Fix refcounting and lifetime management around async python callbacks

* We need to keep a PythonContext (and its globals Dict) around while
  we still have some pending callbacks happening. So now the external
  code creates a PythonContext and then releases it when it's done, but
  the context will hang around until the global redirector object is
  destructed, which is responsible for deleting the context.
* The global redirector is deleted when a refcounting cycle is detected
  and the dict is unreachable, which only happens after the context is
  released.
* Any time a callback is passed to something and converted to a
  std::function we add a reference on the global redirector to keep it
  alive. When the callback has finished executing we remove the ref.
* This way, any pending callbacks that have been called but not finished
  or converted (queued) and not called yet asynchronously will keep the
  context object alive to be able to output, handle exceptions, etc.
* Additionally we need to detect when we're being called asynchronously
  and handle exceptions separately instead of trying to propagate up the
  call chain, because there might not be any more python code up the
  chain (e.g. the render manager calling a python callback).

qrenderdoc/Code/pyrenderdoc/PythonContext.h

@@ -41,10 +41,15 @@ class PythonContext : public QObject
 private:
   Q_OBJECT
 
+  // don't allow destruction from outside, you must heap-allocate the context and let it delete
+  // itself when all references are done. This handles the case where e.g. some Async work is going
+  // on and needs to finish executing after the external code is done with the context
+  ~PythonContext() {}
 public:
   explicit PythonContext(QObject *parent = NULL);
-  ~PythonContext();
+  void Finish();
 
+  PyThreadState *GetExecutingThreadState() { return m_State; }
   static void GlobalInit();
   static void GlobalShutdown();
 
@@ -88,6 +93,10 @@ private:
   // globals are set into and any scripts execute in
   PyObject *context_namespace = NULL;
 
+  // this is set during an execute, so we can identify when a callback happens within our execute or
+  // not
+  PyThreadState *m_State = NULL;
+
   struct
   {
     QString file;
@@ -97,6 +106,7 @@ private:
   void setQtGlobal_internal(const char *varName, const char *typeName, QObject *object);
 
   // Python callbacks
+  static void outstream_del(PyObject *self);
   static PyObject *outstream_write(PyObject *self, PyObject *args);
   static PyObject *outstream_flush(PyObject *self, PyObject *args);
   static int traceEvent(PyObject *obj, PyFrameObject *frame, int what, PyObject *arg);
@@ -109,4 +119,20 @@ template <>
 void PythonContext::setGlobal(const char *varName, QObject *object);
 
 template <>
-void PythonContext::setGlobal(const char *varName, QWidget *object);
+void PythonContext::setGlobal(const char *varName, QWidget *object);
+
+// helper struct to handle dynamically allocating then calling Finish()
+
+struct PythonContextHandle
+{
+public:
+  PythonContextHandle() { m_ctx = new PythonContext; }
+  ~PythonContextHandle() { m_ctx->Finish(); }
+  // don't allow copying
+  PythonContextHandle(const PythonContextHandle &) = delete;
+  PythonContextHandle &operator=(const PythonContextHandle &) = delete;
+
+  PythonContext &ctx() { return *m_ctx; }
+private:
+  PythonContext *m_ctx;
+};