Handle Thread#kill during scheduler shutdown

lib/async/task.rb

@@ -327,6 +327,11 @@ def result
 		# @parameter later [Boolean] Whether to cancel the task later, or immediately.
 		# @parameter cause [Exception] The cause of the cancel operation.
 		def cancel(later = false, cause: $!)
+			# On affected Ruby versions, `Thread#kill` exposes the internal `TAG_FATAL` value through `$!`, which is not a valid exception cause:
+			unless cause.is_a?(::Exception)
+				cause = nil
+			end
+
 			# If no cause is given, we generate one from the current call stack:
 			unless cause
 				cause = Cancel::Cause.for("Cancelling task!")

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - Fixed `Thread#kill` raising a `TypeError` while shutting down a thread with an active `Async::Reactor`.
+
 ## v2.45.1
 
   - Fixed `Scheduler#io_wait` returning `nil` instead of `false` when an explicit timeout expired. Native callers such as `Socket#connect` with `connect_timeout:` distinguish a timeout by checking for `false`, so the `nil` caused `TypeError: no implicit conversion from nil to integer` instead of the intended `IO::TimeoutError`.

test/async/scheduler.rb

@@ -85,6 +85,33 @@
 	end
 
 	with "#close" do
+		it "can close when the owning thread is killed" do
+			ready = Thread::Queue.new
+
+			thread = Thread.new do
+				reactor = Async::Reactor.new
+
+				reactor.async do
+					reactor.async do
+						sleep
+					end
+				end
+
+				ready << true
+				reactor.run
+			end
+			thread.report_on_exception = false
+
+			ready.pop
+			thread.kill
+
+			expect do
+				thread.join
+			end.not.to raise_exception
+
+			expect(thread).not.to be(:alive?)
+		end
+
 		it "runs the event loop until terminated" do
 			scheduler = Async::Scheduler.new
 			Async::Node.new(scheduler)