ORC/ARC async progress (#15370)

* ARC/ORC: fixes memory leak for empty seqs that have a non-zero capacity

* async: minor refactorings

* it helps to finish a refactoring

compiler/ccgexprs.nim

@@ -2164,7 +2164,7 @@ proc genMove(p: BProc; n: PNode; d: var TLoc) =
     # generated by liftdestructors:
     var src: TLoc
     initLocExpr(p, n[2], src)
-    linefmt(p, cpsStmts, "if ($1.len && $1.p != $2.p) {", [rdLoc(a), rdLoc(src)])
+    linefmt(p, cpsStmts, "if ($1.p != $2.p) {", [rdLoc(a), rdLoc(src)])
     genStmts(p, n[3])
     linefmt(p, cpsStmts, "}$n$1.len = $2.len; $1.p = $2.p;$n", [rdLoc(a), rdLoc(src)])
   else:

lib/pure/asyncdispatch.nim

@@ -1198,6 +1198,11 @@ else:
     let p = getGlobalDispatcher()
     not p.selector.isEmpty() or p.timers.len != 0 or p.callbacks.len != 0
 
+  proc prependSeq(dest: var seq[Callback]; src: sink seq[Callback]) =
+    let old = move dest
+    dest = src
+    dest.add old
+
   proc processBasicCallbacks(
     fd: AsyncFD, event: Event
   ): tuple[readCbListCount, writeCbListCount: int] =
@@ -1246,14 +1251,8 @@ else:
     withData(selector, fd.int, fdData) do:
       # Descriptor is still present in the queue.
       case event
-      of Event.Read:
+      of Event.Read: prependSeq(fdData.readList, newList)
-        let oldReadList = move fdData.readList
+      of Event.Write: prependSeq(fdData.writeList, newList)
-        fdData.readList = move newList
-        fdData.readList.add oldReadList
-      of Event.Write:
-        let oldWriteList = move fdData.writeList
-        fdData.writeList = move newList
-        fdData.writeList.add oldWriteList
       else:
         assert false, "Cannot process callbacks for " & $event
 

lib/pure/ioselects/ioselectors_kqueue.nim

@@ -616,7 +616,7 @@ template withData*[T](s: Selector[T], fd: SocketHandle|int, value,
   let fdi = int(fd)
   s.checkFd(fdi)
   if fdi in s:
-    var value = addr(s.getData(fdi))
+    var value = addr(s.fds[fdi].data)
     body
 
 template withData*[T](s: Selector[T], fd: SocketHandle|int, value, body1,
@@ -625,7 +625,7 @@ template withData*[T](s: Selector[T], fd: SocketHandle|int, value, body1,
   let fdi = int(fd)
   s.checkFd(fdi)
   if fdi in s:
-    var value = addr(s.getData(fdi))
+    var value = addr(s.fds[fdi].data)
     body1
   else:
     body2