Preserve tmpdir world-writable guard

Author: ioquatix

lib/tmpdir.rb

@@ -36,10 +36,12 @@ def self.tmpdir
         # We call File.writable?, not stat.writable?, because you can't tell if a dir is actually
         # writable just from stat; OS mechanisms other than user/group/world bits can affect this.
         #
-        # However, in some container environments (e.g. Kubernetes with read-only root filesystem
-        # and emptyDir volumes), File.writable? may return false even though the directory is
-        # actually writable via mounted volumes. Fall back to stat.writable? in that case.
-        if stat.writable?
+        # However, File.writable? can be a false negative, so preserve the
+        # world-writable non-sticky guard below before falling back to stat.writable?.
+        if stat.world_writable? && !stat.sticky?
+          warn "#{name} is world-writable: #{dir}"
+          next
+        elsif stat.writable?
           warn "#{name}: File.writable? reports not writable but file mode bits suggest writable, using anyway: #{dir}"
           break dir
         end

test/test_tmpdir.rb

@@ -90,9 +90,32 @@ def test_writable_fallback_to_stat
     end
   end
 
+  def test_writable_fallback_rejects_world_writable_non_sticky
+    omit "no meaning on this platform" if /mswin|mingw/ =~ RUBY_PLATFORM
+    Dir.mktmpdir do |tmpdir|
+      envs = %w[TMPDIR TMP TEMP]
+      oldenv = envs.each_with_object({}) {|v, h| h[v] = ENV.delete(v)}
+      begin
+        ENV[envs[0]] = tmpdir
+        File.chmod(0777, tmpdir)
+
+        original_writable = File.method(:writable?)
+        File.define_singleton_method(:writable?) do |path|
+          path == tmpdir ? false : original_writable.call(path)
+        end
+
+        assert_not_equal(tmpdir, assert_warn(/is world-writable/) { Dir.tmpdir })
+      ensure
+        File.define_singleton_method(:writable?, original_writable) if original_writable
+        File.chmod(0755, tmpdir)
+        ENV.update(oldenv)
+      end
+    end
+  end
+
   def test_writable_fallback_both_fail
     omit "no meaning on this platform" if /mswin|mingw/ =~ RUBY_PLATFORM
-    omit "root can write to any directory" if Process.uid == 0
+    omit "root can write to any directory" if Process.euid == 0
     Dir.mktmpdir do |tmpdir|
       envs = %w[TMPDIR TMP TEMP]
       oldenv = envs.each_with_object({}) {|v, h| h[v] = ENV.delete(v)}