Fix sleep spurious wakeup from sigchld (ruby#15802)

When sleeping with `sleep`, currently the main thread can get woken up from sigchld
from any thread (subprocess exited). The timer thread wakes up the main thread when this
happens, as it checks for signals. The main thread then executes the ruby sigchld handler
if one is registered and is supposed to go back to sleep immediately. This is not ideal but
it's the way it's worked for a while. In commit 8d8159e I added writes to `th->status`
before and after `wait_running_turn` in `thread_sched_to_waiting_until_wakeup`, which is
called from `sleep`. This is usually the right way to set the thread's status, but `sleep`
is an exception because the writes to `th->status` are done in `sleep_forever`. There's a
loop that checks `th->status` in `sleep_forever`. When the main thread got woken up from
sigchld it saw the changed `th->status` and continued to run the main thread instead of
going back to sleep.

The following script shows the error. It was returning instead of sleeping forever.

```ruby
t = Thread.new do
  sleep 0.3
  `echo hello`  # Spawns subprocess
  puts "Subprocess exited"
end

puts "Main thread sleeping..."
result = sleep  # Should block forever
puts "sleep returned: #{result.inspect}"
```

Fixes [Bug #21812]

Author: luke-gruber

test/ruby/test_sleep.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 require 'test/unit'
 require 'etc'
+require 'timeout'
 
 class TestSleep < Test::Unit::TestCase
   def test_sleep_5sec
@@ -13,4 +14,21 @@ def test_sleep_5sec
       assert_operator(slept, :<=, 6.0, "[ruby-core:18015]: longer than expected")
     end
   end
+
+  def test_sleep_forever_not_woken_by_sigchld
+    begin
+      t = Thread.new do
+        sleep 0.5
+        `echo hello`
+      end
+
+      assert_raise Timeout::Error do
+        Timeout.timeout 2 do
+          sleep # Should block forever
+        end
+      end
+    ensure
+      t.join
+    end
+  end
 end

thread_pthread.c

@@ -1109,10 +1109,9 @@ thread_sched_to_waiting_until_wakeup(struct rb_thread_sched *sched, rb_thread_t
     {
         if (!RUBY_VM_INTERRUPTED(th->ec)) {
             bool can_direct_transfer = !th_has_dedicated_nt(th);
-            th->status = THREAD_STOPPED_FOREVER;
+            // NOTE: th->status is set before and after this sleep outside of this function in `sleep_forever`
             thread_sched_wakeup_next_thread(sched, th, can_direct_transfer);
             thread_sched_wait_running_turn(sched, th, can_direct_transfer);
-            th->status = THREAD_RUNNABLE;
         }
         else {
             RUBY_DEBUG_LOG("th:%u interrupted", rb_th_serial(th));