Track nil `proxy` to Sentry upon field access

We have a late-init scenario where load balancing isn't available before
configure_proxy is called. Logging this condition to sentry makes these
failures easier to track and diagnose.

ee/lib/gitlab/database/load_balancing.rb

@@ -3,9 +3,6 @@
 module Gitlab
   module Database
     module LoadBalancing
-      # The connection proxy to use for load balancing (if enabled).
-      cattr_accessor :proxy
-
       # The exceptions raised for connection errors.
       CONNECTION_ERRORS = if defined?(PG)
                             [
@@ -22,6 +19,21 @@ module Gitlab
                             [].freeze
                           end
 
+      ProxyNotConfiguredError = Class.new(StandardError)
+
+      # The connection proxy to use for load balancing (if enabled).
+      def self.proxy
+        unless @proxy
+          Gitlab::ErrorTracking.track_exception(
+            ProxyNotConfiguredError.new(
+              "Attempting to access the database load balancing proxy, but it wasn't configured.\n" \
+              "Did you forget to call '#{self.name}.configure_proxy'?"
+            ))
+        end
+
+        @proxy
+      end
+
       # Returns a Hash containing the load balancing configuration.
       def self.configuration
         ActiveRecord::Base.configurations[Rails.env]['load_balancing'] || {}
@@ -89,8 +101,8 @@ module Gitlab
       end
 
       # Configures proxying of requests.
-      def self.configure_proxy
-        self.proxy = ConnectionProxy.new(hosts)
+      def self.configure_proxy(proxy = ConnectionProxy.new(hosts))
+        @proxy = proxy
 
         # This hijacks the "connection" method to ensure both
         # `ActiveRecord::Base.connection` and all models use the same load

ee/spec/lib/gitlab/database/load_balancing_spec.rb

@@ -3,6 +3,37 @@
 require 'spec_helper'
 
 describe Gitlab::Database::LoadBalancing do
+  describe '.proxy' do
+    context 'when configured' do
+      before do
+        allow(ActiveRecord::Base.singleton_class).to receive(:prepend)
+        subject.configure_proxy
+      end
+
+      after do
+        subject.configure_proxy(nil)
+      end
+
+      it 'returns the connection proxy' do
+        expect(subject.proxy).to be_an_instance_of(subject::ConnectionProxy)
+      end
+    end
+
+    context 'when not configured' do
+      it 'returns nil' do
+        expect(subject.proxy).to be_nil
+      end
+
+      it 'tracks an error to sentry' do
+        expect(Gitlab::ErrorTracking).to receive(:track_exception).with(
+          an_instance_of(subject::ProxyNotConfiguredError)
+        )
+
+        subject.proxy
+      end
+    end
+  end
+
   describe '.configuration' do
     it 'returns a Hash' do
       config = { 'hosts' => %w(foo) }
@@ -176,14 +207,16 @@ describe Gitlab::Database::LoadBalancing do
 
   describe '.configure_proxy' do
     after do
-      described_class.proxy = nil
+      described_class.configure_proxy(nil)
     end
 
     it 'configures the connection proxy' do
-      expect(ActiveRecord::Base.singleton_class).to receive(:prepend)
-        .with(Gitlab::Database::LoadBalancing::ActiveRecordProxy)
+      allow(ActiveRecord::Base.singleton_class).to receive(:prepend)
 
       described_class.configure_proxy
+
+      expect(ActiveRecord::Base.singleton_class).to have_received(:prepend)
+        .with(Gitlab::Database::LoadBalancing::ActiveRecordProxy)
     end
   end