Make AsioExternalThreadEvent::abandon() not hit assertion error

Destructor of AsioExternalThreadEvent uses assert() to check whether the
state is legal. Introduction of abandon() mechanism makes it possible to
destroy the AsioExternalThreadEvent in the Waiting state and hit the
assertion error.

This diff solves the problem by introducing Abandoned state.

hphp/runtime/ext/asio/asio_external_thread_event.cpp

@@ -15,9 +15,9 @@
    +----------------------------------------------------------------------+
 */
 
+#include "hphp/runtime/ext/asio/asio_external_thread_event.h"
 #include <thread>
 #include "hphp/runtime/ext/ext_asio.h"
-#include "hphp/runtime/ext/asio/asio_external_thread_event.h"
 #include "hphp/runtime/ext/asio/asio_session.h"
 
 namespace HPHP {
@@ -32,6 +32,7 @@ AsioExternalThreadEvent::AsioExternalThreadEvent(ObjectData* priv_data)
 void AsioExternalThreadEvent::abandon() {
   assert(m_state.load() == Waiting);
   assert(m_waitHandle->getCount() == 1);
+  m_state.store(Abandoned);
   m_waitHandle->abandon(false);
 }
 

hphp/runtime/ext/asio/asio_external_thread_event.h

@@ -214,7 +214,11 @@ class AsioExternalThreadEvent {
      * is eventually called.
      */
     virtual ~AsioExternalThreadEvent() {
-      assert(m_state.load() == Finished || m_state.load() == Canceled);
+      assert(
+        m_state.load() == Finished ||
+        m_state.load() == Canceled ||
+        m_state.load() == Abandoned
+      );
     };
 
     /**
@@ -267,6 +271,14 @@ class AsioExternalThreadEvent {
        * eventually calling markAsFinished() that will destruct this object.
        */
       Canceled,
+
+      /**
+       * Web request thread abandoned event before passing to processing thread.
+       *
+       * This object is owned by web request thread, which is trying to abandon
+       * it prior to passing ownership to processing thread.
+       */
+      Abandoned,
     };
 
     AsioSession* m_session;