Handle branch and tag names which are commit ids

Adds a test where a branch name is also a valid commit id. Git, the
binary should create an error message which is difficult to parse and
leading to errors later, as seen in: gitlab-org/gitlab-ce#43222

To catch these cases in the future,
gitlab-test@1942eed5cc108b19c7405106e81fa96125d0be22 was created. Which
a branch name matching the commit

3 files changed, 48 insertions, 10 deletions

diff --git a/changelogs/unreleased/zj-branch-contains-git-message.yml b/changelogs/unreleased/zj-branch-contains-git-message.yml
new file mode 100644
index 00000000000..ce034e7ec87
--- /dev/null
+++ b/changelogs/unreleased/zj-branch-contains-git-message.yml
@@ -0,0 +1,5 @@
+---
+title: Allow branch names to be named the same as the sha it points to
+merge_request:
+author:
+type: fixed
diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index a10bc0dd32b..072ad312ab8 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -1349,7 +1349,7 @@ module Gitlab
           if is_enabled
             gitaly_ref_client.branch_names_contains_sha(sha)
           else
-            refs_contains_sha(:branch, sha)
+            refs_contains_sha('refs/heads/', sha)
           end
         end
       end
@@ -1359,7 +1359,7 @@ module Gitlab
           if is_enabled
             gitaly_ref_client.tag_names_contains_sha(sha)
           else
-            refs_contains_sha(:tag, sha)
+            refs_contains_sha('refs/tags/', sha)
           end
         end
       end
@@ -1458,19 +1458,25 @@ module Gitlab
         end
       end
 
-      def refs_contains_sha(ref_type, sha)
-        args = %W(#{ref_type} --contains #{sha})
-        names = run_git(args).first
+      def refs_contains_sha(refs_prefix, sha)
+        refs_prefix << "/" unless refs_prefix.ends_with?('/')
 
-        return [] unless names.respond_to?(:split)
+        # By forcing the output to %(refname) each line wiht a ref will start with
+        # the ref prefix. All other lines can be discarded.
+        args = %W(for-each-ref --contains=#{sha} --format=%(refname) #{refs_prefix})
+        names, code = run_git(args)
 
-        names = names.split("\n").map(&:strip)
+        return [] unless code.zero?
 
-        names.each do |name|
-          name.slice! '* '
+        refs = []
+        left_slice_count = refs_prefix.length
+        names.lines.each do |line|
+          next unless line.start_with?(refs_prefix)
+
+          refs << line.rstrip[left_slice_count..-1]
         end
 
-        names
+        refs
       end
 
       def rugged_write_config(full_path:)
diff --git a/spec/lib/gitlab/git/repository_spec.rb b/spec/lib/gitlab/git/repository_spec.rb
index 0e9150964fa..9feaaec3f3b 100644
--- a/spec/lib/gitlab/git/repository_spec.rb
+++ b/spec/lib/gitlab/git/repository_spec.rb
@@ -600,6 +600,33 @@ describe Gitlab::Git::Repository, seed_helper: true do
     end
   end
 
+  describe '#branch_names_contains_sha' do
+    shared_examples 'returning the right branches' do
+      let(:head_id) { repository.rugged.head.target.oid }
+      let(:new_branch) { head_id }
+
+      before do
+        repository.create_branch(new_branch, 'master')
+      end
+
+      after do
+        repository.delete_branch(new_branch)
+      end
+
+      it 'displays that branch' do
+        expect(repository.branch_names_contains_sha(head_id)).to include('master', new_branch)
+      end
+    end
+
+    context 'when Gitaly is enabled' do
+      it_behaves_like 'returning the right branches'
+    end
+
+    context 'when Gitaly is disabled', :disable_gitaly do
+      it_behaves_like 'returning the right branches'
+    end
+  end
+
   describe "#refs_hash" do
     subject { repository.refs_hash }