DEV: Fix production sourcemaps with Ember CLI (#16707)

22a7905f restructured how we load Ember CLI assets in production. Unfortunately, it also broke sourcemaps for those assets. This commit fixes that regression via a couple of changes:

- It adds the necessary `.map` paths to `config.assets.precompile`
- It swaps Sprockets' default `SourcemappingUrlProcessor` with an extended version which maintains relative URLs of maps

config/application.rb

@@ -176,6 +176,7 @@ module Discourse
     Sprockets.register_transformer 'text/x-handlebars', 'application/javascript', Ember::Handlebars::Template
 
     require 'discourse_js_processor'
+    require 'discourse_sourcemapping_url_processor'
 
     Sprockets.register_mime_type 'application/javascript', extensions: ['.js', '.es6', '.js.es6'], charset: :unicode
     Sprockets.register_postprocessor 'application/javascript', DiscourseJsProcessor
@@ -184,6 +185,8 @@ module Discourse
       Discourse::Application.initializer :prepend_ember_assets do |app|
         # Needs to be in its own initializer so it runs after the append_assets_path initializer defined by Sprockets
         app.config.assets.paths.unshift "#{app.config.root}/app/assets/javascripts/discourse/dist/assets"
+        Sprockets.unregister_postprocessor 'application/javascript', Sprockets::Rails::SourcemappingUrlProcessor
+        Sprockets.register_postprocessor 'application/javascript', DiscourseSourcemappingUrlProcessor
       end
     end
 

config/initializers/assets.rb

@@ -61,6 +61,7 @@ if EmberCli.enabled?
       scripts/discourse-test-listen-boot
       scripts/discourse-boot
     }
+  Rails.application.config.assets.precompile += EmberCli::ASSETS.map { |name| name.sub('.js', '.map') }
 else
   Rails.application.config.assets.precompile += %w{
     application.js

lib/discourse_sourcemapping_url_processor.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+# This postprocessor rewrites `//sourceMappingURL=` comments to include the hashed filename of the map.
+# As a side effect, the default implementation also replaces relative sourcemap URLs with absolute URLs, including the CDN domain.
+# We want to preserve the relative nature of the URLs, so that compiled JS is portable across sites with differing CDN configurations.
+class DiscourseSourcemappingUrlProcessor < Sprockets::Rails::SourcemappingUrlProcessor
+  def self.sourcemap_asset_path(sourcemap_logical_path, context:)
+    result = super(sourcemap_logical_path, context: context)
+    if File.basename(sourcemap_logical_path) === sourcemap_logical_path
+      # If the original sourcemap reference is relative, keep it relative
+      result = File.basename(result)
+    end
+    result
+  end
+end

lib/ember_cli.rb

@@ -1,6 +1,15 @@
 # frozen_string_literal: true
 
 module EmberCli
+  ASSETS = %w(
+    discourse.js
+    admin.js
+    ember_jquery.js
+    pretty-text-bundle.js
+    start-discourse.js
+    vendor.js
+  )
+
   ALIASES ||= {
     "application" => "discourse",
     "discourse/tests/test-support-rails" => "test-support",
@@ -36,4 +45,9 @@ module EmberCli
     return name if !enabled?
     ALIASES[name] || name
   end
+
+  def self.is_ember_cli_asset?(name)
+    return false if !enabled?
+    ASSETS.include?(name) || name.start_with?("chunk.")
+  end
 end

lib/tasks/assets.rake

@@ -54,7 +54,7 @@ task 'assets:precompile:before' do
   if EMBER_CLI
     # Add ember cli chunks
     Rails.configuration.assets.precompile.push(
-      *EmberCli.script_chunks.values.flatten
+      *EmberCli.script_chunks.values.flatten.flat_map { |name| ["#{name}.js", "#{name}.map"] }
     )
   end
 end
@@ -189,7 +189,7 @@ end
 
 def max_compress?(path, locales)
   return false if Rails.configuration.assets.skip_minification.include? path
-  return false if is_ember_cli_asset?(path)
+  return false if EmberCli.is_ember_cli_asset?(path)
   return true unless path.include? "locales/"
 
   path_locale = path.delete_prefix("locales/").delete_suffix(".js")

spec/lib/discourse_sourcemapping_url_processor_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require 'discourse_sourcemapping_url_processor'
+
+describe DiscourseSourcemappingUrlProcessor do
+  def process(input)
+    env = Sprockets::Environment.new
+    env.context_class.class_eval do
+      def resolve(path, **kargs)
+        "/assets/mapped.js.map"
+      end
+
+      def asset_path(path, options = {})
+        "/assets/mapped-HEXGOESHERE.js.map"
+      end
+    end
+
+    input = { environment: env, data: input, name: 'mapped', filename: 'mapped.js', metadata: {} }
+    DiscourseSourcemappingUrlProcessor.call(input)[:data]
+  end
+
+  it 'maintains relative paths' do
+    output = process "var mapped;\n//# sourceMappingURL=mapped.js.map"
+    expect(output).to eq("var mapped;\n//# sourceMappingURL=mapped-HEXGOESHERE.js.map\n//!\n")
+  end
+
+  it 'uses default behaviour for non-adjacent relative paths' do
+    output = process "var mapped;\n//# sourceMappingURL=/assets/mapped.js.map"
+    expect(output).to eq("var mapped;\n//# sourceMappingURL=/assets/mapped-HEXGOESHERE.js.map\n//!\n")
+  end
+end