Original bug ID: 7286
Reporter: mfp
Status: closed (set by @xavierleroy on 2017-09-24T15:33:06Z)
Resolution: fixed
Priority: normal
Severity: minor
Version: 4.02.3
Fixed in version: 4.03.0
Category: otherlibs
Duplicate of: #7013

Bug description

I've been getting backtraces like the following on a program that uses
Event via Lwt_preemptive.detach:

Thread 1 killed on uncaught exception Invalid_argument("index out of bounds")
Raised by primitive operation at file "event.ml", line 93, characters 4-20
Called from file "src/preemptive/lwt_preemptive.ml", line 79, characters 17-63
Called from file "thread.ml", line 37, characters 8-14

The exception is raised in a Lwt_preemptive worker thread that performs this operation:

let id, task = Event.sync (Event.receive worker.task_channel) in

(BTW. Lwt_preemptive is moving away from Event because of #7158, but this separate
issue stands.)