ensured namespace creation is working concurrently

app/models/concerns/namespace_parent.rb

@@ -9,12 +9,11 @@ module NamespaceParent
 
   def ensure_namespace
     return namespace if namespace.present?
+    return build_namespace unless persisted?
 
-    ns = build_namespace
-    if persisted?
-      ns.save
-      ns.ensure_personal_namespace_administrator! if ns.user_type?
-    end
+    ns = Namespace.create_or_find_by(parent: self)
+    association(:namespace).target = ns
+    ns.ensure_personal_namespace_administrator! if ns.user_type?
     ns
   end
 end

spec/models/concerns/namespace_parent_spec.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe NamespaceParent do
+  describe '#ensure_namespace', :disable_transaction do
+    let!(:user) { create(:user) }
+    let(:thread_count) { [ActiveRecord::Base.connection_pool.size - 1, 2].max }
+
+    it 'creates exactly one namespace when called concurrently from separate connections' do
+      barrier = Concurrent::CyclicBarrier.new(thread_count)
+
+      results = Array.new(thread_count) do
+        Thread.new do
+          ActiveRecord::Base.connection_pool.with_connection do
+            barrier.wait
+            User.find(user.id).ensure_namespace
+          end
+        end
+      end.map(&:value)
+
+      expect(results).to all(be_a(Namespace))
+      expect(results.map(&:id).uniq.size).to eq(1)
+      expect(Namespace.where(parent: user).count).to eq(1)
+    end
+  end
+end