WorkQueueThread: rework Cancel/Shutdown workflow

- Cancel doesn't shut down anymore.
   Allowing it to be used multiple times thoughout the life of
   the WorkQueue
 - Remove Clear, so we only have Cancel semantics
 - Add IsCancelling so work items can abort early if cancelling
 - Replace m_cancelled and m_thread.joinable() guars with m_shutdown.
 - Rename Flush to WaitForCompletion (As it's ambiguous if a function
   called flush should be blocking or not)
 - Add documentation

Source/Core/Common/WorkQueueThread.h

@@ -3,13 +3,13 @@
 
 #pragma once
 
+#include <atomic>
+#include <condition_variable>
 #include <functional>
 #include <queue>
 #include <string>
 #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.
@@ -26,120 +26,104 @@ public:
     Reset(std::move(function));
   }
   ~WorkQueueThread() { Shutdown(); }
+
+  // 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(std::function<void(T)> function)
   {
     Shutdown();
     std::lock_guard lg(m_lock);
-    m_cancelled = false;
+    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)
   {
     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();
   }
 
+  // Adds an item to the work queue
   void Push(T&& item)
   {
     std::lock_guard lg(m_lock);
-    if (m_cancelled)
-    {
+    if (m_shutdown)
       return;
-    }
+
     m_items.push(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_cancelled)
-      {
-        return;
-      }
-      m_items.push(item);
-      m_idle = false;
-      m_worker_cond_var.notify_one();
+    std::lock_guard lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_items.push(item);
+    m_idle = false;
+    m_worker_cond_var.notify_one();
   }
 
-  void Clear()
+  // Empties the queue
+  // If the worker polls IsCanceling(), it can abort it's work when Cancelling
+  void Cancel()
   {
-    std::lock_guard lg(m_lock);
+    std::unique_lock lg(m_lock);
+    if (m_shutdown)
+      return;
+
+    m_cancelling = true;
     m_items = std::queue<T>();
     m_worker_cond_var.notify_one();
   }
 
-  void Cancel()
+  // 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
+  void Shutdown(bool cancel = false)
   {
-    if (!m_thread.joinable())
-    {
-      return;
-    }
-
     {
       std::unique_lock lg(m_lock);
-      m_items = std::queue<T>();
-      m_cancelled = true;
+      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();
   }
 
-  void Shutdown()
+  // Blocks until all items in the queue have been processed (or cancelled)
+  void WaitForCompletion()
   {
-    if (!m_thread.joinable())
-    {
-      return;
-    }
-
-    {
-      std::unique_lock lg(m_lock);
-      m_shutdown = true;
-      m_worker_cond_var.notify_one();
-    }
-    m_thread.join();
-  }
-
-  // Doesn't return until the most recent function invocation has finished.
-  void ClearAndFlush()
-  {
-    if (!m_thread.joinable())
-    {
-      return;
-    }
-
     std::unique_lock lg(m_lock);
-    m_items = std::queue<T>();
-    m_wait_cond_var.wait(lg, [&] {
-        return m_idle;
-    });
-  }
-
-  // Doesn't return until the most recent function invocation has finished.
-  void Flush()
-  {
-    if (!m_thread.joinable())
-    {
+    if (m_idle)  // Only check m_idle, we want this to work even another thread called Shutdown
       return;
-    }
 
-    std::unique_lock lg(m_lock);
-    m_wait_cond_var.wait(lg, [&] {
-        return m_idle;
-    });
+    m_wait_cond_var.wait(lg, [&] { return m_idle; });
   }
 
+  // For the worker to check if it should abort it's work early.
+  bool IsCancelling() const { return m_cancelling.load(); }
+
 private:
   void ThreadLoop()
   {
@@ -151,14 +135,13 @@ private:
       if (m_items.empty())
       {
         m_idle = true;
+        m_cancelling = false;
         m_wait_cond_var.notify_all();
-        m_worker_cond_var.wait(lg, [&] {
-            return m_shutdown || !m_items.empty();
-        });
+        m_worker_cond_var.wait(
+            lg, [&] { return m_shutdown || m_cancelling.load() || !m_items.empty(); });
+
         if (m_shutdown)
-        {
           break;
-        }
         continue;
       }
       T item{std::move(m_items.front())};
@@ -176,9 +159,9 @@ private:
   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;
-  bool m_cancelled = false;
 };
 
 }  // namespace Common

Source/Core/DolphinQt/GameList/GameTracker.cpp

@@ -37,7 +37,7 @@ GameTracker::GameTracker(QObject* parent) : QFileSystemWatcher(parent)
 
   connect(qApp, &QApplication::aboutToQuit, this, [this] {
     m_processing_halted = true;
-    m_load_thread.Cancel();
+    m_load_thread.Shutdown(true);
   });
   connect(this, &QFileSystemWatcher::directoryChanged, this, &GameTracker::UpdateDirectory);
   connect(this, &QFileSystemWatcher::fileChanged, this, &GameTracker::UpdateFile);
@@ -203,7 +203,7 @@ void GameTracker::RemoveDirectory(const QString& dir)
 void GameTracker::RefreshAll()
 {
   m_processing_halted = true;
-  m_load_thread.Clear();
+  m_load_thread.Cancel();
   m_load_thread.EmplaceItem(Command{CommandType::ResumeProcessing, {}});
 
   if (m_needs_purge)