Fix re-entrant calls to Group#wait_for.

Author: samuel-williams-shopify

lib/async/container/group.rb

@@ -239,7 +239,10 @@ def wait_for_children(duration = nil)
 					# Maybe consider using a proper event loop here:
 					if ready = self.select(duration)
 						ready.each do |io|
-							@running[io].resume
+							if fiber = @running[io]
+								# This method can be re-entered. While resuming a fiber, a policy hook may be invoked, which may invoke operations on the container. In that case, select may be called again on the same set of waiting fibers. On returning, those fibers may have already completed and removed themselves from @running, so we need to check for that.
+								fiber.resume
+							end
 						end
 					end
 				end

test/async/container/group.rb

@@ -272,4 +272,32 @@
 			group.health_check!
 		end.not.to raise_exception
 	end
+
+	it "handles nil fiber in @running during iteration (re-entrance scenario)" do
+		# This test simulates a scenario where:
+		# 1. IO.select returns [io1, io2]
+		# 2. While resuming fiber for io1, a re-entrant call completes fiber for io2
+		# 3. When iteration continues to io2, @running[io2] is nil
+		# Without defensive check (&.), this would crash with NoMethodError
+		
+		channel1 = Async::Container::Channel.new
+		channel2 = Async::Container::Channel.new
+		
+		fiber1 = Fiber.new{group.running.delete(channel2.in)}
+		fiber2 = Fiber.new{Fiber.yield}
+		
+		fiber2.resume
+		
+		group.running[channel1.in] = fiber1
+		group.running[channel2.in] = fiber2
+		
+		# Mock select to return both channels:
+		expect(group).to receive(:select).and_return([channel1.in, channel2.in])
+		
+		# This should not crash due to &. operator:
+		group.sleep(0)
+		
+		# Verify fiber2 was removed
+		expect(group.running.key?(channel2.in)).to be == false
+	end
 end