gob: send empty but non-nil maps.

Fixes #2082.

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/4798042

src/pkg/gob/codec_test.go

@@ -575,6 +575,7 @@ func TestEndToEnd(t *testing.T) {
 	type T1 struct {
 		A, B, C  int
 		M        map[string]*float64
+		EmptyMap map[string]int // to check that we receive a non-nil map.
 		N        *[3]float64
 		Strs     *[2]string
 		Int64s   *[]int64
@@ -590,6 +591,7 @@ func TestEndToEnd(t *testing.T) {
 		B:        18,
 		C:        -5,
 		M:        map[string]*float64{"pi": &pi, "e": &e},
+		EmptyMap: make(map[string]int),
 		N:        &[3]float64{1.5, 2.5, 3.5},
 		Strs:     &[2]string{s1, s2},
 		Int64s:   &[]int64{77, 89, 123412342134},
@@ -611,6 +613,13 @@ func TestEndToEnd(t *testing.T) {
 	if !reflect.DeepEqual(t1, &_t1) {
 		t.Errorf("encode expected %v got %v", *t1, _t1)
 	}
+	// Be absolutely sure the received map is non-nil.
+	if t1.EmptyMap == nil {
+		t.Errorf("nil map sent")
+	}
+	if _t1.EmptyMap == nil {
+		t.Errorf("nil map received")
+	}
 }
 
 func TestOverflow(t *testing.T) {

src/pkg/gob/doc.go

@@ -113,6 +113,11 @@ uninterpreted bytes of the value.
 All other slices and arrays are sent as an unsigned count followed by that many
 elements using the standard gob encoding for their type, recursively.
 
+Maps are sent as an unsigned count followed by that man key, element
+pairs. Empty but non-nil maps are sent, so if the sender has allocated
+a map, the receiver will allocate a map even no elements are
+transmitted.
+
 Structs are sent as a sequence of (field number, field value) pairs.  The field
 value is sent using the standard gob encoding for its type, recursively.  If a
 field has the zero value for its type, it is omitted from the transmission.  The

src/pkg/gob/encode.go

@@ -557,7 +557,9 @@ func (enc *Encoder) encOpFor(rt reflect.Type, inProgress map[reflect.Type]*encOp
 				// the iteration.
 				v := reflect.ValueOf(unsafe.Unreflect(t, unsafe.Pointer(p)))
 				mv := reflect.Indirect(v)
-				if !state.sendZero && mv.Len() == 0 {
+				// We send zero-length (but non-nil) maps because the
+				// receiver might want to use the map.  (Maps don't use append.)
+				if !state.sendZero && mv.IsNil() {
 					return
 				}
 				state.update(i)