[rubygems/rubygems] 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

https://github.com/rubygems/rubygems/commit/6056138b6a

2 files changed, 7 insertions, 3 deletions

diff --git a/lib/rubygems/installer.rb b/lib/rubygems/installer.rb
index 1e35e4fe83..aca42a03b0 100644
--- a/lib/rubygems/installer.rb
+++ b/lib/rubygems/installer.rb
@@ -351,6 +351,9 @@ class Gem::Installer
     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 @@ class Gem::Installer
 
   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
diff --git a/test/rubygems/test_gem_commands_install_command.rb b/test/rubygems/test_gem_commands_install_command.rb
index 0c4e1efc78..c38c94e5e2 100644
--- a/test/rubygems/test_gem_commands_install_command.rb
+++ b/test/rubygems/test_gem_commands_install_command.rb
@@ -193,7 +193,7 @@ ERROR:  Could not find a valid gem 'bar' (= 0.5) (required by 'foo' (>= 0)) in a
     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 @@ ERROR:  Could not find a valid gem 'bar' (= 0.5) (required by 'foo' (>= 0)) in a
       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