Merge branch 'fix-pipeline-for-empty-merge-request-diff' into 'master'

Fix pipeline error when trying to read empty merge request diff

When a user pushed something which resulted an empty merge request diff, `st_commits` would be `nil`. Therefore we also need to check if there exists `st_commits`.

We could tell this from:

``` ruby
def commits
  @commits ||= load_commits(st_commits || [])
end
```

and

``` ruby
def save_commits
  new_attributes = {}

  commits = compare.commits

  if commits.present?
    commits = Commit.decorate(commits, merge_request.source_project).reverse
    new_attributes[:st_commits] = dump_commits(commits)
  end

  update_columns_serialized(new_attributes)
end
```

Closes #22438

See merge request !6470

2 files changed, 43 insertions, 6 deletions

diff --git a/spec/models/merge_request_diff_spec.rb b/spec/models/merge_request_diff_spec.rb
index e5b185dc3f6..530a7def553 100644
--- a/spec/models/merge_request_diff_spec.rb
+++ b/spec/models/merge_request_diff_spec.rb
@@ -64,5 +64,27 @@ describe MergeRequestDiff, models: true do
         end
       end
     end
+
+    describe '#commits_sha' do
+      shared_examples 'returning all commits SHA' do
+        it 'returns all commits SHA' do
+          commits_sha = subject.commits_sha
+
+          expect(commits_sha).to eq(subject.commits.map(&:sha))
+        end
+      end
+
+      context 'when commits were loaded' do
+        before do
+          subject.commits
+        end
+
+        it_behaves_like 'returning all commits SHA'
+      end
+
+      context 'when commits were not loaded' do
+        it_behaves_like 'returning all commits SHA'
+      end
+    end
   end
 end
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 433aba7747b..12df6adde44 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -517,7 +517,7 @@ describe MergeRequest, models: true do
 
     context 'with multiple irrelevant merge_request_diffs' do
       before do
-        subject.update(target_branch: 'markdown')
+        subject.update(target_branch: 'v1.0.0')
       end
 
       it_behaves_like 'returning pipelines with proper ordering'
@@ -544,13 +544,28 @@ describe MergeRequest, models: true do
       subject.merge_request_diffs.flat_map(&:commits).map(&:sha).uniq
     end
 
-    before do
-      subject.update(target_branch: 'markdown')
+    shared_examples 'returning all SHA' do
+      it 'returns all SHA from all merge_request_diffs' do
+        expect(subject.merge_request_diffs.size).to eq(2)
+        expect(subject.all_commits_sha).to eq(all_commits_sha)
+      end
+    end
+
+    context 'with a completely different branch' do
+      before do
+        subject.update(target_branch: 'v1.0.0')
+      end
+
+      it_behaves_like 'returning all SHA'
     end
 
-    it 'returns all SHA from all merge_request_diffs' do
-      expect(subject.merge_request_diffs.size).to eq(2)
-      expect(subject.all_commits_sha).to eq(all_commits_sha)
+    context 'with a branch having no difference' do
+      before do
+        subject.update(target_branch: 'v1.1.0')
+        subject.reload # make sure commits were not cached
+      end
+
+      it_behaves_like 'returning all SHA'
     end
   end