encoding/binary: make type error more specific

Right now it says 'invalid type S' for a struct type S.
Instead, say which type inside the struct is the problem.

Fixes #4825.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/7301102

src/pkg/encoding/binary/binary.go

@@ -167,9 +167,9 @@ func Read(r io.Reader, order ByteOrder, data interface{}) error {
 	default:
 		return errors.New("binary.Read: invalid type " + d.Type().String())
 	}
-	size := dataSize(v)
-	if size < 0 {
-		return errors.New("binary.Read: invalid type " + v.Type().String())
+	size, err := dataSize(v)
+	if err != nil {
+		return errors.New("binary.Read: " + err.Error())
 	}
 	d := &decoder{order: order, buf: make([]byte, size)}
 	if _, err := io.ReadFull(r, d.buf); err != nil {
@@ -247,64 +247,68 @@ func Write(w io.Writer, order ByteOrder, data interface{}) error {
 
 	// Fallback to reflect-based encoding.
 	v := reflect.Indirect(reflect.ValueOf(data))
-	size := dataSize(v)
-	if size < 0 {
-		return errors.New("binary.Write: invalid type " + v.Type().String())
+	size, err := dataSize(v)
+	if err != nil {
+		return errors.New("binary.Write: " + err.Error())
 	}
 	buf := make([]byte, size)
 	e := &encoder{order: order, buf: buf}
 	e.value(v)
-	_, err := w.Write(buf)
+	_, err = w.Write(buf)
 	return err
 }
 
 // Size returns how many bytes Write would generate to encode the value v, which
 // must be a fixed-size value or a slice of fixed-size values, or a pointer to such data.
 func Size(v interface{}) int {
-	return dataSize(reflect.Indirect(reflect.ValueOf(v)))
+	n, err := dataSize(reflect.Indirect(reflect.ValueOf(v)))
+	if err != nil {
+		return -1
+	}
+	return n
 }
 
 // dataSize returns the number of bytes the actual data represented by v occupies in memory.
 // For compound structures, it sums the sizes of the elements. Thus, for instance, for a slice
 // it returns the length of the slice times the element size and does not count the memory
 // occupied by the header.
-func dataSize(v reflect.Value) int {
+func dataSize(v reflect.Value) (int, error) {
 	if v.Kind() == reflect.Slice {
-		elem := sizeof(v.Type().Elem())
-		if elem < 0 {
-			return -1
+		elem, err := sizeof(v.Type().Elem())
+		if err != nil {
+			return 0, err
 		}
-		return v.Len() * elem
+		return v.Len() * elem, nil
 	}
 	return sizeof(v.Type())
 }
 
-func sizeof(t reflect.Type) int {
+func sizeof(t reflect.Type) (int, error) {
 	switch t.Kind() {
 	case reflect.Array:
-		n := sizeof(t.Elem())
-		if n < 0 {
-			return -1
+		n, err := sizeof(t.Elem())
+		if err != nil {
+			return 0, err
 		}
-		return t.Len() * n
+		return t.Len() * n, nil
 
 	case reflect.Struct:
 		sum := 0
 		for i, n := 0, t.NumField(); i < n; i++ {
-			s := sizeof(t.Field(i).Type)
-			if s < 0 {
-				return -1
+			s, err := sizeof(t.Field(i).Type)
+			if err != nil {
+				return 0, err
 			}
 			sum += s
 		}
-		return sum
+		return sum, nil
 
 	case reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64,
 		reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64,
 		reflect.Float32, reflect.Float64, reflect.Complex64, reflect.Complex128:
-		return int(t.Size())
+		return int(t.Size()), nil
 	}
-	return -1
+	return 0, errors.New("invalid type " + t.String())
 }
 
 type coder struct {
@@ -514,11 +518,12 @@ func (e *encoder) value(v reflect.Value) {
 }
 
 func (d *decoder) skip(v reflect.Value) {
-	d.buf = d.buf[dataSize(v):]
+	n, _ := dataSize(v)
+	d.buf = d.buf[n:]
 }
 
 func (e *encoder) skip(v reflect.Value) {
-	n := dataSize(v)
+	n, _ := dataSize(v)
 	for i := range e.buf[0:n] {
 		e.buf[i] = 0
 	}

src/pkg/encoding/binary/binary_test.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"math"
 	"reflect"
+	"strings"
 	"testing"
 )
 
@@ -149,8 +150,14 @@ func TestWriteT(t *testing.T) {
 
 	tv := reflect.Indirect(reflect.ValueOf(ts))
 	for i, n := 0, tv.NumField(); i < n; i++ {
+		typ := tv.Field(i).Type().String()
+		if typ == "[4]int" {
+			typ = "int" // the problem is int, not the [4]
+		}
 		if err := Write(buf, BigEndian, tv.Field(i).Interface()); err == nil {
 			t.Errorf("WriteT.%v: have err == nil, want non-nil", tv.Field(i).Type())
+		} else if !strings.Contains(err.Error(), typ) {
+			t.Errorf("WriteT: have err == %q, want it to mention %s", err, typ)
 		}
 	}
 }
@@ -238,7 +245,7 @@ func BenchmarkReadStruct(b *testing.B) {
 	bsr := &byteSliceReader{}
 	var buf bytes.Buffer
 	Write(&buf, BigEndian, &s)
-	n := dataSize(reflect.ValueOf(s))
+	n, _ := dataSize(reflect.ValueOf(s))
 	b.SetBytes(int64(n))
 	t := s
 	b.ResetTimer()