Make the worker pool an optional feature that defaults to off.

.github/workflows/test-worker-pool.yaml

@@ -0,0 +1,38 @@
+name: Test
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+env:
+  CONSOLE_OUTPUT: XTerm
+  ASYNC_SCHEDULER_DEFAULT_WORKER_POOL: true
+
+jobs:
+  test:
+    name: ${{matrix.ruby}} on ${{matrix.os}} / ASYNC_SCHEDULER_DEFAULT_WORKER_POOL=true
+    runs-on: ${{matrix.os}}-latest
+
+    strategy:
+      matrix:
+        os:
+          - ubuntu
+
+        ruby:
+          - head
+
+    steps:
+    - uses: actions/checkout@v3
+    - uses: ruby/setup-ruby@v1
+      with:
+        ruby-version: ${{matrix.ruby}}
+        bundler-cache: true
+
+    - name: Run tests
+      timeout-minutes: 10
+      run: bundle exec bake test
+
+    - name: Run external tests
+      timeout-minutes: 10
+      run: bundle exec bake test:external

lib/async/scheduler.rb

@@ -17,6 +17,10 @@
 module Async
 	# Handles scheduling of fibers. Implements the fiber scheduler interface.
 	class Scheduler < Node
+		DEFAULT_WORKER_POOL = ENV.fetch("ASYNC_SCHEDULER_DEFAULT_WORKER_POOL", nil).then do |value|
+			value == "true" ? true : nil
+		end
+
 		# Raised when an operation is attempted on a closed scheduler.
 		class ClosedError < RuntimeError
 			# Create a new error.
@@ -38,7 +42,7 @@ def self.supported?
 		# @public Since *Async v1*.
 		# @parameter parent [Node | Nil] The parent node to use for task hierarchy.
 		# @parameter selector [IO::Event::Selector] The selector to use for event handling.
-		def initialize(parent = nil, selector: nil)
+		def initialize(parent = nil, selector: nil, worker_pool: DEFAULT_WORKER_POOL)
 			super(parent)
 
 			@selector = selector || ::IO::Event::Selector.new(Fiber.current)
@@ -50,7 +54,15 @@ def initialize(parent = nil, selector: nil)
 			@idle_time = 0.0
 
 			@timers = ::IO::Event::Timers.new
-			@worker_pool = WorkerPool.new
+			if worker_pool == true
+				@worker_pool = WorkerPool.new
+			else
+				@worker_pool = worker_pool
+			end
+
+			if @worker_pool
+				self.singleton_class.prepend(WorkerPool::BlockingOperationWait)
+			end
 		end
 
 		# Compute the scheduler load according to the busy and idle times that are updated by the run loop.
@@ -348,17 +360,6 @@ def process_wait(pid, flags)
 			return @selector.process_wait(Fiber.current, pid, flags)
 		end
 
-		# Wait for the given work to be executed.
-		#
-		# @public Since *Async v2.19* and *Ruby v3.4*.
-		# @asynchronous May be non-blocking.
-		#
-		# @parameter work [Proc] The work to execute on a background thread.
-		# @returns [Object] The result of the work.
-		def blocking_operation_wait(work)
-			@worker_pool.call(work)
-		end
-
 		# Run one iteration of the event loop.
 		#
 		# When terminating the event loop, we already know we are finished. So we don't need to check the task tree. This is a logical requirement because `run_once` ignores transient tasks. For example, a single top level transient task is not enough to keep the reactor running, but during termination we must still process it in order to terminate child tasks.

lib/async/worker_pool.rb

@@ -10,6 +10,19 @@ module Async
 	#
 	# @private
 	class WorkerPool
+		module BlockingOperationWait
+			# Wait for the given work to be executed.
+			#
+			# @public Since *Async v2.19* and *Ruby v3.4*.
+			# @asynchronous May be non-blocking.
+			#
+			# @parameter work [Proc] The work to execute on a background thread.
+			# @returns [Object] The result of the work.
+			def blocking_operation_wait(work)
+				@worker_pool.call(work)
+			end
+		end
+
 		class Promise
 			def initialize(work)
 				@work = work