Test case for #5626 (#8204)

tests/threads/t5626.nim

@@ -0,0 +1,27 @@
+import threadpool
+
+var ch: Channel[int]
+ch.open
+var pch = ch.addr
+
+proc run(f: proc(): int {.gcsafe.}): proc() =
+  let r = spawn f()
+  return proc() = await(r)
+
+var working = false
+
+proc handler(): int =
+  while true:
+    let (h, v) = pch[].tryRecv()
+    if not h:
+      discard cas(working.addr, true, false)
+      break
+  1
+
+proc send(x: int) =
+  ch.send(x)
+  if cas(working.addr, false, true):
+    discard run(handler)
+
+for x in 0..1000000:
+  send(x)