Acquire the GIL in nogil functions to build the traceback

Cython/Compiler/Nodes.py

@@ -1456,7 +1456,16 @@ class FuncDefNode(StatNode, BlockNode):
                 # TODO: Fix exception tracing (though currently unused by cProfile).
                 # code.globalstate.use_utility_code(get_exception_tuple_utility_code)
                 # code.put_trace_exception()
+
+                if lenv.nogil:
+                    code.putln("{")
+                    code.put_ensure_gil()
+
                 code.put_add_traceback(self.entry.qualified_name)
+
+                if lenv.nogil:
+                    code.put_release_ensured_gil()
+                    code.putln("}")
             else:
                 warning(self.entry.pos, "Unraisable exception in function '%s'." \
                             % self.entry.qualified_name, 0)

tests/run/nogil.pyx

@@ -19,3 +19,19 @@ cdef int g(int x) nogil:
         cdef int y
         y = x + 42
         return y
+
+cdef int with_gil_func() except 0 with gil:
+    raise Exception("error!")
+
+cdef int nogil_func() nogil except 0:
+    with_gil_func()
+
+def test_nogil_exception_propagation():
+    """
+    >>> test_nogil_exception_propagation()
+    Traceback (most recent call last):
+       ...
+    Exception: error!
+    """
+    with nogil:
+        nogil_func()