Always avoid loading the full spec when checking extensions

By explicitly defining a couple more methods, we can avoid loading the
full specification in all cases, including pathological ones that
currently lead to stackoverflow errors.

bundler/lib/bundler/stub_specification.rb

@@ -30,7 +30,11 @@ def to_yaml
 
     # This is defined directly to avoid having to loading the full spec
     def missing_extensions?
-      stub.missing_extensions?
+      return false if default_gem?
+      return false if extensions.empty?
+      return false if File.exist? gem_build_complete_path
+
+      true
     end
 
     def activated
@@ -41,6 +45,14 @@ def activated=(activated)
       stub.instance_variable_set(:@activated, activated)
     end
 
+    def extensions
+      stub.extensions
+    end
+
+    def gem_build_complete_path
+      File.join(extension_dir, "gem.build_complete")
+    end
+
     def default_gem?
       stub.default_gem?
     end

bundler/spec/commands/exec_spec.rb

@@ -928,5 +928,24 @@ def bin_path(a,b,c)
         expect(err).to include("custom openssl should not be loaded")
       end
     end
+
+    context "with a git gem that includes extensions" do
+      before do
+        build_git "simple_git_binary", &:add_c_extension
+        bundle! "config set --local path .bundle"
+        install_gemfile! <<-G
+          gem "simple_git_binary", :git => '#{lib_path("simple_git_binary-1.0")}'
+        G
+      end
+
+      it "allows calling bundle install" do
+        bundle! "exec bundle install"
+      end
+
+      it "allows calling bundle install after removing gem.build_complete" do
+        FileUtils.rm_rf Dir[bundled_app(".bundle/**/gem.build_complete")]
+        bundle! "exec #{Gem.ruby} -S bundle install"
+      end
+    end
   end
 end