Process source mapping URLs be set by transpilers (#479)

* Process source mapping URLs be set by transpilers

Rewrites source mapping urls with the digested paths and protect against semicolon appending with a dummy comment line.

* Correct file name

* Correct indention

* Ensure files that can't resolve raise

* Test source mapping url processor

* Warn in the log about removing the sourceMappingURL on missing assets

lib/sprockets/rails/sourcemapping_url_processor.rb

@@ -0,0 +1,22 @@
+module Sprockets
+  module Rails
+    # Rewrites source mapping urls with the digested paths and protect against semicolon appending with a dummy comment line
+    class SourcemappingUrlProcessor
+      REGEX = /\/\/# sourceMappingURL=(.*\.map)/
+
+      def self.call(input)
+        env     = input[:environment]
+        context = env.context_class.new(input)
+        data    = input[:data].gsub(REGEX) do |_match|
+          ensure_file_is_present = context.resolve($1)
+          "//# sourceMappingURL=#{context.asset_path($1)}\n//!\n"
+        rescue Sprockets::FileNotFound
+          env.logger.warn "Removed sourceMappingURL comment for missing asset '#{$1}' from #{input[:filename]}"
+          nil
+        end
+
+        { data: data }
+      end
+    end
+  end
+end

lib/sprockets/railtie.rb

@@ -6,6 +6,7 @@
 require 'sprockets'
 
 require 'sprockets/rails/asset_url_processor'
+require 'sprockets/rails/sourcemapping_url_processor'
 require 'sprockets/rails/context'
 require 'sprockets/rails/helper'
 require 'sprockets/rails/quiet_assets'
@@ -122,6 +123,10 @@ def configure(&block)
       Sprockets.register_postprocessor "text/css", ::Sprockets::Rails::AssetUrlProcessor
     end
 
+    initializer :asset_sourcemap_url_processor do |app|
+      Sprockets.register_postprocessor "application/javascript", ::Sprockets::Rails::SourcemappingUrlProcessor
+    end
+
     config.assets.version     = ""
     config.assets.debug       = false
     config.assets.compile     = true

test/test_sourcemapping_url_processor.rb

@@ -0,0 +1,38 @@
+require 'minitest/autorun'
+require 'sprockets/railtie'
+
+
+Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)
+class TestSourceMappingUrlProcessor < Minitest::Test
+  def setup
+    @env = Sprockets::Environment.new
+  end
+
+  def test_successful
+    @env.context_class.class_eval do
+      def resolve(path, **kargs)
+        "/yes"
+      end
+
+      def asset_path(path, options = {})
+        'mapped-HEXGOESHERE.js.map'
+      end
+    end
+
+    input = { environment: @env, data: "var mapped;\n//# sourceMappingURL=mapped.js.map", filename: 'mapped.js', metadata: {} }
+    output = Sprockets::Rails::SourcemappingUrlProcessor.call(input)
+    assert_equal({ data: "var mapped;\n//# sourceMappingURL=mapped-HEXGOESHERE.js.map\n//!\n" }, output)
+  end
+
+  def test_missing
+    @env.context_class.class_eval do
+      def resolve(path, **kargs)
+        raise Sprockets::FileNotFound
+      end
+    end
+
+    input = { environment: @env, data: "var mapped;\n//# sourceMappingURL=mappedNOT.js.map", filename: 'mapped.js', metadata: {} }
+    output = Sprockets::Rails::SourcemappingUrlProcessor.call(input)
+    assert_equal({ data: "var mapped;\n" }, output)
+  end
+end