Fabric: Lock-free events 4/n: Added an assert in EventTarget

Summary: See the diff for more details.

Reviewed By: sahrens

Differential Revision: D13642593

fbshipit-source-id: 8bdcc91bcc2ea1e4093bcac03d87167b8901cbb4

ReactCommon/fabric/events/EventEmitter.h

@@ -46,7 +46,6 @@ class EventEmitter {
   virtual ~EventEmitter() = default;
 
   /*
-   * `DispatchMutex` must be acquired before calling.
    * Enables/disables event emitter.
    * Enabled event emitter retains a pointer to `eventTarget` strongly (as
    * `std::shared_ptr`) whereas disabled one don't.
@@ -54,6 +53,7 @@ class EventEmitter {
    * a possibility to extract JSI value from it.
    * The enable state is additive; a number of `enable` calls should be equal to
    * a number of `disable` calls to release the event target.
+   * `DispatchMutex` must be acquired before calling.
    */
   void setEnabled(bool enabled) const;
 

ReactCommon/fabric/events/EventTarget.cpp

@@ -31,6 +31,15 @@ void EventTarget::retain(jsi::Runtime &runtime) const {
   }
 
   strongInstanceHandle_ = weakInstanceHandle_.lock(runtime);
+
+  // Having a `null` or `undefined` object here indicates that
+  // `weakInstanceHandle_` was already deallocated. This should *not* happen by
+  // design, and if it happens it's a severe problem. This basically means that
+  // particular implementation of JSI was able to detect this inconsistency and
+  // dealt with it, but some JSI implementation may not support this feature and
+  // that case will lead to a crash in those environments.
+  assert(!strongInstanceHandle_.isNull());
+  assert(!strongInstanceHandle_.isUndefined());
 }
 
 jsi::Value EventTarget::release(jsi::Runtime &runtime) const {