Merge pull request #186 from aws-lumberyard-dev/Atom/mnaumov/ATOM-15296

[ATOM-15296] Fixing deadlock related to thumbnails

Code/Framework/AzToolsFramework/AzToolsFramework/AssetBrowser/Views/EntryDelegate.cpp

@@ -140,7 +140,7 @@ namespace AzToolsFramework
             else
             {
                 QPixmap pixmap = thumbnail->GetPixmap(size);
-                painter->drawPixmap(point.x(), point.y(), size.width(), size.height(), pixmap);
+                painter->drawPixmap(point, pixmap.scaled(size, Qt::IgnoreAspectRatio, Qt::SmoothTransformation));
             }
             return m_iconSize;
         }

Code/Framework/AzToolsFramework/AzToolsFramework/Thumbnails/Thumbnail.cpp

@@ -10,12 +10,14 @@
 *
 */
 
+#include <AzToolsFramework/Thumbnails/ThumbnailerBus.h>
 #include <AzToolsFramework/Thumbnails/Thumbnail.h>
 AZ_PUSH_DISABLE_WARNING(4127 4251 4800 4244, "-Wunknown-warning-option") // 4127: conditional expression is constant
                                                                          // 4251: 'QTextCodec::ConverterState::flags': class 'QFlags<QTextCodec::ConversionFlag>' needs to have dll-interface to be used by clients of struct 'QTextCodec::ConverterState'
                                                                          // 4800: 'QTextBoundaryFinderPrivate *const ': forcing value to bool 'true' or 'false' (performance warning)
                                                                          // 4244: conversion from 'int' to 'qint8', possible loss of data
 #include <QtConcurrent/QtConcurrent>
+#include <QThreadPool>
 AZ_POP_DISABLE_WARNING
 
 namespace AzToolsFramework
@@ -80,7 +82,11 @@ namespace AzToolsFramework
             if (m_state == State::Unloaded)
             {
                 m_state = State::Loading;
-                QFuture<void> future = QtConcurrent::run([this](){ LoadThread(); });
+                QThreadPool* threadPool;
+                ThumbnailContextRequestBus::BroadcastResult(
+                    threadPool,
+                    &ThumbnailContextRequestBus::Handler::GetThreadPool);
+                QFuture<void> future = QtConcurrent::run(threadPool, [this](){ LoadThread(); });
                 m_watcher.setFuture(future);
             }
         }

Code/Framework/AzToolsFramework/AzToolsFramework/Thumbnails/ThumbnailContext.cpp

@@ -28,10 +28,15 @@ namespace AzToolsFramework
             : m_missingThumbnail(new MissingThumbnail(thumbnailSize))
             , m_loadingThumbnail(new LoadingThumbnail(thumbnailSize))
             , m_thumbnailSize(thumbnailSize)
+            , m_threadPool(this)
         {
+            ThumbnailContextRequestBus::Handler::BusConnect();
         }
 
-        ThumbnailContext::~ThumbnailContext() = default;
+        ThumbnailContext::~ThumbnailContext()
+        {
+            ThumbnailContextRequestBus::Handler::BusDisconnect();
+        }
 
         bool ThumbnailContext::IsLoading(SharedThumbnailKey key)
         {
@@ -53,6 +58,11 @@ namespace AzToolsFramework
             AzToolsFramework::AssetBrowser::AssetBrowserViewRequestBus::Broadcast(&AzToolsFramework::AssetBrowser::AssetBrowserViewRequests::Update);
         }
 
+        QThreadPool* ThumbnailContext::GetThreadPool()
+        {
+            return &m_threadPool;
+        }
+
         SharedThumbnail ThumbnailContext::GetThumbnail(SharedThumbnailKey key)
         {
             SharedThumbnail thumbnail;

Code/Framework/AzToolsFramework/AzToolsFramework/Thumbnails/ThumbnailContext.h

@@ -19,6 +19,7 @@
 
 #include <QObject>
 #include <QList>
+#include <QThreadPool>
 #endif
 
 class QString;
@@ -40,6 +41,7 @@ namespace AzToolsFramework
         */
         class ThumbnailContext
             : public QObject
+            , public ThumbnailContextRequestBus::Handler
         {
             Q_OBJECT
         public:
@@ -58,10 +60,13 @@ namespace AzToolsFramework
             void UnregisterThumbnailProvider(const char* providerName);
 
             void RedrawThumbnail();
-            
+
             //! Default context used for most thumbnails
             static constexpr const char* DefaultContext = "Default";
 
+            // ThumbnailContextRequestBus::Handler interface overrides...
+            QThreadPool* GetThreadPool() override;
+
         private:
             struct ProviderCompare {
                 bool operator() (const SharedThumbnailProvider& lhs, const SharedThumbnailProvider& rhs) const
@@ -79,6 +84,9 @@ namespace AzToolsFramework
             SharedThumbnail m_loadingThumbnail;
             //! Thumbnail size (width and height in pixels)
             int m_thumbnailSize;
+            //! There is only a limited number of threads on global threadPool, because there can be many thumbnails rendering at once
+            //! an individual threadPool is needed to avoid deadlocks
+            QThreadPool m_threadPool;
         };
     } // namespace Thumbnailer
 } // namespace AzToolsFramework

Code/Framework/AzToolsFramework/AzToolsFramework/Thumbnails/ThumbnailWidget.cpp

@@ -81,6 +81,9 @@ namespace AzToolsFramework
                 int x = (originalWidth - realWidth) / 2;
                 // pixmap needs to be manually scaled to produce smoother result and avoid looking pixelated
                 // using painter.setRenderHint(QPainter::SmoothPixmapTransform); does not seem to work
+                // Note: there is a potential issue with pixmap.scaled:
+                // it is multithreaded (using global threadPool) and blocking until finished.
+                // A deadlock will happen if global threadPool has no free threads available.
                 painter.drawPixmap(QPoint(x, 0), pixmap.scaled(realWidth, realHeight, Qt::IgnoreAspectRatio, Qt::SmoothTransformation));
             }
             QWidget::paintEvent(event);

Code/Framework/AzToolsFramework/AzToolsFramework/Thumbnails/ThumbnailerBus.h

@@ -17,11 +17,23 @@
 #include <AzToolsFramework/Thumbnails/Thumbnail.h>
 
 class QPixmap;
+class QThreadPool;
 
 namespace AzToolsFramework
 {
     namespace Thumbnailer
     {
+        //! Interaction with thumbnail context
+        class ThumbnailContextRequests
+            : public AZ::EBusTraits
+        {
+        public:
+            //! Get thread pool for drawing thumbnails
+            virtual QThreadPool* GetThreadPool() = 0;
+        };
+
+        using ThumbnailContextRequestBus = AZ::EBus<ThumbnailContextRequests>;
+
         //! Interaction with thumbnailer
         class ThumbnailerRequests
             : public AZ::EBusTraits