[rubygems/rubygems] Fix gem pristine etc resetting gem twice sometimes

If a default version and a regular version of etc are present at the
same time, RubyGems will end up duplicating work and running pristine
twice.

The `etc` gem is special because it's loaded by RubyGems by default.
When doing this, RubyGems will activate the regularly installed version.
The when `gem pristine` runs, it will find to installed specifications
but materialize both to the already activated specification.

Before:

```
$ gem pristine etc --version 1.4.3
Restoring gems to pristine condition...
Building native extensions. This could take a while...
Restored etc-1.4.3
Building native extensions. This could take a while...
Restored etc-1.4.3
```

After:

```
$ gem pristine etc --version 1.4.3
Restoring gems to pristine condition...
Skipped etc-1.4.3, it is a default gem
Building native extensions. This could take a while...
Restored etc-1.4.3
```

https://github.com/rubygems/rubygems/commit/5c279ac56b

lib/rubygems/basic_specification.rb

@@ -100,7 +100,7 @@ class Gem::BasicSpecification
   end
 
   def default_gem?
-    loaded_from &&
+    !loaded_from.nil? &&
       File.dirname(loaded_from) == Gem.default_specifications_dir
   end
 

lib/rubygems/stub_specification.rb

@@ -83,11 +83,7 @@ class Gem::StubSpecification < Gem::BasicSpecification
   # True when this gem has been activated
 
   def activated?
-    @activated ||=
+    @activated ||= !loaded_spec.nil?
-      begin
-        loaded = Gem.loaded_specs[name]
-        loaded && loaded.version == version
-      end
   end
 
   def default_gem?
@@ -187,11 +183,7 @@ class Gem::StubSpecification < Gem::BasicSpecification
   # The full Gem::Specification for this gem, loaded from evalling its gemspec
 
   def spec
-    @spec ||= if @data
+    @spec ||= loaded_spec if @data
-      loaded = Gem.loaded_specs[name]
-      loaded if loaded && loaded.version == version
-    end
-
     @spec ||= Gem::Specification.load(loaded_from)
   end
   alias_method :to_spec, :spec
@@ -231,4 +223,13 @@ class Gem::StubSpecification < Gem::BasicSpecification
   def sort_obj # :nodoc:
     [name, version, Gem::Platform.sort_priority(platform)]
   end
+
+  private
+
+  def loaded_spec
+    spec = Gem.loaded_specs[name]
+    return unless spec && spec.version == version && spec.default_gem? == default_gem?
+
+    spec
+  end
 end

test/rubygems/test_gem_stub_specification.rb

@@ -171,6 +171,35 @@ class TestStubSpecification < Gem::TestCase
     assert_same real_foo, @foo.to_spec
   end
 
+  def test_to_spec_default
+    bar_default_spec = File.join(@gemhome, "specifications", "default", "bar-0.0.2.gemspec")
+    File.open bar_default_spec, "w" do |io|
+      io.write <<~STUB
+        # -*- encoding: utf-8 -*-
+        # stub: bar 0.0.2 ruby lib
+
+        Gem::Specification.new do |s|
+          s.name          = "bar"
+          s.version       = "0.0.2"
+          s.platform      = "ruby"
+          s.require_paths = ["lib"]
+          s.summary       = "A very bar gem"
+        end
+      STUB
+    end
+
+    bar = Gem::StubSpecification.gemspec_stub BAR, @base_dir, @gems_dir
+    bar_default = Gem::StubSpecification.default_gemspec_stub bar_default_spec, @base_dir, @gems_dir
+
+    real_bar = util_spec bar_default.name, bar_default.version
+    real_bar.activate
+
+    assert_equal bar.version, Gem.loaded_specs[bar.name].version,
+                 "sanity check"
+
+    refute_same real_bar, bar_default.to_spec
+  end
+
   def test_to_spec_with_other_specs_loaded_does_not_warn
     real_foo = util_spec @foo.name, @foo.version
     real_foo.activate