Add PageMutex to Publisher

Prevent publishing the same page at the same time. The PageMutex is locking the page for the time it takes until the page is published. After that the lock is released and it can publish again.

app/models/alchemy/page/publisher.rb

@@ -20,21 +20,23 @@ def initialize(page)
       #
       def publish!(public_on:)
         Page.transaction do
-          version = public_version(public_on)
-          DeleteElements.new(version.elements).call
-
-          repository = page.draft_version.element_repository
-          ActiveRecord::Base.no_touching do
-            Element.acts_as_list_no_update do
-              repository.visible.not_nested.each.with_index(1) do |element, position|
-                Alchemy::DuplicateElement.new(element, repository: repository).call(
-                  page_version_id: version.id,
-                  position: position
-                )
+          PageMutex.with_lock!(@page) do
+            version = public_version(public_on)
+            DeleteElements.new(version.elements).call
+
+            repository = page.draft_version.element_repository
+            ActiveRecord::Base.no_touching do
+              Element.acts_as_list_no_update do
+                repository.visible.not_nested.each.with_index(1) do |element, position|
+                  Alchemy::DuplicateElement.new(element, repository: repository).call(
+                    page_version_id: version.id,
+                    position: position
+                  )
+                end
               end
             end
+            page.update(published_at: public_on)
           end
-          page.update(published_at: public_on)
         end
 
         Alchemy.publish_targets.each { |p| p.perform_later(page) }

spec/models/alchemy/page/publisher_spec.rb

@@ -95,5 +95,25 @@
         publish
       end
     end
+
+    context "in parallel" do
+      before do
+        # another publisher - instance created a mutex entry and locked the page
+        Alchemy::PageMutex.create(page: page, created_at: 5.seconds.ago)
+      end
+
+      it "fails, if another process locked the page" do
+        expect { publish }.to raise_error Alchemy::PageMutex::LockFailed
+      end
+
+      context "another page" do
+        let(:another_page) { create(:alchemy_page) }
+        let(:publisher) { described_class.new(another_page) }
+
+        it "should allow the publishing of another page" do
+          expect { publish }.to change { another_page.versions.published.count }.by(1)
+        end
+      end
+    end
   end
 end