Merge pull request #762 from Routineco/bugfix/lwt-wakeup-exn-invalid-arg

Fix Invalid_arg exception on stack overflow when waking up a request.

Author: mseri

CHANGES.md

@@ -1,3 +1,8 @@
+## current
+
+- lwt_jsoo: Fix `Lwt.wakeup_exn` `Invalid_arg` exception when a js
+  stack overflow happens in the XHR completion handler (@mefyl #762).
+
 ## v4.0.0 (2021-03-24)
 
 - cohttp.response: fix malformed status header for custom status codes (@mseri @aalekseyev #752)

cohttp-lwt-jsoo/src/cohttp_lwt_jsoo.ml

@@ -231,7 +231,20 @@ module Make_client_async (P : Params) = Make_api (struct
                  * Remove the type constraint on Lwt.task above and return any old
                  * guff here.  It'll compile and crash in the browser! *)
                 Lwt.wakeup wake (response, body)
-              with e -> Lwt.wakeup_exn wake e)
+              with
+              | e
+              (* If we exhaust the stack, it is possible that
+                 Lwt.wakeup just aboves marks the promise as
+                 completed, but raises Stack_overflow while
+                 running the promise callbacks. In this case
+                 waking calling wakeup_exn on the already
+                 completed promise would raise an Invalid_arg
+                 exception, so although the promise is in a
+                 really bad state we may as well let the actual
+                 Stack_overflow exception go through. *)
+              when Lwt.state res = Lwt.Sleep
+              ->
+                Lwt.wakeup_exn wake e)
           | _ -> ());
 
     (* perform call *)