Observe: Clarify observing of Unknown or Proxy URIs is not supported

man/coap_observe.txt.in

@@ -99,11 +99,13 @@ observable status of the _resource_ by the setting of _mode_.  If _mode_ is
 _resource_ is no longer observable.
 
 *NOTE:* It is not possible for the Unknown Resource, created by
-*coap_resource_unknown_init*(3), to be observable as the Uri-Path is not known
+*coap_resource_unknown_init2*(3), to be observable as the Uri-Path is not known
 when libcoap creates a "fake GET/FETCH request".  The Unknown Resource PUT
 handler must create a new resource and mark the resource as "observable" if
 a specific resource needs to be observable.  The application must then
-manage the deletion of the resource at the appropriate time.
+manage the deletion of the resource at the appropriate time. Similarily,
+it is not possible to observe the resource set up by
+*coap_resource_proxy_uri_init2*() or *coap_resource_reverse_proxy_init*().
 
 *NOTE:* The type (confirmable or non-confirmable) of the triggered observe
 GET response is determined not by the initial GET/FETCH request, but

src/coap_resource.c

@@ -1379,7 +1379,13 @@ coap_resource_release_userdata_handler(coap_context_t *context,
 
 void
 coap_resource_set_get_observable(coap_resource_t *resource, int mode) {
-  resource->observable = mode ? 1 : 0;
+  if (resource->is_unknown || resource->is_proxy_uri) {
+    /* We cannot observe these */
+    coap_log_debug("coap_resource_set_get_observable: Not supported for Unknown or Proxy URIs\n");
+    resource->observable = 0;
+  } else {
+    resource->observable = mode ? 1 : 0;
+  }
 }
 
 coap_str_const_t *