SplitHTTP: Do not produce too large upload (#3691)

transport/pipe/impl.go

@@ -46,6 +46,14 @@ var (
 	errSlowDown   = errors.New("slow down")
 )
 
+func (p *pipe) Len() int32 {
+    data := p.data
+    if data == nil {
+        return 0
+    }
+    return data.Len()
+}
+
 func (p *pipe) getState(forRead bool) error {
 	switch p.state {
 	case open: