fixes #1816

lib/system/channels.nim

@@ -233,6 +233,7 @@ proc tryRecv*[TMsg](c: var TChannel[TMsg]): tuple[dataAvailable: bool,
   var q = cast[PRawChannel](addr(c))
   if q.mask != ChannelDeadMask:
     if tryAcquireSys(q.lock):
+      if q.count > 0:
         llRecv(q, addr(result.msg), cast[PNimType](getTypeInfo(result.msg)))
         result.dataAvailable = true
       releaseSys(q.lock)

tests/threads/ttryrecv.nim

@@ -0,0 +1,35 @@
+discard """
+  outputsub: "channel is empty"
+"""
+
+# bug #1816
+
+from math import random
+from os import sleep
+
+type PComm = ptr TChannel[int]
+
+proc doAction(outC: PComm) {.thread.} =
+  for i in 0.. <5:
+    sleep(random(100))
+    send(outC[], i)
+
+var
+  thr: TThread[PComm]
+  chan: TChannel[int]
+
+open(chan)
+createThread[PComm](thr, doAction, addr(chan))
+
+while true:
+  let (flag, x) = tryRecv(chan)
+  if flag:
+    echo("received from chan: " & $x)
+  else:
+    echo "channel is empty"
+    break
+
+echo "Finished listening"
+
+joinThread(thr)                                                       
+close(chan)