fixes a race condition that caused '^' to hang

lib/pure/concurrency/threadpool.nim

@@ -95,7 +95,7 @@ type
 
   FlowVarBase* = ref FlowVarBaseObj ## untyped base class for 'FlowVar[T]'
   FlowVarBaseObj = object of RootObj
-    ready, usesCondVar: bool
+    ready, usesCondVar, awaited: bool
     cv: CondVar #\
     # for 'awaitAny' support
     ai: ptr AwaitInfo
@@ -129,8 +129,8 @@ type
 proc await*(fv: FlowVarBase) =
   ## waits until the value for the flowVar arrives. Usually it is not necessary
   ## to call this explicitly.
-  if fv.usesCondVar:
-    fv.usesCondVar = false
+  if fv.usesCondVar and not fv.awaited:
+    fv.awaited = true
     await(fv.cv)
     destroyCondVar(fv.cv)
 
@@ -185,7 +185,8 @@ proc nimFlowVarSignal(fv: FlowVarBase) {.compilerProc.} =
     inc fv.ai.cv.counter
     release(fv.ai.cv.L)
     signal(fv.ai.cv.c)
-  if fv.usesCondVar: signal(fv.cv)
+  if fv.usesCondVar: 
+    signal(fv.cv)
 
 proc awaitAndThen*[T](fv: FlowVar[T]; action: proc (x: T) {.closure.}) =
   ## blocks until the ``fv`` is available and then passes its value