From e38fcc8cfdbc9c4691022532dd2cee8dace20a1c Mon Sep 17 00:00:00 2001
From: Sean McGivern <sean@gitlab.com>
Date: Thu, 28 Sep 2017 11:11:10 +0100
Subject: 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                        |  2 -
 app/models/repository.rb                           |  6 ++-
 ...error-undefined-method-sha-for-nil-nilclass.yml |  5 +++
 spec/models/merge_request_spec.rb                  | 34 ++++++++++++++--
 spec/models/repository_spec.rb                     | 47 ++++++++++++++++++++++
 5 files changed, 88 insertions(+), 6 deletions(-)
 create mode 100644 changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 5043711c2ea..74ea71a52c3 100644
--- a/app/models/merge_request.rb
+++ b/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
diff --git a/app/models/repository.rb b/app/models/repository.rb
index 46cb033834d..e55daf48dbb 100644
--- a/app/models/repository.rb
+++ b/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
diff --git a/changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml b/changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml
new file mode 100644
index 00000000000..f3c39827590
--- /dev/null
+++ b/changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml
@@ -0,0 +1,5 @@
+---
+title: Fix 500 error on merged merge requests when GitLab is restored from a backup
+merge_request:
+author:
+type: fixed
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 5baa7c81ecc..0028b60749b 100644
--- a/spec/models/merge_request_spec.rb
+++ b/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
 
diff --git a/spec/models/repository_spec.rb b/spec/models/repository_spec.rb
index 3f815bb38ef..131340cee4c 100644
--- a/spec/models/repository_spec.rb
+++ b/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
-- 
cgit v1.2.1