Avoid race conditions in Scheduler#unblock.

See <ruby/ruby#7460> for more details.

examples/bugs/write_lock.rb

@@ -0,0 +1,41 @@
+#!/usr/bin/env ruby
+
+# This bug only generally shows up on Linux, when using io_uring, as it has more fine-grained locking. The issue is that `puts` can acquire and release a write lock, and if one thread releases that lock while the reactor on the waitq thread is closing, it can call `unblock` with `@selector = nil` which fails or causes odd behaviour.
+
+require_relative '../../lib/async'
+
+def wait_for_interrupt(thread_index, repeat)
+	sequence = []
+
+	events = Thread::Queue.new
+	reactor = Async::Reactor.new
+
+	thread = Thread.new do
+		if events.pop
+			puts "#{thread_index}+#{repeat} Sending Interrupt!"
+			reactor.interrupt
+		end
+	end
+
+	reactor.async do
+		events << true
+		puts "#{thread_index}+#{repeat} Reactor ready!"
+
+		# Wait to be interrupted:
+		sleep(1)
+
+		puts "#{thread_index}+#{repeat} Missing interrupt!"
+	end
+
+	reactor.run
+
+	thread.join
+end
+
+100.times.map do |thread_index|
+	Thread.new do
+		1000.times do |repeat|
+			wait_for_interrupt(thread_index, repeat)
+		end
+	end
+end.each(&:join)

lib/async/scheduler.rb

@@ -50,9 +50,12 @@ def close
 			# We depend on GVL for consistency:
 			# @guard.synchronize do
 
-			@selector&.close
+			# 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
+
 			# end
 
 			consume
@@ -69,9 +72,10 @@ def to_s
 		end
 
 		# Interrupt the event loop and cause it to exit.
+		# @asynchronous May be called from any thread.
 		def interrupt
 			@interrupted = true
-			@selector.wakeup
+			@selector&.wakeup
 		end
 
 		# Transfer from the calling fiber to the event loop.
@@ -127,8 +131,10 @@ def unblock(blocker, fiber)
 			# $stderr.puts "unblock(#{blocker}, #{fiber})"
 
 			# This operation is protected by the GVL:
-			@selector.push(fiber)
-			@selector.wakeup
+			if selector = @selector
+				selector.push(fiber)
+				selector.wakeup
+			end
 		end
 
 		# @asynchronous May be non-blocking..

test-segfault.rb

@@ -0,0 +1,39 @@
+#!/usr/bin/env ruby
+
+require_relative 'lib/async'
+
+def wait_for_interrupt(thread_index, repeat)
+	sequence = []
+
+	events = Thread::Queue.new
+	reactor = Async::Reactor.new
+
+	thread = Thread.new do
+		if events.pop
+			puts "#{thread_index}+#{repeat} Sending Interrupt!"
+			reactor.interrupt
+		end
+	end
+
+	reactor.async do
+		events << true
+		puts "#{thread_index}+#{repeat} Reactor ready!"
+
+		# Wait to be interrupted:
+		sleep(1)
+
+		puts "#{thread_index}+#{repeat} Missing interrupt!"
+	end
+
+	reactor.run
+
+	thread.join
+end
+
+32.times.map do |thread_index|
+	Thread.new do
+		100.times do |repeat|
+			wait_for_interrupt(thread_index, repeat)
+		end
+	end
+end.each(&:join)

test/async/scheduler.rb

@@ -63,4 +63,28 @@
 			expect(duration).to be <= 0.1
 		end
 	end
+
+	with '#interrupt' do
+		it "can interrupt a closed scheduler" do
+			scheduler = Async::Scheduler.new
+			scheduler.close
+			scheduler.interrupt
+		end
+	end
+
+	with '#block' do
+		it "can block and unblock the scheduler after closing" do
+			scheduler = Async::Scheduler.new
+
+			fiber = Fiber.new do
+				scheduler.block(:test, nil)
+			end
+
+			fiber.transfer
+
+			expect do
+				scheduler.close
+			end.to raise_exception(RuntimeError, message: be =~ /Closing scheduler with blocked operations/)
+		end
+	end
 end