[Bug #18933] Make Dir.mktmpdir Ractor-safe

Fix https://bugs.ruby-lang.org/issues/18933

Author: nobu

lib/tmpdir.rb

@@ -13,15 +13,20 @@
 
 class Dir
 
-  @@systmpdir ||= defined?(Etc.systmpdir) ? Etc.systmpdir : '/tmp'
+  # Class variables are inaccessible from non-main Ractor.
+  # And instance variables too, in Ruby 3.0.
+
+  # System-wide temporary directory path
+  SYSTMPDIR = (defined?(Etc.systmpdir) ? Etc.systmpdir.freeze : '/tmp')
+  private_constant :SYSTMPDIR
 
   ##
   # 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|
+    ['TMPDIR', 'TMP', 'TEMP', ['system temporary path', SYSTMPDIR], ['/tmp']*2, ['.']*2].find do |name, dir|
       unless dir
-        next if !(dir = ENV[name]) or dir.empty?
+        next if !(dir = ENV[name] rescue next) or dir.empty?
       end
       dir = File.expand_path(dir)
       stat = File.stat(dir) rescue next
@@ -118,16 +123,17 @@ def tmpdir
     UNUSABLE_CHARS = "^,-.0-9A-Z_a-z~"
 
     # Dedicated random number generator
-    RANDOM = Random.new
+    RANDOM = Object.new
     class << RANDOM # :nodoc:
       # Maximum random number
       MAX = 36**6 # < 0x100000000
 
       # Returns new random string upto 6 bytes
       def next
-        rand(MAX).to_s(36)
+        (::Random.urandom(4).unpack1("L")%MAX).to_s(36)
       end
     end
+    RANDOM.freeze
     private_constant :RANDOM
 
     # Generates and yields random names to create a temporary name

test/test_tmpdir.rb

@@ -115,4 +115,20 @@ def assert_mktmpdir_traversal
       end
     end
   end
+
+  def test_ractor
+    assert_ractor(<<~'end;', require: "tmpdir")
+      r = Ractor.new do
+        Dir.mktmpdir() do |d|
+          Ractor.yield d
+          Ractor.receive
+        end
+      end
+      dir = r.take
+      assert_file.directory? dir
+      r.send true
+      r.take
+      assert_file.not_exist? dir
+    end;
+  end
 end