Skip squashing with only one commit in a merge request

We noticed in https://gitlab.com/gitlab-org/gitaly/issues/2395 that the
backend was unnecessarily attempting to squash merge requests with one
commit. This happened because `SquashService` skips the squash if two
conditions are met:

1. The number of commits < 2
2. The `squash_commit_message` is null

If a `squash_commit_message` is passed, this indicates that the user
wants the squashed commit to have a different commit message.

By default, the frontend always sends `squash_commit_message`,
regardless of whether the squash button is available in the merge
request. With this change, the frontend will only send this parameter if
the button is shown (i.e. when the commit count is >= 2).

Alternatively, we could also change the backend to skip squashing if
only the first condition is met.

Closes https://gitlab.com/gitlab-org/gitlab/issues/198971

app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue

@@ -146,9 +146,15 @@ export default {
         auto_merge_strategy: useAutoMerge ? this.mr.preferredAutoMergeStrategy : undefined,
         should_remove_source_branch: this.removeSourceBranch === true,
         squash: this.squashBeforeMerge,
-        squash_commit_message: this.squashCommitMessage,
       };
 
+      // If users can't alter the squash message (e.g. for 1-commit merge requests),
+      // we shouldn't send the commit message because that would make the backend
+      // do unnecessary work.
+      if (this.shouldShowSquashBeforeMerge) {
+        options.squash_commit_message = this.squashCommitMessage;
+      }
+
       this.isMakingRequest = true;
       this.service
         .merge(options)

changelogs/unreleased/sh-fix-squash-one-commit.yml

+---
+title: Skip squashing with only one commit in a merge request
+merge_request: 23744
+author:
+type: fixed

spec/features/merge_requests/user_squashes_merge_request_spec.rb

@@ -6,6 +6,7 @@ describe 'User squashes a merge request', :js do
   let(:user) { create(:user) }
   let(:project) { create(:project, :repository) }
   let(:source_branch) { 'csv' }
+  let(:protected_source_branch) { false }
 
   let!(:original_head) { project.repository.commit('master') }
 
@@ -40,7 +41,7 @@ describe 'User squashes a merge request', :js do
   def accept_mr
     expect(page).to have_button('Merge')
 
-    uncheck 'Delete source branch'
+    uncheck 'Delete source branch' unless protected_source_branch
     click_on 'Merge'
   end
 
@@ -56,14 +57,34 @@ describe 'User squashes a merge request', :js do
   end
 
   context 'when the MR has only one commit' do
+    let(:source_branch) { 'master' }
+    let(:target_branch) { 'branch-merged' }
+    let(:protected_source_branch) { true }
+    let(:source_sha) { project.commit(source_branch).sha }
+    let(:target_sha) { project.commit(target_branch).sha }
+
     before do
-      merge_request = create(:merge_request, source_project: project, target_project: project, source_branch: 'master', target_branch: 'branch-merged')
+      merge_request = create(:merge_request, source_project: project, target_project: project, source_branch: source_branch, target_branch: target_branch, squash: true)
 
       visit project_merge_request_path(project, merge_request)
     end
 
-    it 'does not show the squash checkbox' do
+    it 'accepts the merge request without issuing a squash request', :sidekiq_inline do
+      expect_next_instance_of(Gitlab::GitalyClient::OperationService) do |instance|
+        expect(instance).not_to receive(:user_squash)
+      end
+
+      expect(project.repository.ancestor?(source_branch, target_branch)).to be_falsey
       expect(page).not_to have_field('squash')
+
+      accept_mr
+
+      expect(page).to have_content('Merged')
+
+      latest_target_commits = project.repository.commits_between(source_sha, target_sha).map(&:raw)
+
+      expect(latest_target_commits.count).to eq(1)
+      expect(project.repository.ancestor?(source_branch, target_branch)).to be_truthy
     end
   end