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

app/models/merge_request_diff.rb

@@ -30,6 +30,10 @@ class MergeRequestDiff < ActiveRecord::Base
     select(column_names - ['st_diffs'])
   end
 
+  def st_commits
+    super || []
+  end
+
   # Collect information about commits and diff from repository
   # and save it to the database as serialized data
   def save_git_content
@@ -83,7 +87,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def commits
-    @commits ||= load_commits(st_commits || [])
+    @commits ||= load_commits(st_commits)
   end
 
   def reload_commits

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

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'
@@ -544,16 +544,31 @@ describe MergeRequest, models: true do
       subject.merge_request_diffs.flat_map(&:commits).map(&:sha).uniq
     end
 
-    before do
-      subject.update(target_branch: 'markdown')
-    end
-
+    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
+
+    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
+
   describe '#participants' do
     let(:project) { create(:project, :public) }