Ensure that QueryAnalyzer handles recursive calls

It is possible that Query Analyzer to unintentionally
trigger SQL query. To prevent Stack Too Deep discover
recursive calls and prevent them.

lib/gitlab/database/query_analyzer.rb

@@ -15,7 +15,11 @@ module Gitlab
 
       def hook!
         @subscriber = ActiveSupport::Notifications.subscribe('sql.active_record') do |event|
-          process_sql(event.payload[:sql], event.payload[:connection])
+          # In some cases analyzer code might trigger another SQL call
+          # to avoid stack too deep this detects recursive call of subscriber
+          with_ignored_recursive_calls do
+            process_sql(event.payload[:sql], event.payload[:connection])
+          end
         end
       end
 
@@ -57,6 +61,17 @@ module Gitlab
 
         nil
       end
+
+      def with_ignored_recursive_calls
+        return if Thread.current[:query_analyzer_recursive]
+
+        begin
+          Thread.current[:query_analyzer_recursive] = true
+          yield
+        ensure
+          Thread.current[:query_analyzer_recursive] = nil
+        end
+      end
     end
   end
 end

spec/lib/gitlab/database/query_analyzer_spec.rb

@@ -19,6 +19,15 @@ RSpec.describe Gitlab::Database::QueryAnalyzer do
 
       Project.connection.execute("SELECT 1 FROM projects")
     end
+
+    it 'does prevent recursive execution' do
+      expect(analyzer).to receive(:enabled?).and_return(true)
+      expect(analyzer).to receive(:analyze) do
+        Project.connection.execute("SELECT 1 FROM projects")
+      end
+
+      Project.connection.execute("SELECT 1 FROM projects")
+    end
   end
 
   describe '#process_sql' do