Optimize parallel channel request map method (#2057)

* optimize parallel channel request map method

* optimize

* optimze request map function

src/brpc/parallel_channel.cpp

@@ -601,7 +601,7 @@ void ParallelChannel::CallMethod(
     for (int i = 0; i < nchan; ++i) {
         SubChan& sub_chan = _chans[i];
         if (sub_chan.call_mapper != NULL) {
-            aps[i] = sub_chan.call_mapper->Map(i, method, request, response);
+            aps[i] = sub_chan.call_mapper->Map(i, nchan, method, request, response);
             // Test is_skip first because it implies is_bad.
             if (aps[i].is_skip()) {
                 --ndone;

src/brpc/parallel_channel.h

@@ -94,10 +94,23 @@ struct SubCall {
 class CallMapper : public SharedObject {
 public:
     virtual SubCall Map(int channel_index/*starting from 0*/,
+		        int channel_count,
                         const google::protobuf::MethodDescriptor* method,
                         const google::protobuf::Message* request,
-                        google::protobuf::Message* response) = 0;
+                        google::protobuf::Message* response) {
+        return Map(channel_index, method, request, response);    
+    }
+
 protected:
+    // TODO: Remove this backward compatibility method.
+    // This method is deprecated. You should override public Map function.
+    virtual SubCall Map(int channel_index/*starting from 0*/,
+                        const google::protobuf::MethodDescriptor* method,
+                        const google::protobuf::Message* request,
+                        google::protobuf::Message* response) {
+        return SubCall::Bad();
+    }
+
     // Only callable by subclasses and butil::intrusive_ptr
     virtual ~CallMapper() {}
 };