Instead of checking writability, try to write

Checking writability is prone to errors. For example:

$ mkdir -p foo/bar
$ chmod -w foo
$ touch foo/bar/baz # succeeds even if foo is not writable

lib/rubygems/installer.rb

@@ -351,6 +351,9 @@ def install
     run_post_install_hooks
 
     spec
+  rescue Errno::EACCES => e
+    # Permission denied - /path/to/foo
+    raise Gem::FilePermissionError, e.message.split(" - ").last
   end
 
   def run_pre_install_hooks # :nodoc:
@@ -716,7 +719,6 @@ def check_that_user_bin_dir_is_in_path # :nodoc:
 
   def verify_gem_home # :nodoc:
     FileUtils.mkdir_p gem_home, :mode => options[:dir_mode] && 0o755
-    raise Gem::FilePermissionError, gem_home unless File.writable?(gem_home)
   end
 
   def verify_spec

test/rubygems/test_gem_commands_install_command.rb

@@ -193,7 +193,7 @@ def test_execute_no_user_install
     pend "skipped in root privilege" if Process.uid.zero?
 
     spec_fetcher do |fetcher|
-      fetcher.gem "a", 2
+      fetcher.download "a", 2
     end
 
     @cmd.options[:user_install] = false
@@ -204,12 +204,14 @@ def test_execute_no_user_install
       FileUtils.chmod 0o755, @userhome
       FileUtils.chmod 0o555, @gemhome
 
-      assert_raise Gem::FilePermissionError do
+      assert_raise Gem::MockGemUi::SystemExitException, @ui.error do
         @cmd.execute
       end
     ensure
       FileUtils.chmod 0o755, @gemhome
     end
+
+    assert_equal %w[a-2], @cmd.installed_specs.map(&:full_name).sort
   end
 
   def test_execute_local_missing