Mitigate race condition (#11)

* Mitigate work complete race condition

* Rearrange race mitigation to have no penalty for regular outcomes

If the job response arrives not too early after submission we wont have any delay penalty

* Use const for work handle delay

client/client.go

@@ -24,6 +24,10 @@ var (
 	NullBytes                    = []byte{Null}
 )
 
+const (
+	WORK_HANDLE_DELAY_MS = 5 // milliseconds delay for re-try processing of work completion requests if handler hasn't been yet stored in hash map.
+)
+
 type connection struct {
 	// This simple wrapper struct is just a convenient way to deal with the
 	// fact that accessing the underlying pointer for an interface value type
@@ -426,14 +430,21 @@ func (client *Client) process(resp *Response) {
 	case rt.PT_WorkData, rt.PT_WorkWarning, rt.PT_WorkStatus:
 		// These alternate conditions should not happen so long as
 		// everyone is following the specification.
-		if handler, ok := client.handlers.Load(resp.Handle); ok {
+		var handler interface{}
+		var ok bool
+		if handler, ok = client.handlers.Load(resp.Handle); !ok {
+			// possibly the response arrived faster than the job handler was added to client.handlers, we'll wait a bit and give it another try
+			time.Sleep(WORK_HANDLE_DELAY_MS * time.Millisecond)
+			if handler, ok = client.handlers.Load(resp.Handle); !ok {
+				client.err(errors.New(fmt.Sprintf("unexpected %s response for \"%s\" with no handler", resp.DataType, resp.Handle)))
+			}
+		}
+		if ok {
 			if h, ok := handler.(ResponseHandler); ok {
 				h(resp)
 			} else {
 				client.err(errors.New(fmt.Sprintf("Could not cast handler to ResponseHandler for %v", resp.Handle)))
 			}
-		} else {
-			client.err(errors.New(fmt.Sprintf("unexpected %s response for \"%s\" with no handler", resp.DataType, resp.Handle)))
 		}
 
 		client.responsePool.Put(resp)