[ruby/tmpdir] Ignore empty environment variables

Fixes https://github.com/ruby/tmpdir/pull/17

https://github.com/ruby/tmpdir/commit/a79c727a5d

lib/tmpdir.rb

@@ -19,8 +19,10 @@ class Dir
   # Returns the operating system's temporary file path.
 
   def self.tmpdir
-    ['TMPDIR', 'TMP', 'TEMP', ['system temporary path', @@systmpdir], ['/tmp']*2, ['.']*2].find do |name, dir = ENV[name]|
-      next if !dir
+    ['TMPDIR', 'TMP', 'TEMP', ['system temporary path', @@systmpdir], ['/tmp']*2, ['.']*2].find do |name, dir|
+      unless dir
+        next if !(dir = ENV[name]) or dir.empty?
+      end
       dir = File.expand_path(dir)
       stat = File.stat(dir) rescue next
       case

test/test_tmpdir.rb

@@ -47,6 +47,18 @@ class TestTmpdir < Test::Unit::TestCase
     end
   end
 
+  def test_tmpdir_not_empty_parent
+    Dir.mktmpdir do |tmpdir|
+      envs = %w[TMPDIR TMP TEMP]
+      oldenv = envs.each_with_object({}) {|v, h| h[v] = ENV.delete(v)}
+      ENV[envs[0]] = ""
+      ENV[envs[1]] = tmpdir
+      assert_equal(tmpdir, Dir.tmpdir)
+    ensure
+      ENV.update(oldenv)
+    end
+  end
+
   def test_no_homedir
     bug7547 = '[ruby-core:50793]'
     home, ENV["HOME"] = ENV["HOME"], nil