Fix exception handling when pybind11::weakref() fails. (#3739)

* Clear Python error state if pybind11::weakref() fails.

The weakref() constructor calls pybind11_fail() without clearing any
Python interpreter error state. If a client catches the C++ exception
thrown by pybind11_fail(), the Python interpreter will be left in an
error state.

* Add test case for failing to create weakref

* Add Debug asserts for pybind11 fail

* Make error handling more pythonic

* Does this fix PyPy?

* Adapt test to PyPy differences

* Simplify test to remove redundancy

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

include/pybind11/detail/common.h

@@ -936,9 +936,11 @@ PYBIND11_RUNTIME_EXCEPTION(cast_error, PyExc_RuntimeError) /// Thrown when pybin
 PYBIND11_RUNTIME_EXCEPTION(reference_cast_error, PyExc_RuntimeError) /// Used internally
 
 [[noreturn]] PYBIND11_NOINLINE void pybind11_fail(const char *reason) {
+    assert(!PyErr_Occurred());
     throw std::runtime_error(reason);
 }
 [[noreturn]] PYBIND11_NOINLINE void pybind11_fail(const std::string &reason) {
+    assert(!PyErr_Occurred());
     throw std::runtime_error(reason);
 }
 

include/pybind11/pytypes.h

@@ -1509,6 +1509,9 @@ public:
     explicit weakref(handle obj, handle callback = {})
         : object(PyWeakref_NewRef(obj.ptr(), callback.ptr()), stolen_t{}) {
         if (!m_ptr) {
+            if (PyErr_Occurred()) {
+                throw error_already_set();
+            }
             pybind11_fail("Could not allocate weak reference!");
         }
     }