Invalidate branches cache on PostReceive

Whenever `PostReceive` is enqueued, `UpdateMergeRequestsWorker`
is enqueued and `MergeRequests::RefreshService` is called, it'll
check if the source branch of each MR asssociated to the push exists
or not via `MergeRequest#source_branch_exists?`. The said method will
call `Repository#branch_exists?` which is cached in `Rails.cache`.

When the cache contains outdated data and the source branch actually
exists, the `MergeRequests#RefreshService` job will close associated
MRs which is not correct.

The fix is to expire the branches cache of the project so we have
updated data during the post receive hook which will help in the
accuracy of the check if we need to close associated MRs or not.

1 files changed, 52 insertions, 0 deletions

diff --git a/spec/lib/gitlab/git_post_receive_spec.rb b/spec/lib/gitlab/git_post_receive_spec.rb
new file mode 100644
index 00000000000..f4a10d8d984
--- /dev/null
+++ b/spec/lib/gitlab/git_post_receive_spec.rb
@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe ::Gitlab::GitPostReceive do
+  let(:project) { create(:project) }
+
+  subject { described_class.new(project, "project-#{project.id}", changes.dup, {}) }
+
+  describe '#branches_exist?' do
+    context 'with no branches' do
+      let(:changes) do
+        <<~EOF
+          654321 210987 refs/nobranches/tag1
+          654322 210986 refs/tags/test1
+          654323 210985 refs/merge-requests/mr1
+        EOF
+      end
+
+      it 'returns false' do
+        expect(subject.branches_exist?).to be_falsey
+      end
+    end
+
+    context 'with branches' do
+      let(:changes) do
+        <<~EOF
+          654322 210986 refs/heads/test1
+          654321 210987 refs/tags/tag1
+          654323 210985 refs/merge-requests/mr1
+        EOF
+      end
+
+      it 'returns true' do
+        expect(subject.branches_exist?).to be_truthy
+      end
+    end
+
+    context 'with malformed changes' do
+      let(:changes) do
+        <<~EOF
+          ref/heads/1 a
+          somebranch refs/heads/2
+        EOF
+      end
+
+      it 'returns false' do
+        expect(subject.branches_exist?).to be_falsey
+      end
+    end
+  end
+end