Handle error when fetching ref for MR with deleted source branch

If the ref doesn't exist, and the source branch is deleted, we can't get it back
easily. Previously, we ignored this error by shelling out, so replicate that
behaviour.

app/models/merge_request.rb

@@ -950,8 +950,6 @@ class MergeRequest < ActiveRecord::Base
       source_project.repository, source_branch) do |commit|
         if commit
           target_project.repository.write_ref(ref_path, commit.sha)
-        else
-          raise Rugged::ReferenceError, 'source repository is empty'
         end
       end
   end

app/models/repository.rb

@@ -996,7 +996,11 @@ class Repository
     if start_repository == self
       yield commit(start_branch_name)
     else
-      sha = start_repository.commit(start_branch_name).sha
+      start_commit = start_repository.commit(start_branch_name)
+
+      return yield nil unless start_commit
+
+      sha = start_commit.sha
 
       if branch_commit = commit(sha)
         yield branch_commit

changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml

+---
+title: Fix 500 error on merged merge requests when GitLab is restored from a backup
+merge_request:
+author:
+type: fixed

spec/models/merge_request_spec.rb

@@ -1660,12 +1660,40 @@ describe MergeRequest do
   end
 
   describe '#fetch_ref' do
-    it 'sets "ref_fetched" flag to true' do
+    before do
       subject.update!(ref_fetched: nil)
+    end
 
-      subject.fetch_ref
+    context 'when the branch exists' do
+      it 'writes the ref' do
+        expect(subject.target_project.repository).to receive(:write_ref).with(subject.ref_path, /\h{40}/)
 
-      expect(subject.reload.ref_fetched).to be_truthy
+        subject.fetch_ref
+      end
+
+      it 'sets "ref_fetched" flag to true' do
+        subject.fetch_ref
+
+        expect(subject.reload.ref_fetched).to be_truthy
+      end
+    end
+
+    context 'when the branch does not exist' do
+      before do
+        subject.source_branch = 'definitely-not-master'
+      end
+
+      it 'does not write the ref' do
+        expect(subject.target_project.repository).not_to receive(:write_ref)
+
+        subject.fetch_ref
+      end
+
+      it 'sets "ref_fetched" flag to true' do
+        subject.fetch_ref
+
+        expect(subject.reload.ref_fetched).to be_truthy
+      end
     end
   end
 

spec/models/repository_spec.rb

@@ -2110,4 +2110,51 @@ describe Repository, models: true do
       end
     end
   end
+
+  describe '#with_repo_branch_commit' do
+    context 'when comparing with the same repository' do
+      let(:start_repository) { repository }
+
+      context 'when the branch exists' do
+        let(:start_branch_name) { 'master' }
+
+        it 'yields the commit' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(an_instance_of(::Commit))
+        end
+      end
+
+      context 'when the branch does not exist' do
+        let(:start_branch_name) { 'definitely-not-master' }
+
+        it 'yields nil' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(nil)
+        end
+      end
+    end
+
+    context 'when comparing with another repository' do
+      let(:forked_project) { create(:project, :repository) }
+      let(:start_repository) { forked_project.repository }
+
+      context 'when the branch exists' do
+        let(:start_branch_name) { 'master' }
+
+        it 'yields the commit' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(an_instance_of(::Commit))
+        end
+      end
+
+      context 'when the branch does not exist' do
+        let(:start_branch_name) { 'definitely-not-master' }
+
+        it 'yields nil' do
+          expect { |b| repository.with_repo_branch_commit(start_repository, start_branch_name, &b) }
+            .to yield_with_args(nil)
+        end
+      end
+    end
+  end
 end