Explain Propshaft::MissingAssetError from a stale asset manifest

lib/reactionview/asset_manifest.rb

@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+require "json"
+
+module ReActionView
+  class AssetManifest
+    def initialize(assets)
+      @assets = assets
+    end
+
+    def path
+      config = @assets&.config
+
+      return unless config.respond_to?(:manifest_path)
+
+      config.manifest_path
+    end
+
+    def mtime
+      return if path.nil?
+
+      File.mtime(path)
+    rescue SystemCallError
+      nil
+    end
+
+    def exist?
+      !mtime.nil?
+    end
+
+    def missing_assets
+      precompiled = logical_paths
+
+      return [] if precompiled.empty?
+
+      ReActionView::Railtie::PRECOMPILE_ASSETS - precompiled
+    end
+
+    def stale?
+      exist? && !missing_assets.empty?
+    end
+
+    def in_use?
+      @assets.respond_to?(:resolver) && @assets.resolver.respond_to?(:manifest_path)
+    end
+
+    def explanation
+      <<~MESSAGE
+        ReActionView's dev tools assets are missing from your precompiled assets.
+        #{status}
+
+        To fix this, delete the precompiled assets:
+
+            bin/rails assets:clobber
+
+        That is safe in development. Without public/assets/, Propshaft serves each asset straight from
+        the directory it lives in, including ReActionView's.
+
+            Missing:  #{missing_assets.join(", ")}
+            Manifest: #{path}
+
+        public/assets/ was most likely left behind by `RAILS_ENV=production bin/rails assets:precompile`,
+        and a production precompile doesn't include ReActionView's dev tools.
+      MESSAGE
+    end
+
+    private
+
+    def status
+      if in_use?
+        "Because public/assets/.manifest.json exists, Propshaft serves every asset from public/assets/\n" \
+          "and doesn't look anywhere else, which is why the lookup failed."
+      else
+        "This server booted before public/assets/.manifest.json existed, so pages still render for now.\n" \
+          "After the next restart Propshaft will serve assets from public/assets/ only, and rendering\n" \
+          "will fail then."
+      end
+    end
+
+    def logical_paths
+      return [] if path.nil?
+
+      JSON.parse(File.read(path)).keys
+    rescue SystemCallError, JSON::ParserError
+      []
+    end
+  end
+end

lib/reactionview/middleware/asset_manifest_check.rb

@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+require_relative "../asset_manifest"
+require_relative "../stale_asset_manifest_error"
+
+module ReActionView
+  module Middleware
+    class AssetManifestCheck
+      def initialize(app, assets: nil, logger: nil)
+        @app = app
+        @assets = assets
+        @logger = logger
+        @mutex = Mutex.new
+        @checked_mtime = nil
+      end
+
+      def call(env)
+        warn_about_upcoming_failure
+
+        @app.call(env)
+      rescue StandardError => e
+        raise unless dev_tools_asset_missing?(e)
+
+        manifest = self.manifest
+
+        raise unless manifest.stale?
+
+        raise StaleAssetManifestError.for(manifest)
+      end
+
+      private
+
+      def assets
+        @assets || (defined?(Rails) && Rails.application&.assets)
+      end
+
+      def logger
+        @logger || (defined?(Rails) && Rails.logger)
+      end
+
+      def manifest
+        AssetManifest.new(assets)
+      end
+
+      def warn_about_upcoming_failure
+        manifest = self.manifest
+        mtime = manifest.mtime
+
+        return if mtime.nil?
+
+        @mutex.synchronize do
+          return if @checked_mtime == mtime
+
+          @checked_mtime = mtime
+
+          return if manifest.in_use? || !manifest.stale?
+
+          log("[ReActionView] #{manifest.explanation}")
+        end
+      end
+
+      def dev_tools_asset_missing?(error)
+        error_class = missing_asset_error_class
+
+        return false if error_class.nil?
+
+        while error
+          if error.is_a?(error_class) &&
+             ReActionView::Railtie::PRECOMPILE_ASSETS.any? { |asset| error.message.include?(asset) }
+            return true
+          end
+
+          error = error.cause
+        end
+
+        false
+      end
+
+      def missing_asset_error_class
+        return unless defined?(::Propshaft::MissingAssetError)
+
+        ::Propshaft::MissingAssetError
+      end
+
+      def log(message)
+        if logger
+          logger.warn(message)
+        else
+          warn(message)
+        end
+      end
+    end
+  end
+end

lib/reactionview/railtie.rb

@@ -23,6 +23,14 @@ class Railtie < Rails::Railtie
       end
     end
 
+    initializer "reactionview.asset_manifest_check" do |app|
+      next unless ReActionView.config.development?
+
+      require_relative "middleware/asset_manifest_check"
+
+      app.middleware.use ReActionView::Middleware::AssetManifestCheck
+    end
+
     initializer "reactionview.register_herb_handler" do
       ActiveSupport.on_load(:action_view) do
         ActionView::Template.register_template_handler :herb, ReActionView::Template::Handlers::Herb

lib/reactionview/stale_asset_manifest_error.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module ReActionView
+  class StaleAssetManifestError < StandardError
+    def self.for(manifest)
+      new(manifest.explanation)
+    end
+  end
+end