Delay donenotify unlock in _wait2 to avoid a race

Author: kpamnany

base/task.jl

@@ -320,7 +320,6 @@ function _wait2(t::Task, waiter::Task)
         lock(t.donenotify)
         if !istaskdone(t)
             push!(t.donenotify.waitq, waiter)
-            unlock(t.donenotify)
             # since _wait2 is similar to schedule, we should observe the sticky
             # bit, even if we aren't calling `schedule` due to this early-return
             if waiter.sticky && Threads.threadid(waiter) == 0 && !GC.in_finalizer()
@@ -333,6 +332,7 @@ function _wait2(t::Task, waiter::Task)
                 tid = Threads.threadid()
                 ccall(:jl_set_task_tid, Cint, (Any, Cint), waiter, tid-1)
             end
+            unlock(t.donenotify)
             return nothing
         else
             unlock(t.donenotify)