event::Emitter: fix reentrant retraction edgecase

modules/event.sjs

@@ -100,7 +100,18 @@ function Emitter() {
         listeners.push(resume);
       }
       retract {
-        __js listeners.splice(listeners.indexOf(resume), 1);
+        __js {
+          // the listeners array gets exchanged from 'emit',
+          // so if this is a concurrent retract, we must take care not to try 
+          // to accidentally call splice(-1,1) on listeners when our listener isn't found.
+          // i.e., we must not do:
+          //   listeners.splice(listeners.indexOf(resume), 1);
+          // see also event-tests.sjs:concurrent retract edgecase
+          var i = listeners.indexOf(resume);
+          if (i >= 0)
+            listeners.splice(i, 1);
+        } // __js
+
       }
       receiver(val);
     }

test/unit/event-tests.sjs

@@ -441,3 +441,42 @@ context('wait()') {||
     rv .. assert.eq(2);
   }
 }
+
+test('concurrent retract edgecase') {||
+  var rv = '';
+
+  var E = event.Emitter();
+  var R;
+
+  waitfor {
+    E .. event.wait();
+    waitfor {
+      E .. event.wait(); // this listener used to be erroneously removed by the listener
+                         // retraction triggered by R(), causing the test to hang forever
+      rv += 'C';
+    }
+    and {
+      R(); // this call will abort the event listener in the clause below
+      rv += 'B';
+    }
+
+    rv += 'D';
+  }
+  and {
+    waitfor() {
+      R = resume;
+      // R() will be called before we receive the event, causing the wait() call to be 
+      // aborted
+      E .. event.wait();
+    }
+    rv += 'A';
+  }
+  and {
+    hold(0);
+    E.emit();
+    hold(0);
+    E.emit();
+    rv += 'E';
+  }
+  rv .. assert.eq('ABCDE');
+}