encoding/binary: fix error message

In the process, simplified internal sizeOf and
dataSize functions. Minor positive impact on
performance. Added test case.

benchmark                         old ns/op     new ns/op     delta
BenchmarkReadSlice1000Int32s      14006         14122         +0.83%
BenchmarkReadStruct               2508          2447          -2.43%
BenchmarkReadInts                 921           928           +0.76%
BenchmarkWriteInts                2086          2081          -0.24%
BenchmarkWriteSlice1000Int32s     13440         13497         +0.42%
BenchmarkPutUvarint32             28.5          26.3          -7.72%
BenchmarkPutUvarint64             81.3          76.7          -5.66%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkReadSlice1000Int32s      285.58       283.24       0.99x
BenchmarkReadStruct               27.90        28.60        1.03x
BenchmarkReadInts                 32.57        32.31        0.99x
BenchmarkWriteInts                14.38        14.41        1.00x
BenchmarkWriteSlice1000Int32s     297.60       296.36       1.00x
BenchmarkPutUvarint32             140.55       151.92       1.08x
BenchmarkPutUvarint64             98.36        104.33       1.06x

Fixes #6818.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/149290045

2 files changed, 53 insertions, 40 deletions

diff --git a/src/encoding/binary/binary.go b/src/encoding/binary/binary.go
index b5a377430f..466bf97c97 100644
--- a/src/encoding/binary/binary.go
+++ b/src/encoding/binary/binary.go
@@ -200,18 +200,17 @@ func Read(r io.Reader, order ByteOrder, data interface{}) error {
 	}
 
 	// Fallback to reflect-based decoding.
-	var v reflect.Value
-	switch d := reflect.ValueOf(data); d.Kind() {
+	v := reflect.ValueOf(data)
+	size := -1
+	switch v.Kind() {
 	case reflect.Ptr:
-		v = d.Elem()
+		v = v.Elem()
+		size = dataSize(v)
 	case reflect.Slice:
-		v = d
-	default:
-		return errors.New("binary.Read: invalid type " + d.Type().String())
+		size = dataSize(v)
 	}
-	size, err := dataSize(v)
-	if err != nil {
-		return errors.New("binary.Read: " + err.Error())
+	if size < 0 {
+		return errors.New("binary.Read: invalid type " + reflect.TypeOf(data).String())
 	}
 	d := &decoder{order: order, buf: make([]byte, size)}
 	if _, err := io.ReadFull(r, d.buf); err != nil {
@@ -324,68 +323,64 @@ func Write(w io.Writer, order ByteOrder, data interface{}) error {
 
 	// Fallback to reflect-based encoding.
 	v := reflect.Indirect(reflect.ValueOf(data))
-	size, err := dataSize(v)
-	if err != nil {
-		return errors.New("binary.Write: " + err.Error())
+	size := dataSize(v)
+	if size < 0 {
+		return errors.New("binary.Write: invalid type " + reflect.TypeOf(data).String())
 	}
 	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.
+// If v is neither of these, Size returns -1.
 func Size(v interface{}) int {
-	n, err := dataSize(reflect.Indirect(reflect.ValueOf(v)))
-	if err != nil {
-		return -1
-	}
-	return n
+	return dataSize(reflect.Indirect(reflect.ValueOf(v)))
 }
 
 // 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, error) {
+// occupied by the header. If the type of v is not acceptable, dataSize returns -1.
+func dataSize(v reflect.Value) int {
 	if v.Kind() == reflect.Slice {
-		elem, err := sizeof(v.Type().Elem())
-		if err != nil {
-			return 0, err
+		if s := sizeof(v.Type().Elem()); s >= 0 {
+			return s * v.Len()
 		}
-		return v.Len() * elem, nil
+		return -1
 	}
 	return sizeof(v.Type())
 }
 
-func sizeof(t reflect.Type) (int, error) {
+// sizeof returns the size >= 0 of variables for the given type or -1 if the type is not acceptable.
+func sizeof(t reflect.Type) int {
 	switch t.Kind() {
 	case reflect.Array:
-		n, err := sizeof(t.Elem())
-		if err != nil {
-			return 0, err
+		if s := sizeof(t.Elem()); s >= 0 {
+			return s * t.Len()
 		}
-		return t.Len() * n, nil
 
 	case reflect.Struct:
 		sum := 0
 		for i, n := 0, t.NumField(); i < n; i++ {
-			s, err := sizeof(t.Field(i).Type)
-			if err != nil {
-				return 0, err
+			s := sizeof(t.Field(i).Type)
+			if s < 0 {
+				return -1
 			}
 			sum += s
 		}
-		return sum, nil
+		return sum
 
 	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()), nil
+		return int(t.Size())
 	}
-	return 0, errors.New("invalid type " + t.String())
+
+	return -1
 }
 
 type coder struct {
@@ -595,12 +590,11 @@ func (e *encoder) value(v reflect.Value) {
 }
 
 func (d *decoder) skip(v reflect.Value) {
-	n, _ := dataSize(v)
-	d.buf = d.buf[n:]
+	d.buf = d.buf[dataSize(v):]
 }
 
 func (e *encoder) skip(v reflect.Value) {
-	n, _ := dataSize(v)
+	n := dataSize(v)
 	for i := range e.buf[0:n] {
 		e.buf[i] = 0
 	}
diff --git a/src/encoding/binary/binary_test.go b/src/encoding/binary/binary_test.go
index c80c90383a..8ee595fa47 100644
--- a/src/encoding/binary/binary_test.go
+++ b/src/encoding/binary/binary_test.go
@@ -289,6 +289,26 @@ func TestUnexportedRead(t *testing.T) {
 	Read(&buf, LittleEndian, &u2)
 }
 
+func TestReadErrorMsg(t *testing.T) {
+	var buf bytes.Buffer
+	read := func(data interface{}) {
+		err := Read(&buf, LittleEndian, data)
+		want := "binary.Read: invalid type " + reflect.TypeOf(data).String()
+		if err == nil {
+			t.Errorf("%T: got no error; want %q", data, want)
+			return
+		}
+		if got := err.Error(); got != want {
+			t.Errorf("%T: got %q; want %q", data, got, want)
+		}
+	}
+	read(0)
+	s := new(struct{})
+	read(&s)
+	p := &s
+	read(&p)
+}
+
 type byteSliceReader struct {
 	remain []byte
 }
@@ -315,8 +335,7 @@ func BenchmarkReadStruct(b *testing.B) {
 	bsr := &byteSliceReader{}
 	var buf bytes.Buffer
 	Write(&buf, BigEndian, &s)
-	n, _ := dataSize(reflect.ValueOf(s))
-	b.SetBytes(int64(n))
+	b.SetBytes(int64(dataSize(reflect.ValueOf(s))))
 	t := s
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {