fixes #11275 (#11276)

tests/parallel/tconvexhull.nim

@@ -52,6 +52,8 @@ proc convex_hull[T](points: var seq[T], cmp: proc(x, y: T): int {.closure.}) : s
   result = concat(^ul[0], ^ul[1])
 
 var s = map(toSeq(0..99999), proc(x: int): Point = (float(x div 1000), float(x mod 1000)))
+# On some runs, this pool size reduction will set the "shutdown" attribute on the
+# worker thread that executes our spawned task, before we can read the flowvars.
 setMaxPoolSize 2
 
 #echo convex_hull[Point](s, cmpPoint)