Start function (iresearch-toolkit#577)

* Start function

* wip

* fix

Author: Andrei Lobov

core/utils/async_utils.cpp

@@ -55,13 +55,19 @@ void busywait_mutex::unlock() noexcept {
 }
 
 template<bool UseDelay>
-ThreadPool<UseDelay>::ThreadPool(size_t threads, basic_string_view<Char> name)
-  : name_{name} {
+ThreadPool<UseDelay>::ThreadPool(size_t threads, basic_string_view<Char> name) {
+  start(threads, name);
+}
+
+template<bool UseDelay>
+void ThreadPool<UseDelay>::start(size_t threads, basic_string_view<Char> name) {
+  IRS_ASSERT(threads_.empty());
   threads_.reserve(threads);
   for (size_t i = 0; i != threads; ++i) {
-    threads_.emplace_back([&] {
-      if (!name_.empty()) {
-        set_thread_name(name_.c_str());
+    threads_.emplace_back([this, name] {
+      if (!name.empty()) {
+        IRS_ASSERT(std::char_traits<Char>::length(name.data()) == name.size());
+        set_thread_name(name.data());
       }
       Work();
     });

core/utils/async_utils.hpp

@@ -57,9 +57,11 @@ class ThreadPool {
   using Clock = std::chrono::steady_clock;
   using Func = fu2::unique_function<void()>;
 
+  ThreadPool() = default;
   explicit ThreadPool(size_t threads, basic_string_view<Char> name = {});
   ~ThreadPool() { stop(true); }
 
+  void start(size_t threads, basic_string_view<Char> name = {});
   bool run(Func&& fn, Clock::duration delay = {});
   void stop(bool skip_pending = false) noexcept;  // always a blocking call
   size_t tasks_active() const {
@@ -95,7 +97,6 @@ class ThreadPool {
 
   bool WasStop() const { return state_ % 2 != 0; }
 
-  basic_string<Char> name_;
   std::vector<std::thread> threads_;
   mutable std::mutex m_;
   std::condition_variable cv_;