,4 +68,27 @@ RSpec.describe ResolveRemoteAccountService do
     expect(account.domain).to eq 'localdomain.com'
     expect(account.remote_url).to eq 'https://webdomain.com/users/foo.atom'
   end
+
+  it 'processes one remote account at a time using locks' do
+    wait_for_start = true
+    fail_occurred  = false
+    return_values  = []
+
+    threads = Array.new(5) do
+      Thread.new do
+        true while wait_for_start
+        begin
+          return_values << subject.call('foo@localdomain.com')
+        rescue ActiveRecord::RecordNotUnique
+          fail_occurred = true
+        end
+      end
+    end
+
+    wait_for_start = false
+    threads.each(&:join)
+
+    expect(fail_occurred).to be false
+    expect(return_values).to_not include(nil)
+  end
 end