encoding/gob: do not use MarshalText, UnmarshalText

This seems to be the best of a long list of bad ways to fix this issue.

Fixes #6760.

R=r
CC=golang-dev
https://golang.org/cl/22770044

doc/go1.2.html

@@ -736,7 +736,8 @@ now treats channel and function fields of structures as if they were unexported,
 even if they are not. That is, it ignores them completely. Previously they would
 trigger an error, which could cause unexpected compatibility problems if an
 embedded structure added such a field.
-The package also now supports the generic encoding interfaces of the
+The package also now supports the generic <code>BinaryMarshaler</code> and
+<code>BinaryUnmarshaler</code> interfaces of the
 <a href="/pkg/encoding/"><code>encoding</code></a> package
 described above.
 </li>

src/pkg/encoding/gob/doc.go

@@ -86,13 +86,13 @@ Functions and channels will not be sent in a gob. Attempting to encode such a va
 at top the level will fail. A struct field of chan or func type is treated exactly
 like an unexported field and is ignored.
 
-Gob can encode a value of any type implementing the GobEncoder,
-encoding.BinaryMarshaler, or encoding.TextMarshaler interfaces by calling the
-corresponding method, in that order of preference.
+Gob can encode a value of any type implementing the GobEncoder or
+encoding.BinaryMarshaler interfaces by calling the corresponding method,
+in that order of preference.
 
-Gob can decode a value of any type implementing the GobDecoder,
-encoding.BinaryUnmarshaler, or encoding.TextUnmarshaler interfaces by calling
-the corresponding method, again in that order of preference.
+Gob can decode a value of any type implementing the GobDecoder or
+encoding.BinaryUnmarshaler interfaces by calling the corresponding method,
+again in that order of preference.
 
 Encoding Details
 

src/pkg/encoding/gob/gobencdec_test.go

@@ -11,6 +11,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net"
 	"strings"
 	"testing"
 	"time"
@@ -767,3 +768,17 @@ func TestGobEncodePtrError(t *testing.T) {
 		t.Fatalf("expected nil, got %v", err2)
 	}
 }
+
+func TestNetIP(t *testing.T) {
+	// Encoding of net.IP{1,2,3,4} in Go 1.1.
+	enc := []byte{0x07, 0x0a, 0x00, 0x04, 0x01, 0x02, 0x03, 0x04}
+
+	var ip net.IP
+	err := NewDecoder(bytes.NewReader(enc)).Decode(&ip)
+	if err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if ip.String() != "1.2.3.4" {
+		t.Errorf("decoded to %v, want 1.2.3.4", ip.String())
+	}
+}

src/pkg/encoding/gob/type.go

@@ -88,18 +88,25 @@ func validUserType(rt reflect.Type) (ut *userTypeInfo, err error) {
 		ut.externalEnc, ut.encIndir = xGob, indir
 	} else if ok, indir := implementsInterface(ut.user, binaryMarshalerInterfaceType); ok {
 		ut.externalEnc, ut.encIndir = xBinary, indir
-	} else if ok, indir := implementsInterface(ut.user, textMarshalerInterfaceType); ok {
-		ut.externalEnc, ut.encIndir = xText, indir
 	}
 
+	// NOTE(rsc): Would like to allow MarshalText here, but results in incompatibility
+	// with older encodings for net.IP. See golang.org/issue/6760.
+	// } else if ok, indir := implementsInterface(ut.user, textMarshalerInterfaceType); ok {
+	// 	ut.externalEnc, ut.encIndir = xText, indir
+	// }
+
 	if ok, indir := implementsInterface(ut.user, gobDecoderInterfaceType); ok {
 		ut.externalDec, ut.decIndir = xGob, indir
 	} else if ok, indir := implementsInterface(ut.user, binaryUnmarshalerInterfaceType); ok {
 		ut.externalDec, ut.decIndir = xBinary, indir
-	} else if ok, indir := implementsInterface(ut.user, textUnmarshalerInterfaceType); ok {
-		ut.externalDec, ut.decIndir = xText, indir
 	}
 
+	// See note above.
+	// } else if ok, indir := implementsInterface(ut.user, textUnmarshalerInterfaceType); ok {
+	// 	ut.externalDec, ut.decIndir = xText, indir
+	// }
+
 	userTypeCache[rt] = ut
 	return
 }