process_wait can hang if reusing FD in rapid succession. (#126)

* Failing test case.

* Unique per-pid/file descriptor.

ext/io/event/selector/epoll.c

@@ -519,7 +519,7 @@ VALUE IO_Event_Selector_EPoll_process_wait(VALUE self, VALUE fiber, VALUE _pid,
 
 	RB_OBJ_WRITTEN(self, Qundef, fiber);
 
-	int result = IO_Event_Selector_EPoll_Waiting_register(selector, 0, descriptor, &waiting);
+	int result = IO_Event_Selector_EPoll_Waiting_register(selector, _pid, descriptor, &waiting);
 
 	if (result == -1) {
 		close(descriptor);

test/io/event/selector.rb

@@ -581,6 +581,32 @@ def transfer
 			expect(events).to be == [:process_finished]
 			expect(result).to be(:success?)
 		end
+
+		it "can wait for two processes sequentially" do
+			result1 = result2 = nil
+			events = []
+
+			fiber = Fiber.new do
+				pid1 = Process.spawn("sleep 0.001")
+				pid2 = Process.spawn("sleep 0.001")
+
+				result1 = selector.process_wait(Fiber.current, pid1, 0)
+				events << :process_finished1
+
+				result2 = selector.process_wait(Fiber.current, pid2, 0)
+				events << :process_finished2
+			end
+
+			fiber.transfer
+
+			while fiber.alive?
+				selector.select(0)
+			end
+
+			expect(events).to be == [:process_finished1, :process_finished2]
+			expect(result1).to be(:success?)
+			expect(result2).to be(:success?)
+		end
 	end
 
 	with "#resume" do