From b63e8d64c1a2d39a7e18f22b526e48bab2c79956 Mon Sep 17 00:00:00 2001
From: Sean McGivern <sean@gitlab.com>
Date: Thu, 28 Sep 2017 11:11:10 +0100
Subject: [PATCH] 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.
---
 ...-undefined-method-sha-for-nil-nilclass.yml |  5 ++
 lib/gitlab/git/repository.rb                  |  9 ++-
 spec/lib/gitlab/git/repository_spec.rb        | 78 +++++++++++++++++++
 3 files changed, 90 insertions(+), 2 deletions(-)
 create mode 100644 changelogs/unreleased/38319-nomethoderror-undefined-method-sha-for-nil-nilclass.yml

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/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index 8c1df650567..6baff362dad 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -947,7 +947,11 @@ module Gitlab
         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
@@ -976,8 +980,9 @@ module Gitlab
         with_repo_branch_commit(source_repository, source_branch) do |commit|
           if commit
             write_ref(local_ref, commit.sha)
+            true
           else
-            raise Rugged::ReferenceError, 'source repository is empty'
+            false
           end
         end
       end
diff --git a/spec/lib/gitlab/git/repository_spec.rb b/spec/lib/gitlab/git/repository_spec.rb
index 4fc26c625a5..5effaf2b043 100644
--- a/spec/lib/gitlab/git/repository_spec.rb
+++ b/spec/lib/gitlab/git/repository_spec.rb
@@ -1332,6 +1332,84 @@ describe Gitlab::Git::Repository, seed_helper: true do
     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(Gitlab::Git::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(:start_repository) { Gitlab::Git::Repository.new('default', TEST_MUTABLE_REPO_PATH, '') }
+
+      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(Gitlab::Git::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
+
+  describe '#fetch_source_branch' do
+    let(:local_ref) { 'refs/merge-requests/1/head' }
+
+    context 'when the branch exists' do
+      let(:source_branch) { 'master' }
+
+      it 'writes the ref' do
+        expect(repository).to receive(:write_ref).with(local_ref, /\h{40}/)
+
+        repository.fetch_source_branch(repository, source_branch, local_ref)
+      end
+
+      it 'returns true' do
+        expect(repository.fetch_source_branch(repository, source_branch, local_ref)).to eq(true)
+      end
+    end
+
+    context 'when the branch does not exist' do
+      let(:source_branch) { 'definitely-not-master' }
+
+      it 'does not write the ref' do
+        expect(repository).not_to receive(:write_ref)
+
+        repository.fetch_source_branch(repository, source_branch, local_ref)
+      end
+
+      it 'returns false' do
+        expect(repository.fetch_source_branch(repository, source_branch, local_ref)).to eq(false)
+      end
+    end
+  end
+
   def create_remote_branch(repository, remote_name, branch_name, source_branch_name)
     source_branch = repository.branches.find { |branch| branch.name == source_branch_name }
     rugged = repository.rugged