Add conditionals and a transaction to the service

Differentiating which post-merge functions are "safe" to run and which
ones could potentially lead to data loss or an unintended effect.

This should be more idempotent now.

app/services/merge_requests/merge_service.rb

@@ -27,6 +27,7 @@ module MergeRequests
           success
         end
       end
+
       log_info("Merge process finished on JID #{merge_jid} with state #{state}")
     rescue MergeError => e
       handle_merge_error(log_message: e.message, save_message_on_model: true)
@@ -54,7 +55,7 @@ module MergeRequests
       error =
         if @merge_request.should_be_rebased?
           'Only fast-forward merge is allowed for your project. Please update your source branch'
-        elsif !@merge_request.mergeable?
+        elsif !@merge_request.merged? && !@merge_request.mergeable?
           'Merge request is not mergeable'
         end
 

app/services/merge_requests/post_merge_service.rb

@@ -8,17 +8,30 @@ module MergeRequests
   #
   class PostMergeService < MergeRequests::BaseService
     def execute(merge_request)
-      merge_request.mark_as_merged
+      return if merge_request.merged? # nothing to do, this worker has already run at least once
+
+      # These operations need to happen transactionally
+      ActiveRecord::Base.transaction do
+        merge_request.mark_as_merged
+        create_event(merge_request)
+        create_note(merge_request)
+
+        # TODO: Make sure these are async operations. If not, move them earlier
+        # Better to have duplicate notifications than no notifications.
+        todo_service.merge_merge_request(merge_request, current_user)
+        notification_service.merge_mr(merge_request, current_user)
+      end
+
+      # These operations are idempotent so can be safely run multiple times
       close_issues(merge_request)
-      todo_service.merge_merge_request(merge_request, current_user)
-      create_event(merge_request)
-      create_note(merge_request)
-      notification_service.merge_mr(merge_request, current_user)
-      execute_hooks(merge_request, 'merge')
       invalidate_cache_counts(merge_request, users: merge_request.assignees)
       merge_request.update_project_counter_caches
       delete_non_latest_diffs(merge_request)
       cleanup_environments(merge_request)
+
+      # Anything after this point will be executed at-most-once. Less important activity only
+      # TODO: make all the work in here a separate sidekiq job so it can go in the transaction
+      execute_hooks(merge_request, 'merge')
     end
 
     private

app/services/merge_requests/squash_service.rb

@@ -7,9 +7,7 @@ module MergeRequests
     def execute
       # If performing a squash would result in no change, then
       # immediately return a success message without performing a squash
-      if merge_request.commits_count < 2 && message.nil?
-        return success(squash_sha: merge_request.diff_head_sha)
-      end
+      return success(squash_sha: merge_request.diff_head_sha) if squash_redundant?
 
       if merge_request.squash_in_progress?
         return error(s_('MergeRequests|Squash task canceled: another squash is already in progress.'))
@@ -20,6 +18,12 @@ module MergeRequests
 
     private
 
+    def squash_redundant?
+      return true if merge_request.merged?
+
+      merge_request.commits_count < 2 && message.nil?
+    end
+
     def squash!
       squash_sha = repository.squash(current_user, merge_request, message || merge_request.default_squash_commit_message)
 

changelogs/unreleased/32737-make-mergeservice-idempotent.yml

+---
+title: Make MergeService idempotent
+merge_request: 24708
+author:
+type: changed

spec/services/merge_requests/merge_service_spec.rb

@@ -47,6 +47,15 @@ describe MergeRequests::MergeService do
         expect(note.note).to include 'merged'
       end
 
+      it 'is idempotent' do
+        # a first invocation of execute is performed on the before block
+        service.execute(merge_request)
+
+        expect(merge_request.merge_error).to be_falsey
+        expect(merge_request).to be_valid
+        expect(merge_request).to be_merged
+      end
+
       context 'when squashing' do
         let(:merge_params) do
           { commit_message: 'Merge commit message',

spec/services/merge_requests/post_merge_service_spec.rb

@@ -17,7 +17,6 @@ describe MergeRequests::PostMergeService do
     it 'refreshes the number of open merge requests for a valid MR', :use_clean_rails_memory_store_caching do
       # Cache the counter before the MR changed state.
       project.open_merge_requests_count
-      merge_request.update!(state: 'merged')
 
       service = described_class.new(project, user, {})
 

spec/services/merge_requests/squash_service_spec.rb

@@ -137,6 +137,17 @@ describe MergeRequests::SquashService do
       include_examples 'the squash succeeds'
     end
 
+    context 'when the merge request has already been merged' do
+      let(:merge_request) { merge_request_with_one_commit }
+
+      it 'idempotently returns a success' do
+        merge_request.mark_as_merged
+        result = service.execute
+
+        expect(result).to match(status: :success, squash_sha: merge_request.diff_head_sha)
+      end
+    end
+
     context 'git errors' do
       let(:merge_request) { merge_request_with_only_new_files }
       let(:error) { 'A test error' }