1 files changed, 56 insertions, 4 deletions

diff --git a/lib/gitlab/checks/matching_merge_request.rb b/lib/gitlab/checks/matching_merge_request.rb
index 2635ad04770..e37cbc0442b 100644
--- a/lib/gitlab/checks/matching_merge_request.rb
+++ b/lib/gitlab/checks/matching_merge_request.rb
@@ -3,22 +3,74 @@
 module Gitlab
   module Checks
     class MatchingMergeRequest
+      TOTAL_METRIC = :gitlab_merge_request_match_total
+      STALE_METRIC = :gitlab_merge_request_match_stale_secondary
+
       def initialize(newrev, branch_name, project)
         @newrev = newrev
         @branch_name = branch_name
         @project = project
       end
 
-      # rubocop: disable CodeReuse/ActiveRecord
       def match?
+        if ::Gitlab::Database::LoadBalancing.enable?
+          # When a user merges a merge request, the following sequence happens:
+          #
+          # 1. Sidekiq: MergeService runs and updates the merge request in a locked state.
+          # 2. Gitaly: The UserMergeBranch RPC runs.
+          # 3. Gitaly (gitaly-ruby): This RPC calls the pre-receive hook.
+          # 4. Rails: This hook makes an API request to /api/v4/internal/allowed.
+          # 5. Rails: This API check does a SQL query for locked merge
+          #    requests with a matching SHA.
+          #
+          # Since steps 1 and 5 will happen on different database
+          # sessions, replication lag could erroneously cause step 5 to
+          # report no matching merge requests. To avoid this, we check
+          # the write location to ensure the replica can make this query.
+          track_session_metrics do
+            ::Gitlab::Database::LoadBalancing::Sticking.select_valid_host(:project, @project.id)
+          end
+        end
+
+        # rubocop: disable CodeReuse/ActiveRecord
         @project.merge_requests
           .with_state(:locked)
           .where(in_progress_merge_commit_sha: @newrev, target_branch: @branch_name)
           .exists?
+        # rubocop: enable CodeReuse/ActiveRecord
+      end
+
+      private
+
+      def track_session_metrics
+        before = ::Gitlab::Database::LoadBalancing::Session.current.use_primary?
+
+        yield
+
+        after = ::Gitlab::Database::LoadBalancing::Session.current.use_primary?
+
+        increment_attempt_count
+
+        if !before && after
+          increment_stale_secondary_count
+        end
+      end
+
+      def increment_attempt_count
+        total_counter.increment
+      end
+
+      def increment_stale_secondary_count
+        stale_counter.increment
+      end
+
+      def total_counter
+        @total_counter ||= ::Gitlab::Metrics.counter(TOTAL_METRIC, 'Total number of merge request match attempts')
+      end
+
+      def stale_counter
+        @stale_counter ||= ::Gitlab::Metrics.counter(STALE_METRIC, 'Total number of merge request match attempts with lagging secondary')
       end
-      # rubocop: enable CodeReuse/ActiveRecord
     end
   end
 end
-
-Gitlab::Checks::MatchingMergeRequest.prepend_mod_with('Gitlab::Checks::MatchingMergeRequest')