p9: fix Twstat encoding and decoding (#61)

* proto: Better error messages for Receive().

* p9: Fix Twstat encoding and decoding.

msg.go

@@ -237,5 +237,43 @@ type Twstat struct {
 	Stat Stat
 }
 
+func (stat Twstat) P9Encode() ([]byte, error) {
+	var buf bytes.Buffer
+
+	err := proto.Write(&buf, stat.FID)
+	if err != nil {
+		return nil, err
+	}
+
+	err = proto.Write(&buf, stat.Stat.size()+2)
+	if err != nil {
+		return nil, err
+	}
+
+	err = proto.Write(&buf, stat.Stat)
+	return buf.Bytes(), err
+}
+
+func (stat *Twstat) P9Decode(r io.Reader) error {
+	err := proto.Read(r, &stat.FID)
+	if err != nil {
+		return err
+	}
+
+	var size uint16
+	err = proto.Read(r, &size)
+	if err != nil {
+		return err
+	}
+
+	r = &util.LimitedReader{
+		R: r,
+		N: uint32(size),
+		E: ErrLargeStat,
+	}
+
+	return proto.Read(r, &stat.Stat)
+}
+
 type Rwstat struct {
 }

proto/proto.go

@@ -78,19 +78,11 @@ func (p Proto) Receive(r io.Reader, msize uint32) (msg interface{}, tag uint16,
 		E: ErrLargeMessage,
 	}
 
-	read := func(v interface{}) {
-		if err != nil {
-			return
-		}
-
-		err = Read(lr, v)
-		if err != nil {
-			err = util.Errorf("receive: %w", err)
-		}
-	}
-
 	var msgType uint8
-	read(&msgType)
+	err = Read(lr, &msgType)
+	if err != nil {
+		return nil, NoTag, util.Errorf("receive: failed to read message type: %w", err)
+	}
 
 	t := p.TypeFromID(msgType)
 	if t == nil {
@@ -102,10 +94,16 @@ func (p Proto) Receive(r io.Reader, msize uint32) (msg interface{}, tag uint16,
 	}
 
 	tag = NoTag
-	read(&tag)
+	err = Read(lr, &tag)
+	if err != nil {
+		return nil, tag, util.Errorf("receive: failed to read tag: %w", err)
+	}
 
 	m := reflect.New(t)
-	read(m.Interface())
+	err = Read(lr, m.Interface())
+	if err != nil {
+		return nil, tag, util.Errorf("receive %v: %w", m.Type().Elem(), err)
+	}
 
 	return m.Elem().Interface(), tag, err
 }
@@ -125,13 +123,13 @@ func (p Proto) Send(w io.Writer, tag uint16, msg interface{}) (err error) {
 
 		err = Write(w, v)
 		if err != nil {
-			err = util.Errorf("send: %w", err)
+			err = util.Errorf("send %T: %w", msg, err)
 		}
 	}
 
 	n, err := Size(msg)
 	if err != nil {
-		return util.Errorf("send: %w", err)
+		return util.Errorf("send %T: %w", msg, err)
 	}
 
 	write(4 + 1 + 2 + n)

stat.go

@@ -15,7 +15,7 @@ import (
 var (
 	// ErrLargeStat is returned during decoding when a stat is larger
 	// than its own declared size.
-	ErrLargeStat = errors.New("stat larger that declared size")
+	ErrLargeStat = errors.New("stat larger than declared size")
 )
 
 // FileMode stores permission and type information about a file or