Handle error when fetching ref for MR with deleted source branch

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

@@ -1660,15 +1660,43 @@ describe MergeRequest do
   end
 
   describe '#fetch_ref' do
-    it 'sets "ref_fetched" flag to true' do
+    before do
       subject.update!(ref_fetched: nil)
+    end
+
+    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}/)
+
+        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
+
   describe '#ref_fetched?' do
     it 'does not perform git operation when value is cached' do
       subject.ref_fetched = true

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