Improve platform specific gem resolution

It resolves rubygems#6247. This changes includes the patches that add (RSpec)
specs for this situation in rubygems#6247.

If there is a platform specific gem but it can't be resolved available
version, Bundler reports an error.

For example,

    @Index = build_index do
      gem "bar", "1.0.0"
      gem "foo", "1.0.0"
      gem "foo", "1.0.0", "x64-mingw32" do
        dep "bar", "< 1"
      end
    end
    dep "foo"
    platforms "x64-mingw32"

raises an error because foo-1.0.0-x64-mingw32 requires bar<1 but there
isn't bar<1.

With this change, foo-1.0.0 (no x64-mingw32) is used as
fallback. Because foo-1.0.0 doesn't depend on bar<1.

lib/bundler/resolver.rb

@@ -146,7 +146,21 @@ def search_for(dependency)
           @gem_version_promoter.sort_versions(dependency, spec_groups)
         end
       end
-      search.select {|sg| sg.for?(platform) }.each {|sg| sg.activate_platform!(platform) }
+      selected_sgs = []
+      search.each do |sg|
+        next unless sg.for?(platform)
+        sg.activate_platform!(platform)
+        if sg.spec(platform).platform != Gem::Platform::RUBY
+          sg_ruby = SpecGroup.new(sg.all_specs)
+          sg_ruby.ignores_bundler_dependencies = sg.ignores_bundler_dependencies
+          if sg_ruby.for?(Gem::Platform::RUBY)
+            sg_ruby.activate_platform!(Gem::Platform::RUBY)
+            selected_sgs << sg_ruby
+          end
+        end
+        selected_sgs << sg
+      end
+      selected_sgs
     end
 
     def index_for(dependency)
@@ -183,9 +197,7 @@ def name_for_locking_dependency_source
     end
 
     def requirement_satisfied_by?(requirement, activated, spec)
-      return false unless requirement.matches_spec?(spec) || spec.source.is_a?(Source::Gemspec)
-      spec.activate_platform!(requirement.__platform) if !@platforms || @platforms.include?(requirement.__platform)
-      true
+      requirement.matches_spec?(spec) || spec.source.is_a?(Source::Gemspec)
     end
 
     def relevant_sources_for_vertex(vertex)

lib/bundler/resolver/spec_group.rb

@@ -5,10 +5,12 @@ class Resolver
     class SpecGroup
       include GemHelpers
 
+      attr_reader :all_specs
       attr_accessor :name, :version, :source
       attr_accessor :ignores_bundler_dependencies
 
       def initialize(all_specs)
+        @all_specs = all_specs
         raise ArgumentError, "cannot initialize with an empty value" unless exemplary_spec = all_specs.first
         @name = exemplary_spec.name
         @version = exemplary_spec.version
@@ -37,9 +39,12 @@ def activate_platform!(platform)
         @activated_platforms << platform
       end
 
+      def spec(platform)
+        @specs[platform]
+      end
+
       def for?(platform)
-        spec = @specs[platform]
-        !spec.nil?
+        !spec(platform).nil?
       end
 
       def to_s

spec/resolver/platform_spec.rb

@@ -54,6 +54,17 @@
     should_resolve_as %w[foo-1.0.0]
   end
 
+  it "prefers the platform specific gem to the ruby version" do
+    @index = build_index do
+      gem "foo", "1.0.0"
+      gem "foo", "1.0.0", "x64-mingw32"
+    end
+    dep "foo"
+    platforms "x64-mingw32"
+
+    should_resolve_as %w[foo-1.0.0-x64-mingw32]
+  end
+
   it "takes the latest ruby gem if the platform specific gem doesn't match the required_ruby_version" do
     @index = build_index do
       gem "foo", "1.0.0"