Properly abort a merge when merge conflicts occur

If somehow a user attempted to accept a merge request that had
conflicts (e.g. the "Accept Merge Request" button or the MR itself was
not updated), `MergeService` did not properly detect that a conflict
occurred. It would assume that the MR went through without any issues
and close the MR as though everything was fine. This could cause
data loss if the source branch were removed.

Closes #20425

CHANGELOG

@@ -38,6 +38,7 @@ v 8.11.0 (unreleased)
 
 v 8.10.3 (unreleased)
   - Fix hooks missing on imported GitLab projects
+  - Properly abort a merge when merge conflicts occur
 
 v 8.10.2
   - User can now search branches by name. !5144

app/services/merge_requests/merge_service.rb

@@ -35,7 +35,13 @@ module MergeRequests
       }
 
       commit_id = repository.merge(current_user, merge_request, options)
-      merge_request.update(merge_commit_sha: commit_id)
+
+      if commit_id
+        merge_request.update(merge_commit_sha: commit_id)
+      else
+        merge_request.update(merge_error: 'Conflicts detected during merge')
+        false
+      end
     rescue GitHooksService::PreReceiveError => e
       merge_request.update(merge_error: e.message)
       false

spec/services/merge_requests/merge_service_spec.rb

@@ -75,6 +75,17 @@ describe MergeRequests::MergeService, services: true do
 
         expect(merge_request.merge_error).to eq("error")
       end
+
+      it 'aborts if there is a merge conflict' do
+        allow_any_instance_of(Repository).to receive(:merge).and_return(false)
+        allow(service).to receive(:execute_hooks)
+
+        service.execute(merge_request)
+
+        expect(merge_request.open?).to be_truthy
+        expect(merge_request.merge_commit_sha).to be_nil
+        expect(merge_request.merge_error).to eq("Conflicts detected during merge")
+      end
     end
   end
 end