Support read-only git gems (fixes #3092)

lib/bundler/source/git.rb

@@ -79,11 +79,12 @@ def name
       def install_path
         @install_path ||= begin
           git_scope = "#{base_name}-#{shortref_for_path(revision)}"
+          path = Bundler.install_path.join(git_scope)
 
-          if Bundler.requires_sudo?
+          if !path.exist? && Bundler.requires_sudo?
             Bundler.user_bundle_path.join(Bundler.ruby_scope).join(git_scope)
           else
-            Bundler.install_path.join(git_scope)
+            path
           end
         end
       end

spec/runtime/setup_spec.rb

@@ -385,6 +385,14 @@
         end
       end
     end
+
+    it "finds git gem when default bundle path becomes read only" do
+      bundle "install"
+
+      with_read_only("#{Bundler.bundle_path}/**/*") do
+        should_be_installed "rack 1.0.0"
+      end
+    end
   end
 
   describe "when specifying local override" do

spec/support/helpers.rb

@@ -344,5 +344,19 @@ def capture_output
     ensure
       $stdout = actual_stdout
     end
+
+    def with_read_only(pattern)
+      chmod = lambda do |dirmode, filemode|
+        lambda do |f|
+          mode = File.directory?(f) ? dirmode : filemode
+          File.chmod(mode, f)
+        end
+      end
+
+      Dir[pattern].each(&chmod[0555, 0444])
+      yield
+    ensure
+      Dir[pattern].each(&chmod[0755, 0644])
+    end
   end
 end