refactor: unify renderer cache staging under PreSeedRendererCache#call(mode:)

Collapse the copy-based PreSeedRendererCache and the symlink-based
PrepareNodeRenderBundles into a single entry point with a `mode:` keyword
argument. Both modes produce the identical <cache>/<hash>/<hash>.js layout;
only the file operation differs (FileUtils.cp vs relative symlink).

Changes:

- ReactOnRailsPro::PreSeedRendererCache.call now accepts mode: :copy
  (default, for Docker/image builds) or mode: :symlink (same-filesystem
  workflows). Rejects unknown modes with ArgumentError.
- mode: :copy now raises a clear error when neither
  RENDERER_SERVER_BUNDLE_CACHE_PATH nor RENDERER_BUNDLE_PATH is set in
  non-dev/test environments. The Node renderer's default lookup can
  differ from the Ruby side (falling back to /tmp when cwd is outside
  the app tree), so silent fallback is unsafe for production-like
  deploys and was a silent-misconfig footgun.
- `react_on_rails_pro:pre_seed_renderer_cache` rake task accepts
  MODE=copy (default) or MODE=symlink.
- ReactOnRailsPro::PrepareNodeRenderBundles is now a thin deprecated
  shim that emits a once-per-process warning and delegates to
  PreSeedRendererCache.call(mode: :symlink). Public class and the
  `pre_stage_bundle_for_node_renderer` rake task remain for backward
  compatibility.
- AssetsPrecompile.call invokes the unified API with mode: :symlink
  after precompile (no behavior change for existing users).
- react_on_rails:doctor scans common deploy-script locations
  (Procfile*, Dockerfile, bin/*) for references to the deprecated
  pre_stage_bundle_for_node_renderer task and surfaces a migration
  warning.
- Docs: docs/pro/node-renderer.md describes the unified API, both
  modes, the non-dev env-var requirement for copy mode, and the
  deprecation story.
- CHANGELOG: new Changed entry describing the unification.
- Tests: mode validation, raise-in-non-dev guard (including symlink
  opt-out), deprecation warning on the shim, and a doctor check test.

Context: stacked on PR #3124 (jg/3122-preseed-renderer-cache).
Refs: #3122

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: justin808

CHANGELOG.md

@@ -28,6 +28,10 @@ After a release, run `/update-changelog` in Claude Code to analyze commits, writ
 
 - **[Pro]** **Pre-seed renderer cache for Docker builds**: New `react_on_rails_pro:pre_seed_renderer_cache` rake task copies compiled server bundles into the Node Renderer's bundle-hash cache directory structure during Docker image builds, eliminating the 410→retry cold-start latency (200ms–1s+) on the first SSR request after deployment. Supports `RENDERER_SERVER_BUNDLE_CACHE_PATH`, RSC bundles, and rolling-deploy guidance centered on current and previous bundle hashes. The legacy `pre_stage_bundle_for_node_renderer` task now stages the same cache layout via symlinks for same-filesystem workflows. **Note:** `RENDERER_BUNDLE_PATH` is now deprecated in favor of `RENDERER_SERVER_BUNDLE_CACHE_PATH` across both tasks. Existing users with `RENDERER_BUNDLE_PATH` set will see a deprecation warning on stderr. [PR 3124](https://github.com/shakacode/react_on_rails/pull/3124) by [justin808](https://github.com/justin808).
 
+#### Changed
+
+- **[Pro]** **Unified renderer cache staging**: `ReactOnRailsPro::PreSeedRendererCache.call(mode: :copy | :symlink)` is now the single entry point for staging the Node Renderer cache. Both modes produce the same `<cache>/<bundleHash>/<bundleHash>.js` layout. The `react_on_rails_pro:pre_seed_renderer_cache` rake task accepts `MODE=copy` (default; Docker/image builds) or `MODE=symlink` (same-filesystem). `MODE=copy` now raises a clear error when neither `RENDERER_SERVER_BUNDLE_CACHE_PATH` nor `RENDERER_BUNDLE_PATH` is set in non-dev/test environments, because the Node renderer's default lookup can differ from the Ruby side and would silently drop pre-seeded bundles in the wrong directory. The legacy `react_on_rails_pro:pre_stage_bundle_for_node_renderer` task and `ReactOnRailsPro::PrepareNodeRenderBundles` class remain as deprecated shims that emit a once-per-process warning and delegate to `mode: :symlink`. `react_on_rails:doctor` flags deploy scripts that still reference the deprecated task.
+
 #### Fixed
 
 - **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).

docs/pro/node-renderer.md

@@ -119,26 +119,35 @@ When a new container starts, the Node Renderer has an empty bundle cache. The fi
 
 ### Pre-seeding the bundle cache
 
-The `pre_seed_renderer_cache` rake task copies compiled server bundles directly into the renderer's cache directory during your Docker build, so the renderer finds them immediately on startup:
+The `pre_seed_renderer_cache` rake task stages compiled server bundles directly into the renderer's cache directory, so the renderer finds them immediately on startup.
+
+It supports two modes, both producing the same on-disk cache layout (`<cache>/<bundleHash>/<bundleHash>.js`):
+
+- **`MODE=copy`** (default) — copies files. Use in Docker/image builds so the cache is baked into an immutable artifact.
+- **`MODE=symlink`** — creates relative symlinks. For same-filesystem workflows (local dev, CI, Heroku-style same-dyno deploys, bundle-caching restores).
 
 ```dockerfile
-# After webpack/assets build step
+# After webpack/assets build step (Docker image build)
+ENV RENDERER_SERVER_BUNDLE_CACHE_PATH=/app/.node-renderer-bundles
 RUN bundle exec rake react_on_rails_pro:pre_seed_renderer_cache
 ```
 
-This copies the bundle into the renderer's expected directory structure (`<cache>/<bundleHash>/<bundleHash>.js`), including any configured `assets_to_copy` and RSC bundles when RSC support is enabled.
+Both modes stage the server bundle, any configured `assets_to_copy`, and (when RSC is enabled) the RSC bundle and its companion manifests.
 
-This is the preferred path for Docker and other image-build workflows. React on Rails Pro has long supported runtime bundle uploads and the older `react_on_rails_pro:pre_stage_bundle_for_node_renderer` task for same-filesystem deployments; `pre_seed_renderer_cache` is the copy-based variant that fits immutable artifacts while using the same bundle-hash cache layout.
+The `pre_seed_renderer_cache` task is also invoked automatically at the end of `assets:precompile` with `MODE=symlink`, so the local/CI/Heroku path has zero new configuration.
+
+> [!NOTE]
+> The older `react_on_rails_pro:pre_stage_bundle_for_node_renderer` rake task and `ReactOnRailsPro::PrepareNodeRenderBundles` class are deprecated in favor of the unified API. Both remain available as thin shims that emit a deprecation warning and delegate to `MODE=symlink`. `react_on_rails:doctor` flags deploy scripts that still reference the deprecated task.
 
 ### Configuration
 
 The task follows the same environment-variable precedence as the Node Renderer, while the default fallback can differ between Ruby and standalone Node environments:
 
 1. `RENDERER_SERVER_BUNDLE_CACHE_PATH` environment variable (preferred)
 2. `RENDERER_BUNDLE_PATH` environment variable (deprecated — emits a warning)
-3. `Rails.root.join(".node-renderer-bundles")` (Rails-side default when env vars are unset)
+3. `Rails.root.join(".node-renderer-bundles")` (Rails-side default when env vars are unset, only accepted for `MODE=symlink` and in dev/test)
 
-Set `RENDERER_SERVER_BUNDLE_CACHE_PATH` in your Dockerfile to match the renderer's configuration:
+In **`MODE=copy`** (Docker image builds) the task requires one of the env vars above to be set in non-dev/test environments. Because the Node renderer's own default can differ (e.g., falling back to `/tmp/react-on-rails-pro-node-renderer-bundles` when its `cwd` sits outside the app tree), relying on the silent fallback risks pre-seeded bundles landing in a directory the renderer never reads. The task raises a clear error if the env var is missing:
 
 ```dockerfile
 ENV RENDERER_SERVER_BUNDLE_CACHE_PATH=/app/.node-renderer-bundles

react_on_rails/lib/react_on_rails/doctor.rb

@@ -2694,6 +2694,7 @@ def check_pro_setup
       ensure_rails_environment_loaded
       check_pro_renderer_mode
       check_base_package_imports
+      check_deprecated_renderer_cache_task
     end
 
     def check_pro_initializer_existence
@@ -2723,6 +2724,42 @@ def check_pro_renderer_mode
       checker.add_warning("⚠️  Could not detect Pro renderer mode: #{e.message}")
     end
 
+    # Scan common deploy-script locations for references to the deprecated
+    # pre_stage_bundle_for_node_renderer rake task, so users on older Procfile/
+    # Dockerfile entries get a clear migration nudge before the task is removed.
+    DEPRECATED_RENDERER_CACHE_TASK = "pre_stage_bundle_for_node_renderer"
+    RENDERER_CACHE_DEPLOY_SCRIPT_PATHS = [
+      "Procfile",
+      "Procfile.dev",
+      "Procfile.dev-static-assets",
+      "Procfile.production",
+      "Dockerfile",
+      "bin/deploy",
+      "bin/release",
+      "bin/docker-entrypoint"
+    ].freeze
+
+    def check_deprecated_renderer_cache_task
+      matches = RENDERER_CACHE_DEPLOY_SCRIPT_PATHS.select do |path|
+        File.exist?(path) && File.read(path).include?(DEPRECATED_RENDERER_CACHE_TASK)
+      end
+
+      return if matches.empty?
+
+      checker.add_warning(<<~MSG.strip)
+        ⚠️  Deprecated rake task '#{DEPRECATED_RENDERER_CACHE_TASK}' referenced in:
+        #{matches.map { |p| "  • #{p}" }.join("\n")}
+
+        Replace with:
+          rake react_on_rails_pro:pre_seed_renderer_cache MODE=symlink
+
+        The unified 'pre_seed_renderer_cache' task uses MODE=copy by default (for
+        Docker/image builds) and MODE=symlink for same-filesystem workflows.
+      MSG
+    rescue StandardError => e
+      checker.add_warning("⚠️  Could not scan for deprecated renderer-cache task references: #{e.message}")
+    end
+
     # The base 'react-on-rails' npm package is a transitive dependency of 'react-on-rails-pro',
     # so `import ... from 'react-on-rails'` resolves silently — loading the base package instead
     # of Pro. Components registered through the base package won't have Pro features (streaming,

react_on_rails/spec/lib/react_on_rails/doctor_spec.rb

@@ -2396,6 +2396,43 @@ class << self
     end
   end
 
+  describe "check_deprecated_renderer_cache_task" do
+    let(:doctor) { described_class.new(verbose: false, fix: false) }
+    let(:checker) { doctor.instance_variable_get(:@checker) }
+
+    context "when a Procfile references the deprecated task" do
+      around do |example|
+        Dir.mktmpdir do |tmpdir|
+          Dir.chdir(tmpdir) do
+            File.write(
+              "Procfile",
+              "web: bundle exec rake react_on_rails_pro:pre_stage_bundle_for_node_renderer && bundle exec puma\n"
+            )
+            example.run
+          end
+        end
+      end
+
+      it "warns with migration guidance" do
+        doctor.send(:check_deprecated_renderer_cache_task)
+        warning_msgs = checker.messages.select { |m| m[:type] == :warning }
+        expect(warning_msgs.any? { |m| m[:content].include?("pre_stage_bundle_for_node_renderer") }).to be(true)
+        expect(warning_msgs.any? { |m| m[:content].include?("MODE=symlink") }).to be(true)
+      end
+    end
+
+    context "when no deploy scripts reference the deprecated task" do
+      around do |example|
+        Dir.mktmpdir { |tmpdir| Dir.chdir(tmpdir) { example.run } }
+      end
+
+      it "adds no warnings" do
+        doctor.send(:check_deprecated_renderer_cache_task)
+        expect(checker.messages.select { |m| m[:type] == :warning }).to be_empty
+      end
+    end
+  end
+
   describe "check_base_package_imports" do
     let(:doctor) { described_class.new(verbose: false, fix: false) }
     let(:checker) { doctor.instance_variable_get(:@checker) }

react_on_rails_pro/lib/react_on_rails_pro/assets_precompile.rb

@@ -63,7 +63,10 @@ def build_bundles
     def self.call
       instance.build_or_fetch_bundles
 
-      ReactOnRailsPro::PrepareNodeRenderBundles.call if ReactOnRailsPro.configuration.node_renderer?
+      # Auto-stage via symlink after asset precompile (same-filesystem default).
+      # Docker/image builds should invoke `rake react_on_rails_pro:pre_seed_renderer_cache`
+      # (MODE=copy, the default) as a separate step.
+      ReactOnRailsPro::PreSeedRendererCache.call(mode: :symlink) if ReactOnRailsPro.configuration.node_renderer?
     end
 
     def build_or_fetch_bundles

react_on_rails_pro/lib/react_on_rails_pro/pre_seed_renderer_cache.rb

@@ -1,47 +1,94 @@
 # frozen_string_literal: true
 
 require "fileutils"
+require "pathname"
 require "react_on_rails_pro/renderer_cache_helpers"
 
 module ReactOnRailsPro
-  # Pre-seeds the Node Renderer bundle cache by copying compiled server bundles
-  # into the renderer's expected directory structure. Designed for Docker builds
-  # where the bundle can be baked into the image, eliminating the 410→retry
-  # cold-start latency on first SSR request after deployment.
+  # Stages the Node Renderer bundle cache in the renderer's expected directory
+  # structure (`<cache>/<bundleHash>/<bundleHash>.js`), including any configured
+  # assets_to_copy and, when RSC support is enabled, the RSC bundle and manifests.
   #
-  # Unlike PrepareNodeRenderBundles (which stages the same cache layout via
-  # symlinks for same-filesystem workflows), this class copies files so the
-  # cache can be baked into an image or other immutable artifact.
+  # Supports two modes:
+  #
+  # * `:copy` (default) — copies bundle and assets. Designed for Docker image
+  #   builds where the cache must be baked into an immutable artifact.
+  # * `:symlink` — creates relative symlinks. For same-filesystem workflows
+  #   (local dev, CI, Heroku-style same-dyno deploys, bundle-caching restores).
+  #
+  # Both modes produce the same on-disk cache layout, matching the renderer's
+  # runtime contract. The 410→retry cold-start round-trip on first SSR request
+  # is eliminated when the pre-seeded bundle is present at renderer startup.
   class PreSeedRendererCache
-    def self.call
-      cache_dir = resolve_cache_dir
-      puts "[ReactOnRailsPro] Pre-seeding renderer cache in: #{cache_dir}"
+    VALID_MODES = %i[copy symlink].freeze
+
+    def self.call(mode: :copy)
+      unless VALID_MODES.include?(mode)
+        raise ArgumentError, "mode must be one of #{VALID_MODES.inspect}, got #{mode.inspect}"
+      end
+
+      cache_dir = resolve_cache_dir(mode)
+      puts "[ReactOnRailsPro] Staging renderer cache (mode: #{mode}) in: #{cache_dir}"
       pool = ReactOnRailsPro::ServerRenderingPool::NodeRenderingPool
 
       assets = RendererCacheHelpers.collect_assets
       rsc_required_paths = RendererCacheHelpers.required_rsc_asset_paths
 
-      RendererCacheHelpers.bundle_sources(pool, "pre-seeding").each do |src_bundle_path, bundle_hash|
-        seed_bundle(src_bundle_path, bundle_hash, cache_dir)
+      RendererCacheHelpers.bundle_sources(pool, action_description(mode)).each do |src_bundle_path, bundle_hash|
+        bundle_dir = File.join(cache_dir, bundle_hash.to_s)
+        stage_bundle(src_bundle_path, bundle_dir, bundle_hash, mode)
         # The Node Renderer serves manifests from whichever bundle dir it loaded,
         # so both server and RSC dirs need the manifests present.
-        copy_assets(assets, File.join(cache_dir, bundle_hash.to_s), rsc_required_paths)
+        stage_assets(assets, bundle_dir, rsc_required_paths, mode)
       end
     end
 
-    def self.resolve_cache_dir
+    def self.resolve_cache_dir(mode)
+      enforce_cache_dir_env_var!(mode)
       ReactOnRailsPro::Utils.resolve_renderer_cache_dir
     end
     private_class_method :resolve_cache_dir
 
-    def self.seed_bundle(src_path, bundle_hash, cache_dir)
-      bundle_dir = File.join(cache_dir, bundle_hash.to_s)
+    # In copy mode (Docker image builds), silent fallback to Rails.root/.node-renderer-bundles
+    # is a footgun: the renderer process may run from a different cwd and resolve its default
+    # cache directory to a different path (e.g., /tmp/react-on-rails-pro-node-renderer-bundles),
+    # causing pre-seeded bundles to land somewhere the renderer never reads. Require an
+    # explicit env var in non-dev/test environments.
+    def self.enforce_cache_dir_env_var!(mode)
+      return unless mode == :copy
+      return if ENV["RENDERER_SERVER_BUNDLE_CACHE_PATH"].present? || ENV["RENDERER_BUNDLE_PATH"].present?
+      return if Rails.env.development? || Rails.env.test?
+
+      raise ReactOnRailsPro::Error, <<~MSG.strip
+        Pre-seeding the renderer cache in copy mode (#{Rails.env}) requires an explicit
+        cache directory. Set RENDERER_SERVER_BUNDLE_CACHE_PATH in your environment, e.g.
+        in your Dockerfile:
+
+          ENV RENDERER_SERVER_BUNDLE_CACHE_PATH=/app/.node-renderer-bundles
+
+        The Node Renderer's default cache directory resolution differs between the Ruby
+        and standalone Node environments, so relying on the default in production-like
+        deploys can cause pre-seeded bundles to land in a path the renderer never reads.
+      MSG
+    end
+    private_class_method :enforce_cache_dir_env_var!
+
+    def self.action_description(mode)
+      mode == :copy ? "pre-seeding" : "pre-staging"
+    end
+    private_class_method :action_description
+
+    def self.stage_bundle(src_path, bundle_dir, bundle_hash, mode)
       dest_file = File.join(bundle_dir, "#{bundle_hash}.js")
-      FileUtils.mkdir_p(bundle_dir)
-      FileUtils.cp(src_path, dest_file)
-      puts "[ReactOnRailsPro] Pre-seeded renderer cache: #{dest_file}"
+      if mode == :copy
+        FileUtils.mkdir_p(bundle_dir)
+        FileUtils.cp(src_path, dest_file)
+        puts "[ReactOnRailsPro] Pre-seeded renderer cache: #{dest_file}"
+      else
+        make_relative_symlink(src_path, dest_file)
+      end
     end
-    private_class_method :seed_bundle
+    private_class_method :stage_bundle
 
     # RSC manifests are required when RSC is enabled — a missing manifest would cause
     # the renderer to fail at runtime with a hard-to-diagnose error. User-configured
@@ -50,23 +97,41 @@ def self.seed_bundle(src_path, bundle_hash, cache_dir)
     # in assets_to_copy cannot trigger a false-positive "required" error. Expand
     # against Rails.root to match how RendererCacheHelpers.required_rsc_asset_paths
     # builds its Set.
-    def self.copy_assets(assets, bundle_dir, rsc_required_paths)
+    def self.stage_assets(assets, bundle_dir, rsc_required_paths, mode)
       assets.each do |asset_path|
         expanded = File.expand_path(asset_path.to_s, Rails.root)
         unless File.exist?(expanded)
           if rsc_required_paths.include?(expanded)
             raise ReactOnRailsPro::Error, "Required RSC asset not found: #{asset_path}. " \
-                                          "Build your bundles before pre-seeding the renderer cache."
+                                          "Build your bundles before #{action_description(mode)} the renderer cache."
           end
           warn "[ReactOnRailsPro] Asset not found #{asset_path}"
           next
         end
 
         dest = File.join(bundle_dir, File.basename(expanded))
-        FileUtils.cp(expanded, dest)
-        puts "[ReactOnRailsPro] Copied asset: #{dest}"
+        if mode == :copy
+          FileUtils.cp(expanded, dest)
+          puts "[ReactOnRailsPro] Copied asset: #{dest}"
+        else
+          make_relative_symlink(expanded, dest)
+        end
       end
     end
-    private_class_method :copy_assets
+    private_class_method :stage_assets
+
+    def self.make_relative_symlink(source, destination)
+      destination_dir = Pathname.new(destination).dirname
+      FileUtils.mkdir_p(destination_dir)
+      FileUtils.rm_f(destination)
+
+      # Canonicalize both sides so paths like /var -> /private/var do not
+      # produce broken relative symlinks when the cache dir comes from tmpdir.
+      source_path = Pathname.new(source).realpath
+      relative_source_path = source_path.relative_path_from(destination_dir.realpath)
+      File.symlink(relative_source_path, destination)
+      puts "[ReactOnRailsPro] Symlinked #{relative_source_path} to #{destination}"
+    end
+    private_class_method :make_relative_symlink
   end
 end