From 6834143de9b922584ef478377dea3dc9a21f07f7 Mon Sep 17 00:00:00 2001
From: Samuel Williams <samuel.williams@shopify.com>
Date: Fri, 28 Nov 2025 20:46:02 +1300
Subject: [PATCH] Add support for `Process.fork` within an active scheduler.

---
 lib/async/fork_handler.rb | 31 ++++++++++++++++++++++++++
 lib/async/node.rb         |  3 ++-
 lib/async/scheduler.rb    | 44 +++++++++++++++++++++++++++---------
 releases.md               |  4 ++++
 test/process/fork.rb      | 47 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 117 insertions(+), 12 deletions(-)
 create mode 100644 lib/async/fork_handler.rb
 create mode 100644 test/process/fork.rb

diff --git a/lib/async/fork_handler.rb b/lib/async/fork_handler.rb
new file mode 100644
index 00000000..a4ce673e
--- /dev/null
+++ b/lib/async/fork_handler.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2025, by Samuel Williams.
+
+module Async
+	# Private module that hooks into Process._fork to handle fork events.
+	#
+	# If `Scheduler#process_fork` hook is adopted in Ruby 4, this code can be removed after Ruby < 4 is no longer supported.
+	module ForkHandler
+		def _fork(&block)
+			result = super
+			
+			if result.zero?
+				# Child process:
+				if Fiber.scheduler.respond_to?(:process_fork)
+					Fiber.scheduler.process_fork
+				end
+			end
+			
+			return result
+		end
+	end
+	
+	private_constant :ForkHandler
+	
+	# Hook into Process._fork to handle fork events automatically:
+	unless (Fiber.const_get(:SCHEDULER_PROCESS_FORK) rescue false)
+		::Process.singleton_class.prepend(ForkHandler)
+	end
+end
diff --git a/lib/async/node.rb b/lib/async/node.rb
index d64aa446..78d59cc7 100644
--- a/lib/async/node.rb
+++ b/lib/async/node.rb
@@ -214,7 +214,8 @@ def parent=(parent)
 		end
 		
 		protected def remove_child(child)
-			@children.remove(child)
+			# In the case of a fork, the children list may be nil:
+			@children&.remove(child)
 			child.set_parent(nil)
 		end
 		
diff --git a/lib/async/scheduler.rb b/lib/async/scheduler.rb
index 47d418e6..0935da9d 100644
--- a/lib/async/scheduler.rb
+++ b/lib/async/scheduler.rb
@@ -9,6 +9,7 @@
 require_relative "clock"
 require_relative "task"
 require_relative "timeout"
+require_relative "fork_handler"
 
 require "io/event"
 
@@ -146,24 +147,26 @@ def terminate
 		# Terminate all child tasks and close the scheduler.
 		# @public Since *Async v1*.
 		def close
-			self.run_loop do
-				until self.terminate
-					self.run_once!
+			unless @children.nil?
+				self.run_loop do
+					until self.terminate
+						self.run_once!
+					end
 				end
 			end
 			
 			Kernel.raise "Closing scheduler with blocked operations!" if @blocked > 0
 		ensure
 			# We want `@selector = nil` to be a visible side effect from this point forward, specifically in `#interrupt` and `#unblock`. If the selector is closed, then we don't want to push any fibers to it.
-			selector = @selector
-			@selector = nil
-			
-			selector&.close
-			
-			worker_pool = @worker_pool
-			@worker_pool = nil
+			if selector = @selector
+				@selector = nil
+				selector.close
+			end
 			
-			worker_pool&.close
+			if worker_pool = @worker_pool
+				@worker_pool = nil
+				worker_pool.close
+			end
 			
 			consume
 		end
@@ -642,5 +645,24 @@ def timeout_after(duration, exception, message, &block)
 				yield duration
 			end
 		end
+		
+		# Handle fork in the child process. This method is called automatically when `Process.fork` is invoked.
+		#
+		# 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
+			
+			@children = nil
+			@selector = nil
+			@timers = nil
+			
+			# Close the scheduler:
+			Fiber.set_scheduler(nil)
+		end
 	end
 end
diff --git a/releases.md b/releases.md
index fabc8067..e7248682 100644
--- a/releases.md
+++ b/releases.md
@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - `Process.fork` is now properly handled by the Async fiber scheduler, ensuring that the scheduler state is correctly reset in the child process after a fork. This prevents issues where the child process inherits the scheduler state from the parent, which could lead to unexpected behavior.
+
 ## v2.34.0
 
 ### `Kernel::Barrier` Convenience Interface
diff --git a/test/process/fork.rb b/test/process/fork.rb
new file mode 100644
index 00000000..4ff15f97
--- /dev/null
+++ b/test/process/fork.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2025, by Samuel Williams.
+
+require "sus/fixtures/async"
+require "async"
+
+describe Process do
+	describe ".fork" do
+		it "can fork with block form" do
+			r, w = IO.pipe
+			
+			Async do
+				pid = Process.fork do
+					# Child process:
+					w.write("hello")
+				end
+				
+				# Parent process:
+				w.close
+				expect(r.read).to be == "hello"
+			ensure
+				Process.waitpid(pid) if pid
+			end
+		end
+		
+		it "can fork with non-block form" do
+			r, w = IO.pipe
+			
+			Async do
+				unless pid = Process.fork
+					# Child process:
+					w.write("hello")
+					
+					exit!
+				end
+				
+				# Parent process:
+				w.close
+				expect(r.read).to be == "hello"
+			ensure
+				Process.waitpid(pid) if pid
+			end
+		end
+	end
+end