net/rpc: improve error report messages

Updates #19957

Change-Id: I8e2e3837db9e5e69b7102f9bd5831fe78ac60cfc
Reviewed-on: https://go-review.googlesource.com/87335
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/net/rpc/server.go

@@ -296,7 +296,7 @@ func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
 		// Method needs three ins: receiver, *args, *reply.
 		if mtype.NumIn() != 3 {
 			if reportErr {
-				log.Println("method", mname, "has wrong number of ins:", mtype.NumIn())
+				log.Printf("rpc.Register: method %q has %d input parameters; needs exactly three\n", mname, mtype.NumIn())
 			}
 			continue
 		}
@@ -304,7 +304,7 @@ func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
 		argType := mtype.In(1)
 		if !isExportedOrBuiltinType(argType) {
 			if reportErr {
-				log.Println(mname, "argument type not exported:", argType)
+				log.Printf("rpc.Register: argument type of method %q is not exported: %q\n", mname, argType)
 			}
 			continue
 		}
@@ -312,28 +312,28 @@ func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {
 		replyType := mtype.In(2)
 		if replyType.Kind() != reflect.Ptr {
 			if reportErr {
-				log.Println("method", mname, "reply type not a pointer:", replyType)
+				log.Printf("rpc.Register: reply type of method %q is not a pointer: %q\n", mname, replyType)
 			}
 			continue
 		}
 		// Reply type must be exported.
 		if !isExportedOrBuiltinType(replyType) {
 			if reportErr {
-				log.Println("method", mname, "reply type not exported:", replyType)
+				log.Printf("rpc.Register: reply type of method %q is not exported: %q\n", mname, replyType)
 			}
 			continue
 		}
 		// Method needs one out.
 		if mtype.NumOut() != 1 {
 			if reportErr {
-				log.Println("method", mname, "has wrong number of outs:", mtype.NumOut())
+				log.Printf("rpc.Register: method %q has %d output parameters; needs exactly one\n", mname, mtype.NumOut())
 			}
 			continue
 		}
 		// The return type of the method must be error.
 		if returnType := mtype.Out(0); returnType != typeOfError {
 			if reportErr {
-				log.Println("method", mname, "returns", returnType.String(), "not error")
+				log.Printf("rpc.Register: return type of method %q is %q, must be error\n", mname, returnType)
 			}
 			continue
 		}