Flush the current Ractor's stdout/stderr before spawning

prefork() flushed rb_stdout and rb_stderr, which are the main Ractor's IOs.
From another Ractor that takes the write_lock of an IO the Ractor does not
own. When a main Ractor thread holds it (any thread logging to $stdout will
do), the spawning Ractor's only thread sleeps on the mutex and
rb_check_deadlock, which only looks at the Ractor's own threads, raises
"No live threads left. Deadlock?" (fatal) from Kernel#spawn/system/popen.

Use rb_ractor_stdout()/rb_ractor_stderr() so each Ractor flushes its own.

[Bug #22434]

process.c

@@ -3669,9 +3669,11 @@ handle_fork_error(int err, struct rb_process_status *status, int *ep, volatile i
     return -1;
 }
 
+/* Flush the current Ractor's own stdio: taking the write_lock of an IO owned
+ * by another Ractor can block on a thread that rb_check_deadlock can't see. */
 #define prefork() (		\
-        rb_io_flush(rb_stdout), \
-        rb_io_flush(rb_stderr)	\
+        rb_io_flush(rb_ractor_stdout()), \
+        rb_io_flush(rb_ractor_stderr())	\
         )
 
 /*

test/ruby/test_ractor.rb

@@ -2,6 +2,17 @@
 require 'test/unit'
 
 class TestRactor < Test::Unit::TestCase
+  # Spawning from a Ractor must not block on the main Ractor's $stdout
+  def test_spawn_does_not_contend_on_main_ractors_stdout
+    assert_separately([], <<~'RUBY', timeout: 60)
+      $stdout = File.open(File::NULL, "w")
+      writer = Thread.new { loop { $stdout.write("x" * 100 + "\n"); $stdout.flush } }
+      r = Ractor.new { 20.times { system(EnvUtil.rubybin, "-e", "") }; :done }
+      assert_equal :done, r.value
+      writer.kill.join
+    RUBY
+  end
+
   def test_shareability_of_iseq_proc
     assert_raise Ractor::IsolationError do
       foo = []