Add checks for branch existence before changing HEAD

app/models/project.rb

@@ -1046,13 +1046,18 @@ class Project < ActiveRecord::Base
   end
 
   def change_head(branch)
-    repository.before_change_head
-    repository.rugged.references.create('HEAD',
-                                        "refs/heads/#{branch}",
-                                        force: true)
-    repository.copy_gitattributes(branch)
-    repository.after_change_head
-    reload_default_branch
+    if repository.branch_exists?(branch)
+      repository.before_change_head
+      repository.rugged.references.create('HEAD',
+                                          "refs/heads/#{branch}",
+                                          force: true)
+      repository.copy_gitattributes(branch)
+      repository.after_change_head
+      reload_default_branch
+    else
+      errors.add(:base, "Could not change HEAD: branch '#{branch}' does not exist")
+      false
+    end
   end
 
   def forked_from?(project)

app/services/projects/update_service.rb

@@ -10,7 +10,7 @@ module Projects
       end
 
       if changing_default_branch?
-        project.change_head(params[:default_branch])
+        return error("Could not set the default branch") unless project.change_head(params[:default_branch])
       end
 
       if project.update_attributes(params.except(:default_branch))

changelogs/unreleased/36010-api-v4-allows-setting-a-branch-that-doesn-t-exist-as-the-default-one.yml

+---
+title: Add checks for branch existence before changing HEAD
+merge_request: 13359
+author: Vitaliy @blackst0ne Klachkov

spec/models/project_spec.rb

@@ -1832,6 +1832,11 @@ describe Project do
   describe '#change_head' do
     let(:project) { create(:project, :repository) }
 
+    it 'returns error if branch does not exist' do
+      expect(project.change_head('unexisted-branch')).to be false
+      expect(project.errors.size).to eq(1)
+    end
+
     it 'calls the before_change_head and after_change_head methods' do
       expect(project.repository).to receive(:before_change_head)
       expect(project.repository).to receive(:after_change_head)

spec/services/projects/update_service_spec.rb

@@ -101,6 +101,13 @@ describe Projects::UpdateService, '#execute' do
 
       expect(Project.find(project.id).default_branch).to eq 'feature'
     end
+
+    it 'does not change a default branch' do
+      # The branch 'unexisted-branch' does not exist.
+      update_project(project, admin, default_branch: 'unexisted-branch')
+
+      expect(Project.find(project.id).default_branch).to eq 'master'
+    end
   end
 
   context 'when updating a project that contains container images' do