Fix Barrier when parent.async() yields early

fixtures/async/chainable_async.rb

@@ -9,7 +9,9 @@ module Async
 		let(:chainable) {subject.new(parent: parent)}
 
 		it "should chain async to parent" do
-			expect(parent).to receive(:async).and_return(nil)
+			expect(parent).to receive(:async).and_return{|*arguments, **options, &block|
+				Async(*arguments, **options, &block)
+			}
 
 			chainable.async do
 				# Nothing.

lib/async/barrier.rb

@@ -18,6 +18,7 @@ class Barrier
 		def initialize(parent: nil)
 			@tasks = List.new
 			@finished = Queue.new
+			@condition = Condition.new
 
 			@parent = parent
 		end
@@ -42,18 +43,32 @@ def size
 
 		# Execute a child task and add it to the barrier.
 		# @asynchronous Executes the given block concurrently.
+		# @returns [Task] The task which was created to execute the block.
 		def async(*arguments, parent: (@parent or Task.current), **options, &block)
 			raise "Barrier is stopped!" if @finished.closed?
 
 			waiting = nil
 
-			parent.async(*arguments, **options) do |task, *arguments|
-				waiting = TaskNode.new(task)
-				@tasks.append(waiting)
+			task = parent.async(*arguments, **options) do |task, *arguments|
+				# Create a new list node for the task and add it to the list of waiting tasks:
+				node = TaskNode.new(task)
+				@tasks.append(node)
+
+				# Signal the outer async block that we have added the task to the list of waiting tasks, and that it can now wait for it to finish:
+				waiting = node
+				@condition.signal
+
+				# Invoke the block, which may raise an error. If it does, we will still signal that the task has finished:
 				block.call(task, *arguments)
 			ensure
-				@finished.signal(waiting) unless @finished.closed?
+				# Signal that the task has finished, which will unblock the waiting task:
+				@finished.signal(node) unless @finished.closed?
 			end
+
+			# `parent.async` may yield before the child block executes, so we wait here until the child has appended itself to `@tasks`, ensuring `wait` cannot return early and miss tracking it:
+			@condition.wait while waiting.nil?
+
+			return task
 		end
 
 		# Whether there are any tasks being held by the barrier.

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Fix `Barrier#async` when `parent.async` yields before the child block executes. Previously, `Barrier#wait` could return early and miss tracking the task entirely, because the task had not yet appended itself to the barrier's task list.
+
 ## v2.38.0
 
   - Rename `Task#stop` to `Task#cancel` for better clarity and consistency with common concurrency terminology. The old `stop` method is still available as an alias for backward compatibility, but it is recommended to use `cancel` going forward.

test/async/barrier.rb

@@ -133,6 +133,31 @@
 				barrier.stop
 			end
 		end
+
+		it "waits even if the child task yields immediately" do
+			class Yielder
+				def async(*arguments, **options, &block)
+					Async(*arguments, **options) do |task, *arguments|
+						task.yield
+						block.call(task, *arguments)
+					end
+				end
+			end
+
+			parent = Yielder.new
+
+			3.times do |i|
+				barrier.async(parent:){i}
+			end
+
+			expect(barrier.size).to be == 3
+
+			results = []
+			barrier.wait do |task|
+				results << task.wait
+			end
+			expect(results.sort).to be == [0, 1, 2]
+		end
 	end
 
 	with "#stop" do