assert the presence of the KeepAlive functionality with VirtualExecutor

Summary: title

Reviewed By: yfeldblum, andriigrynenko

Differential Revision: D8386575

fbshipit-source-id: c900c8692b10004f2d9fbd95fa8ca96b3b8517d2

folly/Executor.h

@@ -158,6 +158,16 @@ class Executor {
   }
 
  protected:
+  /**
+   * Returns true if the KeepAlive is constructed from an executor that does
+   * not support the keep alive ref-counting functionality
+   */
+  template <typename ExecutorT>
+  static bool isKeepAliveDummy(const KeepAlive<ExecutorT>& keepAlive) {
+    return reinterpret_cast<intptr_t>(keepAlive.executorAndDummyFlag_) &
+        KeepAlive<ExecutorT>::kDummyFlag;
+  }
+
   // Acquire a keep alive token. Should return false if keep-alive mechanism
   // is not supported.
   virtual bool keepAliveAcquire();

folly/VirtualExecutor.h

@@ -32,7 +32,9 @@ namespace folly {
 class VirtualExecutor : public DefaultKeepAliveExecutor {
  public:
   explicit VirtualExecutor(KeepAlive<> executor)
-      : executor_(std::move(executor)) {}
+      : executor_(std::move(executor)) {
+    assert(!isKeepAliveDummy(executor_));
+  }
 
   explicit VirtualExecutor(Executor* executor)
       : VirtualExecutor(getKeepAliveToken(executor)) {}