Merge pull request #11539 from phire/improve_workqueuethread

Various WorkQueueThread improvements

Source/Core/Common/WorkQueueThread.h

@@ -3,12 +3,14 @@
 
 #pragma once
 
+#include <atomic>
+#include <condition_variable>
 #include <functional>
 #include <queue>
+#include <string>
+#include <string_view>
 #include <thread>
 
-#include "Common/Event.h"
-#include "Common/Flag.h"
 #include "Common/Thread.h"
 
 // A thread that executes the given function for every item placed into its queue.
@@ -20,89 +22,151 @@ class WorkQueueThread
 {
 public:
   WorkQueueThread() = default;
-  WorkQueueThread(std::function<void(T)> function) { Reset(std::move(function)); }
+  WorkQueueThread(const std::string_view name, std::function<void(T)> function)
+  {
+    Reset(name, std::move(function));
+  }
   ~WorkQueueThread() { Shutdown(); }
-  void Reset(std::function<void(T)> function)
+
+  // Shuts the current work thread down (if any) and starts a new thread with the given function
+  // Note: Some consumers of this API push items to the queue before starting the thread.
+  void Reset(const std::string_view name, std::function<void(T)> function)
   {
     Shutdown();
-    m_shutdown.Clear();
-    m_cancelled.Clear();
+    std::lock_guard lg(m_lock);
+    m_thread_name = name;
+    m_shutdown = false;
     m_function = std::move(function);
     m_thread = std::thread(&WorkQueueThread::ThreadLoop, this);
   }
 
+  // Adds an item to the work queue
   template <typename... Args>
   void EmplaceItem(Args&&... args)
   {
-    if (!m_cancelled.IsSet())
-    {
-      std::lock_guard lg(m_lock);
-      m_items.emplace(std::forward<Args>(args)...);
-    }
-    m_wakeup.Set();
+    std::lock_guard lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_items.emplace(std::forward<Args>(args)...);
+    m_idle = false;
+    m_worker_cond_var.notify_one();
   }
 
-  void Clear()
+  // Adds an item to the work queue
+  void Push(T&& item)
   {
-    {
-      std::lock_guard lg(m_lock);
-      m_items = std::queue<T>();
-    }
-    m_wakeup.Set();
+    std::lock_guard lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_items.push(std::move(item));
+    m_idle = false;
+    m_worker_cond_var.notify_one();
   }
 
+  // Adds an item to the work queue
+  void Push(const T& item)
+  {
+    std::lock_guard lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_items.push(item);
+    m_idle = false;
+    m_worker_cond_var.notify_one();
+  }
+
+  // Empties the queue
+  // If the worker polls IsCanceling(), it can abort it's work when Cancelling
   void Cancel()
   {
-    m_cancelled.Set();
-    Clear();
-    Shutdown();
+    std::unique_lock lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_cancelling = true;
+    m_items = std::queue<T>();
+    m_worker_cond_var.notify_one();
   }
 
-  bool IsCancelled() const { return m_cancelled.IsSet(); }
-
-  void Shutdown()
+  // Tells the worker to shut down when it's queue is empty
+  // Blocks until the worker thread exits.
+  // If cancel is true, will Cancel before before telling the worker to exit
+  // Otherwise, all currently queued items will complete before the worker exits
+  void Shutdown(bool cancel = false)
   {
-    if (m_thread.joinable())
     {
-      m_shutdown.Set();
-      m_wakeup.Set();
-      m_thread.join();
+      std::unique_lock lg(m_lock);
+      if (m_shutdown || !m_thread.joinable())
+        return;
+
+      if (cancel)
+      {
+        m_cancelling = true;
+        m_items = std::queue<T>();
+      }
+
+      m_shutdown = true;
+      m_worker_cond_var.notify_one();
     }
+
+    m_thread.join();
   }
 
+  // Blocks until all items in the queue have been processed (or cancelled)
+  void WaitForCompletion()
+  {
+    std::unique_lock lg(m_lock);
+    // don't check m_shutdown, because it gets set to request a shutdown, and we want to wait until
+    // after the shutdown completes.
+    // We also check m_cancelling, because we want to ensure the worker acknowledges our cancel.
+    if (m_idle && !m_cancelling.load())
+      return;
+
+    m_wait_cond_var.wait(lg, [&] { return m_idle && m_cancelling.load(); });
+  }
+
+  // If the worker polls IsCanceling(), it can abort its work when Cancelling
+  bool IsCancelling() const { return m_cancelling.load(); }
+
 private:
   void ThreadLoop()
   {
-    Common::SetCurrentThreadName("WorkQueueThread");
+    Common::SetCurrentThreadName(m_thread_name.c_str());
 
     while (true)
     {
-      m_wakeup.Wait();
-
-      while (true)
+      std::unique_lock lg(m_lock);
+      while (m_items.empty())
       {
-        std::unique_lock lg(m_lock);
-        if (m_items.empty())
-          break;
-        T item{std::move(m_items.front())};
-        m_items.pop();
-        lg.unlock();
+        m_idle = true;
+        m_cancelling = false;
+        m_wait_cond_var.notify_all();
+        if (m_shutdown)
+          return;
 
-        m_function(std::move(item));
+        m_worker_cond_var.wait(
+            lg, [&] { return !m_items.empty() || m_shutdown || m_cancelling.load(); });
       }
+      T item{std::move(m_items.front())};
+      m_items.pop();
+      lg.unlock();
 
-      if (m_shutdown.IsSet())
-        break;
+      m_function(std::move(item));
     }
   }
 
   std::function<void(T)> m_function;
+  std::string m_thread_name;
   std::thread m_thread;
-  Common::Event m_wakeup;
-  Common::Flag m_shutdown;
-  Common::Flag m_cancelled;
   std::mutex m_lock;
   std::queue<T> m_items;
+  std::condition_variable m_wait_cond_var;
+  std::condition_variable m_worker_cond_var;
+  std::atomic<bool> m_cancelling = false;
+  bool m_idle = true;
+  bool m_shutdown = false;
 };
 
 }  // namespace Common