Fix TypeError when processing a detached watcher

ext/cool.io/loop.c

@@ -113,6 +113,12 @@ void Coolio_Loop_process_event(VALUE watcher, int revents)
   /* The Global VM lock isn't held right now, but hopefully
    * we can still do this safely */
   watcher_data = Coolio_Watcher_ptr(watcher);
+
+  if (watcher_data->enabled == 0) {
+    /* Ignore event because watcher was already detached. */
+    return;
+  }
+
   loop_data = Coolio_Loop_ptr(watcher_data->loop);
 
   /*  Well, what better place to explain how this all works than

ext/cool.io/watcher.h

@@ -33,6 +33,10 @@
   if(watcher_data->loop == Qnil) \
     rb_raise(rb_eRuntimeError, "not attached to a loop"); \
   \
+  if (watcher_data->enabled == 0) { \
+    /* Ignore because watcher was already detached. */ \
+    return Qnil; \
+  } \
   loop_data = Coolio_Loop_ptr(watcher_data->loop); \
   \
   ev_##watcher_type##_stop(loop_data->ev_loop, &watcher_data->event_types.ev_##watcher_type); \

spec/detach_race_condition_spec.rb

@@ -0,0 +1,50 @@
+require 'spec_helper'
+
+describe Cool.io::Loop do
+  class Victim < Cool.io::IOWatcher
+    def initialize(io)
+      super
+      @io = io
+    end
+
+    def on_readable
+      begin
+        @io.read_nonblock(1024)
+      rescue IO::WaitReadable, EOFError
+      end
+    end
+  end
+
+  # https://github.com/socketry/cool.io/issues/87
+  it "does not raise TypeError when a watcher is detached while an event is pending" do
+    loop = Cool.io::Loop.default
+
+    iterations = 200
+
+    expect {
+      iterations.times do
+        r_victim, w_victim = IO.pipe
+        victim_watcher = Victim.new(r_victim)
+        victim_watcher.attach(loop)
+
+        t1 = Thread.new do
+          sleep 0.01
+          w_victim.write("dummy\n")
+        end
+
+        t2 = Thread.new do
+          sleep 0.01
+          victim_watcher.detach
+        end
+
+        loop.run_once
+
+        t1.join
+        t2.join
+
+        r_victim.close
+        w_victim.close
+      end
+    }.not_to raise_error
+  end
+end