fixes #15076 (#16143)

* fixes #15076

* heapqueue: optimized for ARC

* added another test case [backport:1.4]

* code cleanup

tests/arc/tasyncleak4.nim

@@ -0,0 +1,21 @@
+discard """
+  cmd: "nim c --gc:orc -d:useMalloc $file"
+  output: '''ok'''
+  valgrind: "leaks"
+"""
+
+# bug #15076
+import asyncdispatch
+
+var futures: seq[Future[void]]
+
+for i in 1..20:
+  futures.add sleepAsync 1
+  futures.add sleepAsync 1
+
+  futures.all.waitFor()
+  futures.setLen 0
+
+setGlobalDispatcher nil
+GC_fullCollect()
+echo "ok"

tests/arc/tdestroy_in_loopcond.nim

@@ -0,0 +1,74 @@
+discard """
+  output: '''400 true'''
+  cmd: "nim c --gc:orc $file"
+"""
+
+type HeapQueue*[T] = object
+  data: seq[T]
+
+
+proc len*[T](heap: HeapQueue[T]): int {.inline.} =
+  heap.data.len
+
+proc `[]`*[T](heap: HeapQueue[T], i: Natural): T {.inline.} =
+  heap.data[i]
+
+proc push*[T](heap: var HeapQueue[T], item: T) =
+  heap.data.add(item)
+
+proc pop*[T](heap: var HeapQueue[T]): T =
+  result = heap.data.pop
+
+proc clear*[T](heap: var HeapQueue[T]) = heap.data.setLen 0
+
+
+type
+  Future = ref object of RootObj
+    s: string
+    callme: proc()
+
+var called = 0
+
+proc consume(f: Future) =
+  inc called
+
+proc newFuture(s: string): Future =
+  var r: Future
+  r = Future(s: s, callme: proc() =
+    consume r)
+  result = r
+
+var q: HeapQueue[tuple[finishAt: int64, fut: Future]]
+
+proc sleep(f: int64): Future =
+  q.push (finishAt: f, fut: newFuture("async-sleep"))
+
+proc processTimers =
+  # Pop the timers in the order in which they will expire (smaller `finishAt`).
+  var count = q.len
+  let t = high(int64)
+  while count > 0 and t >= q[0].finishAt:
+    q.pop().fut.callme()
+    dec count
+
+var futures: seq[Future]
+
+proc main =
+  for i in 1..200:
+    futures.add sleep(56020904056300)
+    futures.add sleep(56020804337500)
+    #futures.add sleep(2.0)
+
+    #futures.add sleep(4.0)
+
+    processTimers()
+    #q.pop()[1].callme()
+    #q.pop()[1].callme()
+
+    futures.setLen 0
+
+  q.clear()
+
+main()
+GC_fullCollect()
+echo called, " ", getOccupiedMem() < 160