thread_sync.c (mutex_ptr): only reinitalize waitqueue at fork

Mutexes need to remain locked after forking.

This fixes "[BUG] invalid keeping_mutexes: Attempt to unlock a
mutex which is locked by another thread" and should
fix test_fork_while_parent_locked failures in CI

[ruby-core:90581] [Bug #15424]
[ruby-core:90595] [Bug #15430]

Fixes: r66230 ("handle mutexes held by parent threads in children")

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66438 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_thread.rb

@@ -1260,6 +1260,21 @@ q.pop
     assert_equal 0, failures, '[ruby-core:90312] [Bug #15383]'
   end
 
+  def test_fork_while_mutex_locked_by_forker
+    skip 'needs fork' unless Process.respond_to?(:fork)
+    m = Mutex.new
+    m.synchronize do
+      pid = fork do
+        exit!(2) unless m.locked?
+        m.unlock rescue exit!(3)
+        m.synchronize {} rescue exit!(4)
+        exit!(0)
+      end
+      _, st = Timeout.timeout(30) { Process.waitpid2(pid) }
+      assert_predicate st, :success?, '[ruby-core:90595] [Bug #15430]'
+    end
+  end
+
   def test_subclass_no_initialize
     t = Module.new do
       break eval("class C\u{30b9 30ec 30c3 30c9} < Thread; self; end")

thread_sync.c

@@ -130,8 +130,6 @@ mutex_ptr(VALUE obj)
         /* forked children can't reach into parent thread stacks */
         mutex->fork_gen = fork_gen;
         list_head_init(&mutex->waitq);
-        mutex->next_mutex = 0;
-        mutex->th = 0;
     }
 
     return mutex;