[PATCH] Fix sleep spurious wakeup from sigchld (#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 8d8159e7d8 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]