Improve handling of Process.fork on Ruby 4+.

Author: samuel-williams-shopify

lib/async/fork_handler.rb

@@ -26,7 +26,7 @@ def _fork(&block)
 	private_constant :ForkHandler
 
 	# Hook into Process._fork to handle fork events automatically:
-	unless (Fiber.const_get(:SCHEDULER_PROCESS_FORK) rescue false)
+	unless RUBY_VERSION > "4"
 		::Process.singleton_class.prepend(ForkHandler)
 	end
 end

lib/async/scheduler.rb

@@ -646,17 +646,13 @@ def timeout_after(duration, exception, message, &block)
 			end
 		end
 
-		# Handle fork in the child process. This method is called automatically when `Process.fork` is invoked.
+		# Handle fork in the child process. This method is called automatically when `Process.fork` is invoked on Ruby versions < 4 and cleans up the scheduler state. On Ruby 4+, the scheduler is automatically cleaned up by the Ruby runtime.
 		#
 		# The child process starts with a clean slate - no scheduler is set. Users can create a new scheduler if needed.
 		#
 		# @public Since *Async v2.35*.
 		def process_fork
-			if profiler = @profiler
-				@profiler = nil
-				profiler.stop
-			end
-			
+			@profiler = nil
 			@children = nil
 			@selector = nil
 			@timers = nil

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Improved handling of `Process.fork` on Ruby 4+.
+
 ## v2.35.1
 
   - Fix incorrect handling of spurious wakeups in `Async::Promise#wait`, which could lead to premature (incorrect) resolution of the promise.