encoding/binary: document that Read requires exported struct fields

Add a test for the current behaviour.

Fixes #7482.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/95160043

diff --git a/src/pkg/encoding/binary/binary.go b/src/pkg/encoding/binary/binary.go
index f3466b9af0313e279dcf93de8cea6bcf84e307b1..a5694876ac46eff5f80679c725cf3fd0699afaeb 100644 (file)
--- a/src/pkg/encoding/binary/binary.go
+++ b/src/pkg/encoding/binary/binary.go
@@ -133,6 +133,7 @@ func (bigEndian) GoString() string { return "binary.BigEndian" }
 // When reading into structs, the field data for fields with
 // blank (_) field names is skipped; i.e., blank field names
 // may be used for padding.
+// When reading into a struct, all non-blank fields must be exported.
 func Read(r io.Reader, order ByteOrder, data interface{}) error {
        // Fast path for basic types and slices.
        if n := intDataSize(data); n != 0 {

diff --git a/src/pkg/encoding/binary/binary_test.go b/src/pkg/encoding/binary/binary_test.go
index 1aa6ecd2486ba007cd565209e49bbd05928bff70..c80c90383afbeadb3cf30620f8e878cbfda4c293 100644 (file)
--- a/src/pkg/encoding/binary/binary_test.go
+++ b/src/pkg/encoding/binary/binary_test.go
@@ -265,6 +265,30 @@ func TestBlankFields(t *testing.T) {
        }
 }
 
+// An attempt to read into a struct with an unexported field will
+// panic.  This is probably not the best choice, but at this point
+// anything else would be an API change.
+
+type Unexported struct {
+       a int32
+}
+
+func TestUnexportedRead(t *testing.T) {
+       var buf bytes.Buffer
+       u1 := Unexported{a: 1}
+       if err := Write(&buf, LittleEndian, &u1); err != nil {
+               t.Fatal(err)
+       }
+
+       defer func() {
+               if recover() == nil {
+                       t.Fatal("did not panic")
+               }
+       }()
+       var u2 Unexported
+       Read(&buf, LittleEndian, &u2)
+}
+
 type byteSliceReader struct {
        remain []byte
 }