netchan: fix bug for imported send.

Also add a bit of debugging and sanitation code.
Fixes #769.

R=adg
CC=golang-dev
https://golang.org/cl/2206042

src/pkg/netchan/common.go

@@ -21,6 +21,16 @@ const (
 	Send
 )
 
+func (dir Dir) String() string {
+	switch dir {
+	case Recv:
+		return "Recv"
+	case Send:
+		return "Send"
+	}
+	return "???"
+}
+
 // Payload types
 const (
 	payRequest = iota // request structure follows

src/pkg/netchan/export.go

@@ -19,7 +19,7 @@
 */
 package netchan
 
-// BUG: can't use range clause to receive when using ImportNValues with N non-zero.
+// BUG: can't use range clause to receive when using ImportNValues to limit the count.
 
 import (
 	"log"
@@ -94,6 +94,7 @@ func (client *expClient) run() {
 	reqValue := reflect.NewValue(req)
 	error := new(error)
 	for {
+		*hdr = header{}
 		if err := client.decode(hdrValue); err != nil {
 			log.Stderr("error decoding client header:", err)
 			break

src/pkg/netchan/import.go

@@ -64,6 +64,7 @@ func (imp *Importer) run() {
 	err := new(error)
 	errValue := reflect.NewValue(err)
 	for {
+		*hdr = header{}
 		if e := imp.decode(hdrValue); e != nil {
 			log.Stderr("importer header:", e)
 			imp.shutdown()
@@ -152,7 +153,7 @@ func (imp *Importer) ImportNValues(name string, chT interface{}, dir Dir, n int)
 	}
 	if dir == Send {
 		go func() {
-			for i := 0; n == 0 || i < n; i++ {
+			for i := 0; n == -1 || i < n; i++ {
 				val := ch.Recv()
 				if err := imp.encode(hdr, payData, val.Interface()); err != nil {
 					log.Stderr("error encoding client response:", err)