encoding/gob: handle encoding of different indirects of GobEncoder

Fixes #4647.

R=r, golang-dev
CC=golang-dev
https://golang.org/cl/7085051

src/pkg/encoding/gob/encoder.go

@@ -137,8 +137,8 @@ func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Typ
 	ut := userType(origt)
 	if ut.isGobEncoder {
 		// The rules are different: regardless of the underlying type's representation,
-		// we need to tell the other side that this exact type is a GobEncoder.
-		return enc.sendActualType(w, state, ut, ut.user)
+		// we need to tell the other side that the base type is a GobEncoder.
+		return enc.sendActualType(w, state, ut, ut.base)
 	}
 
 	// It's a concrete value, so drill down to the base type.

src/pkg/encoding/gob/gobencdec_test.go

@@ -142,6 +142,18 @@ type GobTest5 struct {
 	V *ValueGobber
 }
 
+type GobTest6 struct {
+	X int // guarantee we have  something in common with GobTest*
+	V ValueGobber
+	W *ValueGobber
+}
+
+type GobTest7 struct {
+	X int // guarantee we have  something in common with GobTest*
+	V *ValueGobber
+	W ValueGobber
+}
+
 type GobTestIgnoreEncoder struct {
 	X int // guarantee we have  something in common with GobTest*
 }
@@ -360,6 +372,61 @@ func TestGobEncoderValueEncoder(t *testing.T) {
 	}
 }
 
+// Test that we can use a value then a pointer type of a GobEncoder
+// in the same encoded value.  Bug 4647.
+func TestGobEncoderValueThenPointer(t *testing.T) {
+	v := ValueGobber("forty-two")
+	w := ValueGobber("six-by-nine")
+
+	// this was a bug: encoding a GobEncoder by value before a GobEncoder
+	// pointer would cause duplicate type definitions to be sent.
+
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	if err := enc.Encode(GobTest6{42, v, &w}); err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	x := new(GobTest6)
+	if err := dec.Decode(x); err != nil {
+		t.Fatal("decode error:", err)
+	}
+	if got, want := x.V, v; got != want {
+		t.Errorf("v = %q, want %q", got, want)
+	}
+	if got, want := v.W, w; got == nil {
+		t.Errorf("w = nil, want %q", want)
+	} else if *got != want {
+		t.Errorf("w = %q, want %q", *got, want)
+	}
+}
+
+// Test that we can use a pointer then a value type of a GobEncoder
+// in the same encoded value.
+func TestGobEncoderPointerThenValue(t *testing.T) {
+	v := ValueGobber("forty-two")
+	w := ValueGobber("six-by-nine")
+
+	b := new(bytes.Buffer)
+	enc := NewEncoder(b)
+	if err := enc.Encode(GobTest7{42, &v, w}); err != nil {
+		t.Fatal("encode error:", err)
+	}
+	dec := NewDecoder(b)
+	x := new(GobTest7)
+	if err := dec.Decode(x); err != nil {
+		t.Fatal("decode error:", err)
+	}
+	if got, want := x.V, v; got == nil {
+		t.Errorf("v = nil, want %q", want)
+	} else if *got != want {
+		t.Errorf("v = %q, want %q", got, want)
+	}
+	if got, want := v.W, w; got != want {
+		t.Errorf("w = %q, want %q", got, want)
+	}
+}
+
 func TestGobEncoderFieldTypeError(t *testing.T) {
 	// GobEncoder to non-decoder: error
 	b := new(bytes.Buffer)