fixed crash in thread pool and added test

Author: vladstepanyuk

cloud/storage/core/libs/common/thread_pool.cpp

@@ -88,6 +88,7 @@ class TThreadPool final
             worker.Name = TStringBuilder() << threadName << i++;
             worker.Thread = std::make_unique<TWorkerThread>(*this, worker);
         }
+        AtomicSet(RunningWorkers, NumWorkers);
     }
 
     ~TThreadPool() override
@@ -97,8 +98,6 @@ class TThreadPool final
 
     void Start() override
     {
-        AtomicSet(RunningWorkers, NumWorkers);
-
         for (auto& worker: Workers) {
             worker.Thread->Start();
         }

cloud/storage/core/libs/common/thread_pool_ut.cpp

@@ -6,6 +6,8 @@
 
 #include <util/generic/scope.h>
 
+#include <thread>
+
 namespace NCloud {
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -30,6 +32,37 @@ Y_UNIT_TEST_SUITE(TThreadPoolTest)
 
         UNIT_ASSERT_EQUAL(future.GetValue(WaitTimeout), 42);
     }
+
+    Y_UNIT_TEST(ShouldExecuteTaskEnqueuedBeforeStart)
+    {
+        auto threadPool = CreateThreadPool("thread", 1);
+
+        auto promise = NThreading::NewPromise();
+
+        auto future = promise.GetFuture();
+
+        std::thread thread(
+            [threadPool, promise = std::move(promise)]() mutable
+            {
+                promise.SetValue();
+                auto future = threadPool->Execute([] { return 42; });
+
+                UNIT_ASSERT_EQUAL(future.GetValue(WaitTimeout), 42);
+            });
+
+        future.GetValueSync();
+
+        // Sleep to be sure that task will be enqueued before start.
+        Sleep(TDuration::Seconds(1));
+
+        threadPool->Start();
+        Y_DEFER
+        {
+            threadPool->Stop();
+        };
+
+        thread.join();
+    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////