Use addCallback rather than callback= in asyncfutures.all() (#6850)

* Use addCallback rather than callback= in asyncfutures.all()

Addresses part of #6849

* Stop using do notation for #6849

* Update example style

lib/pure/asyncdispatch.nim

@@ -59,9 +59,10 @@ export asyncfutures, asyncstreams
 ##
 ##   .. code-block::nim
 ##      var future = socket.recv(100)
-##      future.callback =
+##      future.addCallback(
 ##        proc () =
 ##          echo(future.read)
+##      )
 ##
 ## All asynchronous functions returning a ``Future`` will not block. They
 ## will not however return immediately. An asynchronous function will have

lib/pure/asyncfutures.nim

@@ -333,7 +333,7 @@ proc all*[T](futs: varargs[Future[T]]): auto =
     let totalFutures = len(futs)
 
     for fut in futs:
-      fut.callback = proc(f: Future[T]) =
+      fut.addCallback proc (f: Future[T]) =
         inc(completedFutures)
         if not retFuture.finished:
           if f.failed:
@@ -355,7 +355,7 @@ proc all*[T](futs: varargs[Future[T]]): auto =
 
     for i, fut in futs:
       proc setCallback(i: int) =
-        fut.callback = proc(f: Future[T]) =
+        fut.addCallback proc (f: Future[T]) =
           inc(completedFutures)
           if not retFuture.finished:
             if f.failed:

tests/async/tasyncall.nim

@@ -40,6 +40,16 @@ proc testVarargs(x, y, z: int): seq[int] =
 
   result = waitFor all(a, b, c)
 
+proc testWithDupes() =
+  var
+    tasks = newSeq[Future[void]](taskCount)
+    fut = futureWithoutValue()
+
+  for i in 0..<taskCount:
+    tasks[i] = fut
+
+  waitFor all(tasks)
+
 block:
     let
       startTime = cpuTime()
@@ -57,6 +67,13 @@ block:
 
     doAssert execTime * 1000 < taskCount * sleepDuration
 
+block:
+    let startTime = cpuTime()
+    testWithDupes()
+    let execTime = cpuTime() - startTime
+
+    doAssert execTime * 1000 < taskCount * sleepDuration
+
 block:
     let
       startTime = cpuTime()