From bca5559c6afaa1d3c3d53a5f27fc17ff430cf69e Mon Sep 17 00:00:00 2001
From: Eugene Kabanov <eugene.kabanov@status.im>
Date: Fri, 27 Nov 2020 00:50:55 +0200
Subject: [PATCH] Race() call (#142)

* Add `race` procedure call which extends `one` with FutureBase.

* Fix race() and add test procedures.
---
 chronos/asyncfutures2.nim |  46 +++++++++
 tests/testfut.nim         | 197 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 243 insertions(+)

diff --git a/chronos/asyncfutures2.nim b/chronos/asyncfutures2.nim
index 231d4973..56613a65 100644
--- a/chronos/asyncfutures2.nim
+++ b/chronos/asyncfutures2.nim
@@ -932,3 +932,49 @@ proc one*[T](futs: varargs[Future[T]]): Future[Future[T]] =
 
   retFuture.cancelCallback = cancellation
   return retFuture
+
+proc race*(futs: varargs[FutureBase]): Future[FutureBase] =
+  ## Returns a future which will complete and return completed FutureBase,
+  ## when one of the futures in ``futs`` will be completed, failed or canceled.
+  ##
+  ## If the argument is empty, the returned future FAILS immediately.
+  ##
+  ## On success returned Future will hold finished FutureBase.
+  ##
+  ## On cancel futures in ``futs`` WILL NOT BE cancelled.
+  var retFuture = newFuture[FutureBase]("chronos.race()")
+
+  # Because we can't capture varargs[T] in closures we need to create copy.
+  var nfuts = @futs
+
+  proc cb(udata: pointer) {.gcsafe.} =
+    if not(retFuture.finished()):
+      var res: FutureBase
+      var rfut = cast[FutureBase](udata)
+      for i in 0..<len(nfuts):
+        if nfuts[i] != rfut:
+          nfuts[i].removeCallback(cb)
+        else:
+          res = nfuts[i]
+      retFuture.complete(res)
+
+  proc cancellation(udata: pointer) {.gcsafe.} =
+    # On cancel we remove all our callbacks only.
+    for i in 0..<len(nfuts):
+      if not(nfuts[i].finished()):
+        nfuts[i].removeCallback(cb)
+
+  # If one of the Future[T] already finished we return it as result
+  for fut in nfuts:
+    if fut.finished():
+      retFuture.complete(fut)
+      return retFuture
+
+  for fut in nfuts:
+    fut.addCallback(cb, cast[pointer](fut))
+
+  if len(nfuts) == 0:
+    retFuture.fail(newException(ValueError, "Empty Future[T] list"))
+
+  retFuture.cancelCallback = cancellation
+  return retFuture
diff --git a/tests/testfut.nim b/tests/testfut.nim
index a96e10cc..2d903d79 100644
--- a/tests/testfut.nim
+++ b/tests/testfut.nim
@@ -1113,6 +1113,192 @@ suite "Future[T] behavior test suite":
 
     waitFor testWait()
 
+  proc testRaceZero(): bool =
+    var tseq = newSeq[FutureBase]()
+    var fut1 = race(tseq)
+    var fut2 = race()
+    var fut3 = race([])
+    fut1.failed() and fut2.failed() and fut3.failed()
+
+  proc testRaceVarargs(): bool =
+    proc vlient1() {.async.} =
+      await sleepAsync(100.milliseconds)
+
+    proc vlient2() {.async.} =
+      await sleepAsync(200.milliseconds)
+
+    proc vlient3() {.async.} =
+      await sleepAsync(300.milliseconds)
+
+    proc ilient1(): Future[int] {.async.} =
+      await sleepAsync(100.milliseconds)
+      result = 10
+
+    proc ilient2(): Future[int] {.async.} =
+      await sleepAsync(200.milliseconds)
+      result = 20
+
+    proc ilient3(): Future[int] {.async.} =
+      await sleepAsync(300.milliseconds)
+      result = 30
+
+    proc slient1(): Future[string] {.async.} =
+      await sleepAsync(100.milliseconds)
+      result = "sclient1"
+
+    proc slient2(): Future[string] {.async.} =
+      await sleepAsync(200.milliseconds)
+      result = "sclient2"
+
+    proc slient3(): Future[string] {.async.} =
+      await sleepAsync(300.milliseconds)
+      result = "sclient3"
+
+    var fut11 = vlient1()
+    var fut12 = ilient2()
+    var fut13 = slient3()
+    var res1 = waitFor(race(fut11, fut12, fut13))
+
+    var fut21 = vlient2()
+    var fut22 = ilient1()
+    var fut23 = slient3()
+    var res2 = waitFor(race(fut21, fut22, fut23))
+
+    var fut31 = vlient3()
+    var fut32 = ilient2()
+    var fut33 = slient1()
+    var res3 = waitFor(race(fut31, fut32, fut33))
+
+    var fut41 = vlient1()
+    var fut42 = slient2()
+    var fut43 = ilient3()
+    var res4 = waitFor(race(fut41, fut42, fut43))
+
+    if (FutureBase(fut11) != res1) or
+       (FutureBase(fut22) != res2) or
+       (FutureBase(fut33) != res3) or
+       (FutureBase(fut41) != res4):
+      return false
+
+    result = true
+
+  proc testRaceSeq(): bool =
+    proc vlient1() {.async.} =
+      await sleepAsync(100.milliseconds)
+
+    proc vlient2() {.async.} =
+      await sleepAsync(200.milliseconds)
+
+    proc vlient3() {.async.} =
+      await sleepAsync(300.milliseconds)
+
+    proc ilient1(): Future[int] {.async.} =
+      await sleepAsync(100.milliseconds)
+      result = 10
+
+    proc ilient2(): Future[int] {.async.} =
+      await sleepAsync(200.milliseconds)
+      result = 20
+
+    proc ilient3(): Future[int] {.async.} =
+      await sleepAsync(300.milliseconds)
+      result = 30
+
+    proc slient1(): Future[string] {.async.} =
+      await sleepAsync(100.milliseconds)
+      result = "slient1"
+
+    proc slient2(): Future[string] {.async.} =
+      await sleepAsync(200.milliseconds)
+      result = "slient2"
+
+    proc slient3(): Future[string] {.async.} =
+      await sleepAsync(300.milliseconds)
+      result = "slient3"
+
+    var v10 = vlient1()
+    var v11 = ilient2()
+    var v12 = slient3()
+    var res1 = waitFor(race(@[FutureBase(v10), FutureBase(v11),
+                              FutureBase(v12)]))
+
+    var v20 = vlient2()
+    var v21 = ilient1()
+    var v22 = slient3()
+    var res2 = waitFor(race(@[FutureBase(v20), FutureBase(v21),
+                              FutureBase(v22)]))
+
+    var v30 = vlient3()
+    var v31 = ilient2()
+    var v32 = slient1()
+    var res3 = waitFor(race(@[FutureBase(v30), FutureBase(v31),
+                              FutureBase(v32)]))
+
+    var v40 = vlient1()
+    var v41 = slient2()
+    var v42 = ilient3()
+    var res4 = waitFor(race(@[FutureBase(v40), FutureBase(v41),
+                              FutureBase(v42)]))
+
+    if res1 != FutureBase(v10) or
+       res2 != FutureBase(v21) or
+       res3 != FutureBase(v32) or
+       res4 != FutureBase(v40):
+      return false
+
+    result = true
+
+  proc testRaceCompleted(): bool =
+    proc client1(): Future[int] {.async.} =
+      result = 1
+
+    proc client2() {.async.} =
+      if true:
+        raise newException(ValueError, "")
+
+    proc client3(): Future[string] {.async.} =
+      await sleepAsync(100.milliseconds)
+      result = "client3"
+
+    var f10 = client1()
+    var f20 = client2()
+    var f30 = client3()
+    var fut1 = race(f30, f10, f20)
+    var f11 = client1()
+    var f21 = client2()
+    var f31 = client3()
+    var fut2 = race(f31, f21, f11)
+
+    result = (fut1.done() and fut1.read() == FutureBase(f10)) and
+             (fut2.done() and fut2.read() == FutureBase(f21))
+
+  proc testRaceCancelled(): bool =
+    proc client1() {.async.} =
+      await sleepAsync(100.milliseconds)
+
+    proc client2(): Future[int] {.async.} =
+      await sleepAsync(200.milliseconds)
+      return 10
+
+    proc client3(): Future[string] {.async.} =
+      await sleepAsync(300.milliseconds)
+      return "client3"
+
+    var f1 = client1()
+    var f2 = client2()
+    var f3 = client3()
+    var fut = race(f1, f2, f3)
+    waitFor(cancelAndWait(fut))
+
+    if f1.finished() or f2.finished() or f3.finished():
+      return false
+
+    waitFor(sleepAsync(400.milliseconds))
+    if not(f1.finished()) or not(f2.finished()) or not(f3.finished()):
+      return false
+
+    return true
+
   test "Async undefined behavior (#7758) test":
     check test1() == true
   test "Immediately completed asynchronous procedure test":
@@ -1152,6 +1338,17 @@ suite "Future[T] behavior test suite":
   test "or() already completed test":
     check testOrCompleted() == true
 
+  test "race(zero) test":
+    check testRaceZero() == true
+  test "race(varargs) test":
+    check testRaceVarargs() == true
+  test "race(seq) test":
+    check testRaceSeq() == true
+  test "race() already completed test":
+    check testRaceCompleted() == true
+  test "race() cancellation test":
+    check testRaceCancelled() == true
+
   test "cancel() async procedure test":
     check testCancelIter() == true
   test "cancelAndWait() test":