Don't run scheduler when there is a pending exception. (#262)

fixtures/child_process.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2023, by Samuel Williams.
+
+module ChildProcess
+	def self.spawn(code)
+		lib_path = File.expand_path("../lib", __dir__)
+
+		IO.popen(["ruby", "-I#{lib_path}"], "r+", err: [:child, :out]) do |process|
+			process.write(code)
+			process.close_write
+
+			return process.read
+		end
+	end
+end

lib/async/scheduler.rb

@@ -41,7 +41,10 @@ def initialize(parent = nil, selector: nil)
 		end
 
 		def scheduler_close
-			self.run
+			# If the execution context (thread) was handling an exception, we want to exit as quickly as possible:
+			unless $!
+				self.run
+			end
 		ensure
 			self.close
 		end

test/fiber.rb

@@ -4,6 +4,7 @@
 # Copyright, 2022, by Samuel Williams.
 
 require 'async/reactor'
+require 'child_process'
 
 describe Fiber do
 	with '.schedule' do
@@ -26,5 +27,51 @@
 
 			expect(sequence).to be == [0, 1, 2]
 		end
+
+		it 'correctly handles exceptions in process' do
+			buffer = ChildProcess.spawn(<<~RUBY)
+				require 'async'
+				
+				scheduler = Async::Scheduler.new
+				Fiber.set_scheduler(scheduler)
+				
+				Fiber.schedule do
+					sleep(1)
+					puts "Finished sleeping!"
+				end
+				
+				raise "Boom!"
+			RUBY
+
+			expect(buffer).to be(:include?, "Boom!")
+			expect(buffer).not.to be(:include?, "Finished sleeping!")
+		end
+
+		it 'correctly handles exceptions' do
+			finished_sleeping = nil
+
+			thread = Thread.new do
+				# Stop the thread logging on exception:
+				Thread.current.report_on_exception = false
+
+				scheduler = Async::Scheduler.new
+				Fiber.set_scheduler(scheduler)
+
+				finished_sleeping = false
+
+				Fiber.schedule do
+					sleep(10)
+					finished_sleeping = true
+				end
+
+				raise "Boom!"
+			end
+
+			expect{thread.join}.to raise_exception(RuntimeError,
+				message: be == "Boom!"
+			)
+
+			expect(finished_sleeping).to be == false
+		end
 	end
 end