fix: serialize generated pack regeneration

CHANGELOG.md

@@ -34,6 +34,7 @@ After a release, run `/update-changelog` in Claude Code to analyze commits, writ
 
 #### Fixed
 
+- **Generated pack regeneration is now serialized**: `generate_packs_if_stale` now uses a Rails `tmp/` lock file, re-checks staleness after waiting, and avoids concurrent cleanup/regeneration races when multiple processes trigger auto-bundling at the same time. Fixes [Issue 1627](https://github.com/shakacode/react_on_rails/issues/1627). [PR 3231](https://github.com/shakacode/react_on_rails/pull/3231) by [justin808](https://github.com/justin808).
 - **[Pro]** **Node renderer now exposes `performance` when `supportModules: true`**: React 19's development build of `React.lazy` calls `performance.now()`, which previously threw `ReferenceError: performance is not defined` inside the node renderer's VM context unless users manually added `performance` via `additionalContext`. `performance` is now included in the default globals alongside `Buffer`, `process`, etc. Fixes [Issue 3154](https://github.com/shakacode/react_on_rails/issues/3154). [PR 3158](https://github.com/shakacode/react_on_rails/pull/3158) by [justin808](https://github.com/justin808).
 - **Client startup now recovers if initialization begins during `interactive` after `DOMContentLoaded` already fired**: React on Rails now still initializes the page when the client bundle starts in the browser timing window after `DOMContentLoaded` but before the document reaches `complete`. Fixes [Issue 3150](https://github.com/shakacode/react_on_rails/issues/3150). [PR 3151](https://github.com/shakacode/react_on_rails/pull/3151) by [ihabadham](https://github.com/ihabadham).
 - **Doctor accepts TypeScript server bundle entrypoints**: `react_on_rails:doctor` now resolves common source entrypoint suffixes (`.js`, `.jsx`, `.ts`, `.tsx`, `.mjs`, `.cjs`) before warning that the server bundle is missing, preventing false positives when apps use `server-bundle.ts`. [PR 3111](https://github.com/shakacode/react_on_rails/pull/3111) by [justin808](https://github.com/justin808).

react_on_rails/lib/react_on_rails/packs_generator.rb

@@ -32,6 +32,7 @@ class PacksGenerator
     # Auto-registration requires nested_entries support which was added in 7.0.0
     # Note: The gemspec requires Shakapacker >= 6.0 for basic functionality
     MINIMUM_SHAKAPACKER_VERSION_FOR_AUTO_BUNDLING = "7.0.0"
+    GENERATED_PACKS_LOCK_TTL_SECONDS = 120
 
     def self.instance
       @instance ||= PacksGenerator.new
@@ -48,26 +49,69 @@ def generate_packs_if_stale
 
       verbose = ENV["REACT_ON_RAILS_VERBOSE"] == "true"
 
-      add_generated_pack_to_server_bundle
+      with_generated_packs_lock(verbose: verbose) do
+        add_generated_pack_to_server_bundle
 
-      # Clean any non-generated files from directories
-      clean_non_generated_files_with_feedback(verbose: verbose)
+        # Clean any non-generated files from directories
+        clean_non_generated_files_with_feedback(verbose: verbose)
 
-      are_generated_files_present_and_up_to_date = Dir.exist?(generated_packs_directory_path) &&
-                                                   File.exist?(generated_server_bundle_file_path) &&
-                                                   !stale_or_missing_packs?
+        if generated_files_present_and_up_to_date?
+          puts Rainbow("✅ Generated packs are up to date, no regeneration needed").green if verbose
+          return
+        end
 
-      if are_generated_files_present_and_up_to_date
-        puts Rainbow("✅ Generated packs are up to date, no regeneration needed").green if verbose
-        return
+        clean_generated_directories_with_feedback(verbose: verbose)
+        generate_packs(verbose: verbose)
       end
-
-      clean_generated_directories_with_feedback(verbose: verbose)
-      generate_packs(verbose: verbose)
     end
 
     private
 
+    def generated_files_present_and_up_to_date?
+      Dir.exist?(generated_packs_directory_path) &&
+        File.exist?(generated_server_bundle_file_path) &&
+        !stale_or_missing_packs?
+    end
+
+    def with_generated_packs_lock(verbose: false)
+      lock_path = generated_packs_lock_path
+      FileUtils.mkdir_p(lock_path.dirname)
+      remove_stale_generated_packs_lock(lock_path, verbose: verbose)
+
+      File.open(lock_path, File::RDWR | File::CREAT, 0o644) do |lock_file|
+        puts Rainbow("🔒 Waiting for generated packs lock at #{lock_path}").yellow if verbose
+        lock_file.flock(File::LOCK_EX)
+        lock_file.rewind
+        lock_file.truncate(0)
+        lock_file.write("pid=#{Process.pid}\nstarted_at=#{Time.now.utc}\n")
+        lock_file.flush
+
+        yield
+      ensure
+        lock_file&.flock(File::LOCK_UN)
+      end
+    end
+
+    def remove_stale_generated_packs_lock(lock_path, verbose: false)
+      return unless File.exist?(lock_path)
+      return unless File.mtime(lock_path) < Time.now - GENERATED_PACKS_LOCK_TTL_SECONDS
+
+      File.open(lock_path, File::RDWR) do |lock_file|
+        next unless lock_file.flock(File::LOCK_EX | File::LOCK_NB)
+
+        FileUtils.rm_f(lock_path)
+        puts Rainbow("🧹 Removed stale generated packs lock at #{lock_path}").yellow if verbose
+      ensure
+        lock_file&.flock(File::LOCK_UN)
+      end
+    rescue Errno::ENOENT
+      nil
+    end
+
+    def generated_packs_lock_path
+      Rails.root.join("tmp", "react_on_rails_generate_packs.lock")
+    end
+
     def generate_packs(verbose: false)
       # Check for name conflicts between components and stores
       check_for_component_store_name_conflicts

react_on_rails/spec/dummy/spec/packs_generator_spec.rb

@@ -45,6 +45,7 @@ def self.configuration
     after do
       ReactOnRails.configuration.server_bundle_js_file = old_server_bundle
       ReactOnRails.configuration.components_subdirectory = old_subdirectory
+      ReactOnRails.configuration.auto_load_bundle = old_auto_load_bundle
 
       FileUtils.rm_rf "#{packer_source_entry_path}/generated"
       FileUtils.rm_rf generated_server_bundle_file_path
@@ -410,6 +411,44 @@ def self.rsc_support_enabled?
         expect(File.read(server_bundle_js_file_path).scan(/(?=#{test_string})/).count).to equal(1)
       end
 
+      it "serializes concurrent generation and rechecks staleness after waiting" do
+        generator = described_class.new
+        generation_started = Queue.new
+        release_generation = Queue.new
+        second_completed = Queue.new
+        generation_count = 0
+
+        allow(generator).to receive(:add_generated_pack_to_server_bundle)
+        allow(generator).to receive(:clean_non_generated_files_with_feedback)
+        allow(generator).to receive(:clean_generated_directories_with_feedback)
+        allow(generator).to receive(:generated_files_present_and_up_to_date?).and_return(false, true)
+        allow(generator).to receive(:generate_packs) do
+          generation_count += 1
+          generation_started << true
+          release_generation.pop
+        end
+
+        first_thread = Thread.new { generator.generate_packs_if_stale }
+        generation_started.pop
+
+        second_thread = Thread.new do
+          generator.generate_packs_if_stale
+          second_completed << true
+        end
+
+        sleep 0.1
+        expect { second_completed.pop(true) }.to raise_error(ThreadError)
+
+        release_generation << true
+        first_thread.join
+        second_thread.join
+
+        expect(generation_count).to eq(1)
+      ensure
+        first_thread&.kill if first_thread&.alive?
+        second_thread&.kill if second_thread&.alive?
+      end
+
       it "generate packs if a new component is added" do
         create_new_component("NewComponent")