Fix PostReceive failing for project mirrors missing local branch

For some reason, if the local branch is deleted but the remote branch
exists, PostReceive fails with a nil error.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50843

ee/app/models/ee/repository.rb

@@ -42,7 +42,7 @@ module EE
       branch_commit = commit("refs/heads/#{branch_name}")
       upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{branch_name}")
 
-      if upstream_commit
+      if branch_commit && upstream_commit
         !raw_repository.ancestor?(branch_commit.id, upstream_commit.id)
       else
         false
@@ -53,7 +53,7 @@ module EE
       branch_commit = commit("refs/heads/#{branch_name}")
       upstream_commit = commit("refs/remotes/#{remote_ref}/#{branch_name}")
 
-      if upstream_commit
+      if branch_commit && upstream_commit
         !raw_repository.ancestor?(upstream_commit.id, branch_commit.id)
       else
         false
@@ -64,7 +64,7 @@ module EE
       branch_commit = commit("refs/heads/#{branch_name}")
       upstream_commit = commit("refs/remotes/#{MIRROR_REMOTE}/#{branch_name}")
 
-      if upstream_commit
+      if branch_commit && upstream_commit
         ancestor?(branch_commit.id, upstream_commit.id)
       else
         false

ee/changelogs/unreleased/sh-fix-issue-50843.yml

+---
+title: Fix PostReceive failing for project mirrors missing local branch
+merge_request: 8495
+author:
+type: fixed

ee/spec/services/ee/git_push_service_spec.rb

+require 'spec_helper'
+
+describe GitPushService do
+  include RepoHelpers
+
+  set(:user)     { create(:user) }
+  let(:blankrev) { Gitlab::Git::BLANK_SHA }
+  let(:oldrev)   { sample_commit.parent_id }
+  let(:newrev)   { sample_commit.id }
+  let(:ref)      { 'refs/heads/master' }
+
+  context 'with pull project' do
+    set(:project) { create(:project, :repository, :mirror) }
+
+    subject do
+      described_class.new(project, user, oldrev: oldrev, newrev: newrev, ref: ref)
+    end
+
+    context 'deleted branch' do
+      let(:newrev) { blankrev }
+
+      it 'handles when remote branch exists' do
+        allow(project.repository).to receive(:commit).and_call_original
+        allow(project.repository).to receive(:commit).with("master").and_return(nil)
+        expect(project.repository).to receive(:commit).with("refs/remotes/upstream/master").and_return(sample_commit)
+
+        subject.execute
+      end
+    end
+  end
+end