Test disabling of tracing returning None from trace function

tests/run/line_trace.pyx

@@ -49,7 +49,7 @@ cdef int trace_trampoline(PyObject* _traceobj, PyFrameObject* _frame, int what,
 
     if result is None:
         PyEval_SetTrace(NULL, None)
-        return -1
+        return 0
     else:
         return 0
 
@@ -87,6 +87,16 @@ def _create__failing_line_trace_func(trace):
         return _trace_func
     return _trace_func
 
+def _create_disable_tracing(trace):
+    func = _create_trace_func(trace)
+    def _trace_func(frame, event, arg):
+        if frame.f_lineno - frame.f_code.co_firstlineno == 2:
+            return None
+
+        func(frame, event, arg)
+        return _trace_func
+
+    return _trace_func
 
 def cy_add(a,b):
     x = a + b     # 1
@@ -198,3 +208,24 @@ def fail_on_line_trace(fail_func):
     else:
         assert x == 5
     return trace
+
+
+def disable_trace(func, *args):
+    """
+    >>> def py_add(a,b):
+    ...     x = a+b
+    ...     return x
+    >>> disable_trace(py_add, 1, 2)
+    [('call', 0), ('line', 1)]
+    >>> disable_trace(cy_add, 1, 2)
+    [('call', 0), ('line', 1)]
+    >>> disable_trace(cy_add_with_nogil, 1, 2)
+    [('call', 0), ('line', 1)]
+    """
+    trace = []
+    PyEval_SetTrace(<Py_tracefunc>trace_trampoline, _create_disable_tracing(trace))
+    try:
+        func(*args)
+    finally:
+        PyEval_SetTrace(NULL, None)
+    return trace