Python: fixing incorrect function object dereference.

The __call__ method can be native and not be a PyFunction type.  A type check
is thus required before accessing op_code and other fields.

Reproduced on Ubuntu 21.04, Python 3.9.4 and Falcon framework: here, the
App.__call__ method is compiled with Cython, so accessing op_code->co_flags is
invalid; accidentally, the COROUTINE bit is set which forces the Python module
into the ASGI mode.

The workaround is explicit protocol specification.

Note: it is impossible to specify the legacy mode for ASGI.

docs/changes.xml

@@ -43,6 +43,13 @@ some Spring Boot applications failed to start, notably with Grails.
 </para>
 </change>
 
+<change type="bugfix">
+<para>
+incorrect Python protocol auto detection (ASGI or WSGI) for native callable
+object, notably with Falcon.
+</para>
+</change>
+
 </changes>
 
 

src/python/nxt_python_asgi.c

@@ -117,15 +117,20 @@ nxt_python_asgi_get_func(PyObject *obj)
     if (PyMethod_Check(call)) {
         obj = PyMethod_GET_FUNCTION(call);
 
-        Py_INCREF(obj);
+        if (PyFunction_Check(obj)) {
-        Py_DECREF(call);
+            Py_INCREF(obj);
 
-        return obj;
+        } else {
+            obj = NULL;
+        }
+
+    } else {
+        obj = NULL;
     }
 
     Py_DECREF(call);
 
-    return NULL;
+    return obj;
 }
 
 
@@ -161,8 +166,9 @@ nxt_python_asgi_init(nxt_unit_init_t *init, nxt_python_proto_t *proto)
     for (i = 0; i < nxt_py_targets->count; i++) {
         func = nxt_python_asgi_get_func(nxt_py_targets->target[i].application);
         if (nxt_slow_path(func == NULL)) {
-            nxt_unit_alert(NULL, "Python cannot find function for callable");
+            nxt_unit_debug(NULL, "asgi: cannot find function for callable, "
-            return NXT_UNIT_ERROR;
+                                 "unable to check for legacy mode (#%d)", i);
+            continue;
         }
 
         code = (PyCodeObject *) PyFunction_GET_CODE(func);