tf: Ensure we drop the GIL if we hold it _before_ taking the singleto…

…n creation

lock.  Otherwise if another thread takes the GIL during singleton creation and
then attempts to GetInstance(), we'll deadlock.

Fixes #3087

(Internal change: 2329963)
(Internal change: 2329985)

pxr/base/tf/instantiateSingleton.h

@@ -25,10 +25,25 @@
 #include "pxr/base/tf/mallocTag.h"
 #include "pxr/base/arch/demangle.h"
 
+#include <memory>
 #include <thread>
 
 PXR_NAMESPACE_OPEN_SCOPE
 
+// This GIL-releasing helper is implemented in singleton.cpp.  We do it this way
+// to avoid including the Python headers here.
+struct Tf_SingletonPyGILDropper
+{
+    TF_API
+    Tf_SingletonPyGILDropper();
+    TF_API
+    ~Tf_SingletonPyGILDropper();
+private:
+#ifdef PXR_PYTHON_SUPPORT_ENABLED
+    std::unique_ptr<class TfPyLock> _pyLock;
+#endif // PXR_PYTHON_SUPPORT_ENABLED
+};
+
 template <class T> std::atomic<T *> TfSingleton<T>::_instance;
 
 template <class T>
@@ -51,6 +66,10 @@ TfSingleton<T>::_CreateInstance(std::atomic<T *> &instance)
     TfAutoMallocTag2 tag("Tf", "TfSingleton::_CreateInstance",
                          "Create Singleton " + ArchGetDemangled<T>());
 
+    // Drop the GIL if we have it, before possibly locking to create the
+    // singleton instance.
+    Tf_SingletonPyGILDropper dropGIL;
+
     // Try to take isInitializing false -> true.  If we do it, then check to see
     // if we don't yet have an instance.  If we don't, then we get to create it.
     // Otherwise we just wait until the instance shows up.

pxr/base/tf/singleton.cpp

@@ -4,4 +4,30 @@
 // Licensed under the terms set forth in the LICENSE.txt file available at
 // https://openusd.org/license.
 //
+
+#include "pxr/pxr.h"
 #include "pxr/base/tf/singleton.h"
+#include "pxr/base/tf/instantiateSingleton.h"
+#include "pxr/base/tf/pyLock.h"
+
+#include <memory>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+// This GIL-releasing helper is declared in tf/instantiateSingleton.h.  It
+// exists so that users of instantiateSingleton.h don't need to pull in the
+// Python headers via tf/pyLock.h.
+
+Tf_SingletonPyGILDropper::Tf_SingletonPyGILDropper()
+{
+#ifdef PXR_PYTHON_SUPPORT_ENABLED
+    if (PyGILState_Check()) {
+        _pyLock = std::make_unique<TfPyLock>();
+        _pyLock->BeginAllowThreads();
+    }
+#endif // PXR_PYTHON_SUPPORT_ENABLED
+}
+
+Tf_SingletonPyGILDropper::~Tf_SingletonPyGILDropper() = default;
+
+PXR_NAMESPACE_CLOSE_SCOPE