Fix hang when movie plugins invoke python

Signed-off-by: Roger Nelson <roger.nelson@autodesk.com>

Author: rogernelson

src/lib/app/RvApp/RvSession.cpp

@@ -6,6 +6,7 @@
 //
 //
 #include <RvApp/CommandsModule.h>
+#include <Python.h>
 #include <RvApp/Options.h>
 #include <RvApp/RvNodeDefinitions.h>
 #include <RvApp/RvSession.h>
@@ -243,6 +244,22 @@ namespace Rv
             //          new Shader::Function -> initGLSLVersion() -> glGetString
             //          -> crashes because no context
             IPCore::Application::instance()->loadOptionNodeDefinitions();
+
+            // Set TwkMovie's blocking callbacks to release (and re-acquire) the Python GIL while it waits for its
+            // plugins to initialize in separate threads. If any of them need Python, they will try to acquire the
+            // GIL and deadlock with the main thread.
+            //
+            thread_local PyThreadState* preloadGilState = nullptr;
+            TwkMovie::GenericIO::setBlockingWaitCallbacks([]() { preloadGilState = PyGILState_Check() ? PyEval_SaveThread() : nullptr; },
+                                                          []()
+                                                          {
+                                                              if (preloadGilState)
+                                                              {
+                                                                  PyEval_RestoreThread(preloadGilState);
+                                                                  preloadGilState = nullptr;
+                                                              }
+                                                          });
+
             firstTime = false;
         }
 

src/lib/image/TwkMovie/MovieIO.cpp

@@ -212,8 +212,14 @@ namespace TwkMovie
         if (reader->isFinishedLoading())
             return;
 
+        if (GenericIO::m_onBlockingWaitRelease)
+            GenericIO::m_onBlockingWaitRelease();
+
         std::unique_lock<std::mutex> lock(m_mainThread_mutex);
         m_mainThread_cv.wait(lock, [&reader] { return reader->isFinishedLoading(); });
+
+        if (GenericIO::m_onBlockingWaitAcquire)
+            GenericIO::m_onBlockingWaitAcquire();
     }
 
     MovieReader* GenericIO::Preloader::getReader(std::string_view filename, const MovieInfo& info, Movie::ReadRequest& request)
@@ -534,6 +540,14 @@ namespace TwkMovie
     bool GenericIO::m_loadedAll = false;
     bool GenericIO::m_dnxhdDecodingAllowed = true;
     GenericIO::Preloader GenericIO::m_preloader;
+    GenericIO::WaitCallback GenericIO::m_onBlockingWaitRelease;
+    GenericIO::WaitCallback GenericIO::m_onBlockingWaitAcquire;
+
+    void GenericIO::setBlockingWaitCallbacks(WaitCallback onRelease, WaitCallback onAcquire)
+    {
+        m_onBlockingWaitRelease = std::move(onRelease);
+        m_onBlockingWaitAcquire = std::move(onAcquire);
+    }
 
     void GenericIO::init()
     {

src/lib/image/TwkMovie/TwkMovie/MovieIO.h

@@ -413,6 +413,21 @@ namespace TwkMovie
 
         static void init();
 
+        ///
+        ///  Optional callbacks invoked in Preloader::waitForFinishedLoading
+        ///  immediately before and after the blocking condition variable wait.
+        ///
+        ///  Applications that hold resouces they shared with workers (e.g. Python's
+        ///  GIL) should set these to release their lock before the wait and reacquire
+        ///  it afterward, so that background threads can avoid deadlocking the main thread
+        ///  when they access the shared resources.
+        ///
+        ///  Both callbacks default to nullptr (no-op).
+        ///
+
+        using WaitCallback = std::function<void()>;
+        static void setBlockingWaitCallbacks(WaitCallback onRelease, WaitCallback onAcquire);
+
         ///
         ///  Shutdown and free generic movieio plugins.
         ///  This needs to be called in the application main thread in main.
@@ -539,6 +554,8 @@ namespace TwkMovie
         static bool m_loadedAll;
         static bool m_dnxhdDecodingAllowed;
         static Preloader m_preloader;
+        static WaitCallback m_onBlockingWaitRelease;
+        static WaitCallback m_onBlockingWaitAcquire;
     };
 
 } // namespace TwkMovie