From eec2ba3623f0ad74e3f6747c9dcb959d28321edd Mon Sep 17 00:00:00 2001
From: Mads Buvik Sandvei <madssandvei@protonmail.com>
Date: Fri, 18 Dec 2020 18:15:14 +0000
Subject: [PATCH] Loading screen initialdrawcallback 4th time's the charm

---
 apps/openmw/mwgui/loadingscreen.cpp | 34 ++++++++++++++++++++++++-----
 apps/openmw/mwgui/loadingscreen.hpp |  1 +
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/apps/openmw/mwgui/loadingscreen.cpp b/apps/openmw/mwgui/loadingscreen.cpp
index 5f0e9f33a3..7ddc8c5507 100644
--- a/apps/openmw/mwgui/loadingscreen.cpp
+++ b/apps/openmw/mwgui/loadingscreen.cpp
@@ -46,6 +46,7 @@ namespace MWGui
         , mProgress(0)
         , mShowWallpaper(true)
         , mOldCallback(nullptr)
+        , mHasCallback(false)
     {
         mMainWidget->setSize(MyGUI::RenderManager::getInstance().getViewSize());
 
@@ -139,13 +140,19 @@ namespace MWGui
     {
     public:
         CopyFramebufferToTextureCallback(osg::Texture2D* texture)
-            : mTexture(texture)
-            , mOneshot(true)
+            : mOneshot(true)
+            , mTexture(texture)
         {
         }
 
         void operator () (osg::RenderInfo& renderInfo) const override
         {
+            {
+                std::unique_lock<std::mutex> lock(mMutex);
+                mOneshot = false;
+            }
+            mSignal.notify_all();
+
             int w = renderInfo.getCurrentCamera()->getViewport()->width();
             int h = renderInfo.getCurrentCamera()->getViewport()->height();
             mTexture->copyTexImage2D(*renderInfo.getState(), 0, 0, w, h);
@@ -164,6 +171,18 @@ namespace MWGui
                 mSignal.wait(lock);
         }
 
+        void waitUntilInvoked()
+        {
+            std::unique_lock<std::mutex> lock(mMutex);
+            while (mOneshot)
+                mSignal.wait(lock);
+        }
+
+        void reset()
+        {
+            mOneshot = true;
+        }
+
     private:
         mutable bool mOneshot;
         mutable std::mutex mMutex;
@@ -349,6 +368,8 @@ namespace MWGui
         mOldCallback = mViewer->getCamera()->getInitialDrawCallback();
         mViewer->getCamera()->setInitialDrawCallback(mCopyFramebufferToTextureCallback);
 #endif
+        mCopyFramebufferToTextureCallback->reset();
+        mHasCallback = true;
 
         mBackgroundImage->setBackgroundImage("");
         mBackgroundImage->setVisible(false);
@@ -391,16 +412,19 @@ namespace MWGui
         mViewer->renderingTraversals();
         mViewer->advance(mViewer->getFrameStamp()->getSimulationTime());
 
-        if (mCopyFramebufferToTextureCallback)
+        if (mHasCallback)
         {
-            mCopyFramebufferToTextureCallback->wait();
+            mCopyFramebufferToTextureCallback->waitUntilInvoked();
+
+            // Note that we are removing the callback before the draw thread has returned from it.
+            // This is OK as we are retaining the ref_ptr.
 #if OSG_VERSION_GREATER_OR_EQUAL(3, 5, 10)
             mViewer->getCamera()->removeInitialDrawCallback(mCopyFramebufferToTextureCallback);
 #else
             // TODO: Remove once we officially end support for OSG versions pre 3.5.10
             mViewer->getCamera()->setInitialDrawCallback(mOldCallback);
 #endif
-            mCopyFramebufferToTextureCallback = nullptr;
+            mHasCallback = false;
         }
 
         mLastRenderTime = mTimer.time_m();
diff --git a/apps/openmw/mwgui/loadingscreen.hpp b/apps/openmw/mwgui/loadingscreen.hpp
index d58899eae4..5d86ed3896 100644
--- a/apps/openmw/mwgui/loadingscreen.hpp
+++ b/apps/openmw/mwgui/loadingscreen.hpp
@@ -88,6 +88,7 @@ namespace MWGui
         osg::ref_ptr<osg::Texture2D> mTexture;
         osg::ref_ptr<CopyFramebufferToTextureCallback> mCopyFramebufferToTextureCallback;
         osg::ref_ptr<osg::Camera::DrawCallback> mOldCallback;
+        bool mHasCallback;
         std::unique_ptr<MyGUI::ITexture> mGuiTexture;
 
         void changeWallpaper();