Do not ensure HEAD is default_branch on snippets

Since create_repository_if_exists takes a default_branch argument, use
that in the initial call to create the repository rather than changing
it after the fact. This avoids potential racy calls to WriteRef, as well
as saves a roundtrip to Gitaly.

Changelog: changed

app/models/snippet.rb

@@ -350,20 +350,6 @@ class Snippet < ApplicationRecord
     snippet_repository&.shard_name || Repository.pick_storage_shard
   end
 
-  # Repositories are created with a default branch. This branch
-  # can be different from the default branch set in the platform.
-  # This method changes the `HEAD` file to point to the existing
-  # default branch in case it's different.
-  def change_head_to_default_branch
-    return unless repository.exists?
-    # 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
 

lib/gitlab/git_access_snippet.rb

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

spec/lib/gitlab/git_access_snippet_spec.rb

@@ -397,38 +397,6 @@ 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_not_found(message_key)

spec/models/snippet_spec.rb

@@ -893,58 +893,4 @@ 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 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