Eagerly set the download strategy

Library/Homebrew/resource.rb

@@ -9,7 +9,8 @@ class Resource
   include FileUtils
 
   attr_reader :checksum, :mirrors, :specs, :using
-  attr_writer :url, :checksum, :version, :download_strategy
+  attr_writer :url, :checksum, :version
+  attr_accessor :download_strategy
 
   # Formula name must be set after the DSL, as we have no access to the
   # formula name before initialization of the formula
@@ -34,10 +35,6 @@ class Resource
     name.nil? ? owner.name : "#{owner.name}--#{name}"
   end
 
-  def download_strategy
-    @download_strategy ||= DownloadStrategyDetector.detect(url, using)
-  end
-
   def cached_download
     downloader.cached_location
   end
@@ -104,6 +101,7 @@ class Resource
     @url = val
     @specs.merge!(specs)
     @using = @specs.delete(:using)
+    @download_strategy = DownloadStrategyDetector.detect(url, using)
   end
 
   def version val=nil

Library/Homebrew/test/test_dependency_collector.rb

@@ -128,7 +128,8 @@ class DependencyCollectorTests < Homebrew::TestCase
 
   def test_resource_dep_raises_for_unknown_classes
     resource = Resource.new
-    resource.url "foo", :using => Class.new
+    resource.url "foo"
+    resource.download_strategy = Class.new
     assert_raises(TypeError) { @d.add(resource) }
   end
 end

Library/Homebrew/test/test_resource.rb

@@ -38,6 +38,10 @@ class ResourceTests < Homebrew::TestCase
     assert_equal GitDownloadStrategy, @resource.download_strategy
   end
 
+  def test_raises_for_unknown_download_strategy_class
+    assert_raises(TypeError) { @resource.url("foo", :using => Class.new) }
+  end
+
   def test_does_not_mutate_specs_hash
     specs = { :using => :git, :branch => 'master' }
     @resource.url('foo', specs)