hold worker busy state in renderbackend rather than in renderworker

Workers run in different threads and the backends can poll whether the worker
is currently busy or not. However the previous iteration has the worker (and an
atomic int) provide the busy state which could easily desync with the main
thread (since all workers run in different threads). By holding the busy states
in the main thread, the main thread will always be able to poll the busy state
accurately.

app/render/backend/opengl/openglbackend.cpp

@@ -6,7 +6,8 @@
 #include "functions.h"
 
 OpenGLBackend::OpenGLBackend(QObject *parent) :
-  VideoRenderBackend(parent)
+  VideoRenderBackend(parent),
+  last_download_thread_(0)
 {
 }
 
@@ -135,6 +136,8 @@ bool OpenGLBackend::TimeIsCached(const TimeRange &time)
 
 void OpenGLBackend::ThreadCompletedFrame(NodeDependency path, QByteArray hash, NodeValueTable table)
 {
+  SetWorkerBusyState(static_cast<RenderWorker*>(sender()), false);
+
   QVariant value = table.Get(NodeParam::kTexture);
   OpenGLTexturePtr texture = value.value<OpenGLTexturePtr>();
 
@@ -146,19 +149,14 @@ void OpenGLBackend::ThreadCompletedFrame(NodeDependency path, QByteArray hash, N
     QString cache_fn = frame_cache()->CachePathName(hash);
 
     // Find an available worker to download this texture
-    foreach (RenderWorker* worker, processors_) {
-      // Check if one is available, but worst case if none of them are available, just queue it on the last worker since
-      // it's the least likely to get work
-      if (worker->IsAvailable() || worker == processors_.last()) {
-        QMetaObject::invokeMethod(worker,
-                                  "Download",
-                                  Q_ARG(NodeDependency, path),
-                                  Q_ARG(QByteArray, hash),
-                                  Q_ARG(QVariant, QVariant::fromValue(texture)),
-                                  Q_ARG(QString, cache_fn));
-        break;
-      }
-    }
+    QMetaObject::invokeMethod(processors_.at(last_download_thread_%processors_.size()),
+                              "Download",
+                              Q_ARG(NodeDependency, path),
+                              Q_ARG(QByteArray, hash),
+                              Q_ARG(QVariant, QVariant::fromValue(texture)),
+                              Q_ARG(QString, cache_fn));
+
+    last_download_thread_++;
   }
 
   // Set as push texture
@@ -184,6 +182,7 @@ void OpenGLBackend::ThreadCompletedDownload(NodeDependency dep, QByteArray hash)
 void OpenGLBackend::ThreadSkippedFrame(NodeDependency dep, QByteArray hash)
 {
   frame_cache()->SetHash(dep.in(), hash);
+  SetWorkerBusyState(static_cast<RenderWorker*>(sender()), false);
 
   // Queue up a new frame for this worker
   CacheNext();
@@ -192,6 +191,7 @@ void OpenGLBackend::ThreadSkippedFrame(NodeDependency dep, QByteArray hash)
 void OpenGLBackend::ThreadHashAlreadyExists(NodeDependency dep, QByteArray hash)
 {
   ThreadCompletedDownload(dep, hash);
+  SetWorkerBusyState(static_cast<RenderWorker*>(sender()), false);
 
   // Queue up a new frame for this worker
   CacheNext();