Fix bug when creating snippet and default branch not master

When a new snippet repository is created, the ref inside
HEAD is `master` even when the default branch is different.

This also happens with projects and wikis. These resources fix
this by explicitly rewrite the HEAD reference and we're applying
the same behavior to snippets.

When the repository is pushed or pulled we update the reference
inside the HEAD file.

app/models/snippet.rb

@@ -20,6 +20,7 @@ class Snippet < ApplicationRecord
   extend ::Gitlab::Utils::Override
 
   MAX_FILE_COUNT = 10
+  MASTER_BRANCH = 'master'
 
   cache_markdown_field :title, pipeline: :single_line
   cache_markdown_field :description
@@ -311,13 +312,27 @@ class Snippet < ApplicationRecord
 
   override :default_branch
   def default_branch
-    super || 'master'
+    super || MASTER_BRANCH
   end
 
   def repository_storage
     snippet_repository&.shard_name || self.class.pick_repository_storage
   end
 
+  # Repositories are created by default with the `master` branch.
+  # This method changes the `HEAD` file to point to the existing
+  # default branch in case it's not master.
+  def change_head_to_default_branch
+    return unless repository.exists?
+    return if default_branch == MASTER_BRANCH
+    # All snippets must have at least 1 file. Therefore, if
+    # `HEAD` is empty is because it's pointing to the wrong
+    # default branch
+    return unless repository.empty? || list_files('HEAD').empty?
+
+    repository.raw_repository.write_ref('HEAD', "refs/heads/#{default_branch}")
+  end
+
   def create_repository
     return if repository_exists? && snippet_repository
 

changelogs/unreleased/292017-fj-fix-bug-when-cloning-snippet-different-from-master.yml

+---
+title: Fix bug with snippets in HEAD when default branch is not master
+merge_request: 50366
+author:
+type: fixed

lib/gitlab/git_access_snippet.rb

@@ -30,7 +30,10 @@ module Gitlab
     def check(cmd, changes)
       check_snippet_accessibility!
 
-      super
+      super.tap do |_|
+        # Ensure HEAD points to the default branch in case it is not master
+        snippet.change_head_to_default_branch
+      end
     end
 
     override :download_ability

spec/lib/gitlab/git_access_snippet_spec.rb

@@ -390,6 +390,38 @@ RSpec.describe Gitlab::GitAccessSnippet do
     end
   end
 
+  describe 'HEAD realignment' do
+    let_it_be(:snippet) { create(:project_snippet, :private, :repository, project: project) }
+
+    shared_examples 'HEAD is updated to the snippet default branch' do
+      let(:actor) { snippet.author }
+
+      specify do
+        expect(snippet).to receive(:change_head_to_default_branch).and_call_original
+
+        subject
+      end
+
+      context 'when an error is raised' do
+        let(:actor) { nil }
+
+        it 'does not realign HEAD' do
+          expect(snippet).not_to receive(:change_head_to_default_branch).and_call_original
+
+          expect { subject }.to raise_error(described_class::ForbiddenError)
+        end
+      end
+    end
+
+    it_behaves_like 'HEAD is updated to the snippet default branch' do
+      subject { push_access_check }
+    end
+
+    it_behaves_like 'HEAD is updated to the snippet default branch' do
+      subject { pull_access_check }
+    end
+  end
+
   private
 
   def raise_snippet_not_found

spec/models/snippet_spec.rb

@@ -796,4 +796,90 @@ RSpec.describe Snippet do
   it_behaves_like 'can move repository storage' do
     let_it_be(:container) { create(:snippet, :repository) }
   end
+
+  describe '#change_head_to_default_branch' do
+    let(:head_path) { Rails.root.join(TestEnv.repos_path, "#{snippet.disk_path}.git", 'HEAD') }
+
+    subject { snippet.change_head_to_default_branch }
+
+    context 'when repository does not exist' do
+      let(:snippet) { create(:snippet) }
+
+      it 'does nothing' do
+        expect(snippet.repository_exists?).to eq false
+        expect(snippet.repository.raw_repository).not_to receive(:write_ref)
+
+        subject
+      end
+    end
+
+    context 'when repository is empty' do
+      let(:snippet) { create(:snippet, :empty_repo) }
+
+      before do
+        allow(Gitlab::CurrentSettings).to receive(:default_branch_name).and_return(default_branch)
+      end
+
+      context 'when default branch in settings is "master"' do
+        let(:default_branch) { 'master' }
+
+        it 'does nothing' do
+          expect(File.read(head_path).squish).to eq 'ref: refs/heads/master'
+
+          expect(snippet.repository.raw_repository).not_to receive(:write_ref)
+
+          subject
+        end
+      end
+
+      context 'when default branch in settings is different from "master"' do
+        let(:default_branch) { 'main' }
+
+        it 'changes the HEAD reference to the default branch' do
+          expect(File.read(head_path).squish).to eq 'ref: refs/heads/master'
+
+          subject
+
+          expect(File.read(head_path).squish).to eq "ref: refs/heads/#{default_branch}"
+        end
+      end
+    end
+
+    context 'when repository is not empty' do
+      let(:snippet) { create(:snippet, :empty_repo) }
+
+      before do
+        populate_snippet_repo
+      end
+
+      context 'when HEAD branch is empty' do
+        it 'changes HEAD to default branch' do
+          File.write(head_path, 'ref: refs/heads/non_existen_branch')
+          expect(File.read(head_path).squish).to eq 'ref: refs/heads/non_existen_branch'
+
+          subject
+
+          expect(File.read(head_path).squish).to eq 'ref: refs/heads/main'
+          expect(snippet.list_files('HEAD')).not_to be_empty
+        end
+      end
+
+      context 'when HEAD branch is not empty' do
+        it 'does nothing' do
+          File.write(head_path, 'ref: refs/heads/main')
+
+          expect(snippet.repository.raw_repository).not_to receive(:write_ref)
+
+          subject
+        end
+      end
+
+      def populate_snippet_repo
+        allow(Gitlab::CurrentSettings).to receive(:default_branch_name).and_return('main')
+
+        data = [{ file_path: 'new_file_test', content: 'bar' }]
+        snippet.snippet_repository.multi_files_action(snippet.author, data, branch_name: 'main', message: 'foo')
+      end
+    end
+  end
 end