use mutexes to prevent threads from interfering with each other while sharing shaders

app/render/backend/opengl/openglshader.cpp

@@ -230,3 +230,13 @@ QString OpenGLShader::CodeAlphaAssociate(const QString &function_name)
                         "  return vec4(col.rgb * col.a, col.a);\n"
                         "}\n").arg(function_name);
 }
+
+void OpenGLShader::Lock()
+{
+  lock_.lock();
+}
+
+void OpenGLShader::Unlock()
+{
+  lock_.unlock();
+}

app/render/backend/opengl/openglshader.h

@@ -2,6 +2,7 @@
 #define OPENGLSHADER_H
 
 #include <memory>
+#include <QMutex>
 #include <QOpenGLShaderProgram>
 
 #include <OpenColorIO/OpenColorIO.h>
@@ -32,7 +33,11 @@ public:
   static QString CodeAlphaReassociate(const QString& function_name);
   static QString CodeAlphaAssociate(const QString& function_name);
 
+  void Lock();
+  void Unlock();
+
 private:
+  QMutex lock_;
 };
 
 #endif // OPENGLSHADER_H

app/render/backend/opengl/openglworker.cpp

@@ -157,6 +157,8 @@ void OpenGLWorker::RunNodeAccelerated(const Node *node, const NodeValueDatabase
 
   buffer_.Bind();
 
+  // Lock the shader so no other thread interferes as we set parameters and draw (and we don't interfere with any others)
+  shader->Lock();
   shader->bind();
 
   unsigned int input_texture_count = 0;
@@ -266,6 +268,11 @@ void OpenGLWorker::RunNodeAccelerated(const Node *node, const NodeValueDatabase
   // Blit this texture through this shader
   olive::gl::Blit(shader);
 
+  // Make sure all OpenGL functions are complete by this point before unlocking the shader (or another thread may
+  // change its parameters before our drawing in this thread is done)
+  functions_->glFinish();
+  shader->Unlock();
+
   // Release any textures we bound before
   while (input_texture_count > 0) {
     input_texture_count--;
@@ -281,8 +288,6 @@ void OpenGLWorker::RunNodeAccelerated(const Node *node, const NodeValueDatabase
 
   buffer_.Detach();
 
-  functions_->glFinish();
-
   output_params->Push(NodeParam::kTexture, QVariant::fromValue(output));
 }