FiberManager: numTasks() to query for total number of active fibers

Reviewed By: andriigrynenko

Differential Revision: D15366589

fbshipit-source-id: e8d1c17e158437a3ca882f51f12071bf666c1a6e

folly/fibers/FiberManagerInternal.h

@@ -209,6 +209,15 @@ class FiberManager : public ::folly::Executor {
    */
   bool hasTasks() const;
 
+  /**
+   * @return The number of currently active fibers (ready to run or blocked).
+   * Does not include the number of remotely enqueued tasks that have not been
+   * run yet.
+   */
+  size_t numActiveTasks() const noexcept {
+    return fibersActive_;
+  }
+
   /**
    * @return true if there are tasks ready to run.
    */