encoding/json: add "overflow" struct tag option

Fixes #6213.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://golang.org/cl/13180043

1 files changed, 59 insertions, 6 deletions

diff --git a/src/pkg/encoding/json/encode.go b/src/pkg/encoding/json/encode.go
index f951250e98..590010f3b2 100644
--- a/src/pkg/encoding/json/encode.go
+++ b/src/pkg/encoding/json/encode.go
@@ -109,6 +109,10 @@ import (
 // an anonymous struct field in both current and earlier versions, give the field
 // a JSON tag of "-".
 //
+// The "overflow" option may be used with a struct field of a map type to
+// indicate that the map contents should be marshalled as if the keys are part
+// of the struct object itself.
+//
 // Map values encode as JSON objects.
 // The map's key type must be string; the object keys are used directly
 // as map keys.
@@ -239,6 +243,32 @@ var hex = "0123456789abcdef"
 type encodeState struct {
 	bytes.Buffer // accumulated output
 	scratch      [64]byte
+	overflow     int
+}
+
+func (e *encodeState) startOverflow() {
+	e.overflow = e.Len()
+}
+
+func (e *encodeState) endOverflow() {
+	if e.overflow == 0 {
+		panic("endOverflow called before startOverflow")
+	}
+	start, end := e.overflow, e.Len()
+	b := e.Bytes()
+	if b[start] == '{' && b[end-1] == '}' {
+		// Remove surrounding { and }.
+		copy(b[start:], b[start+1:])
+		e.Truncate(end - 2)
+	} else if bytes.Equal(b[start:end], []byte("null")) {
+		// Drop "null".
+		e.Truncate(start)
+	}
+	// Remove trailing comma if overflow value was null or {}.
+	if start > 0 && e.Len() == start && b[start-1] == ',' {
+		e.Truncate(start - 1)
+	}
+	e.overflow = 0
 }
 
 // TODO(bradfitz): use a sync.Cache here
@@ -582,7 +612,7 @@ func (se *structEncoder) encode(e *encodeState, v reflect.Value, quoted bool) {
 	e.WriteByte('{')
 	first := true
 	for i, f := range se.fields {
-		fv := fieldByIndex(v, f.index)
+		fv := fieldByIndex(v, f.index, false)
 		if !fv.IsValid() || f.omitEmpty && isEmptyValue(fv) {
 			continue
 		}
@@ -591,6 +621,16 @@ func (se *structEncoder) encode(e *encodeState, v reflect.Value, quoted bool) {
 		} else {
 			e.WriteByte(',')
 		}
+		if f.overflow {
+			if tenc := se.fieldEncs[i]; tenc != nil {
+				e.startOverflow()
+				tenc(e, fv, f.quoted)
+				e.endOverflow()
+			} else {
+				panic("no encoder for " + fv.String())
+			}
+			continue
+		}
 		e.string(f.name)
 		e.WriteByte(':')
 		if tenc := se.fieldEncs[i]; tenc != nil {
@@ -610,7 +650,7 @@ func newStructEncoder(t reflect.Type, vx reflect.Value) encoderFunc {
 		fieldEncs: make([]encoderFunc, len(fields)),
 	}
 	for i, f := range fields {
-		vxf := fieldByIndex(vx, f.index)
+		vxf := fieldByIndex(vx, f.index, false)
 		if vxf.IsValid() {
 			se.fieldEncs[i] = typeEncoder(vxf.Type(), vxf)
 		}
@@ -750,11 +790,16 @@ func isValidTag(s string) bool {
 	return true
 }
 
-func fieldByIndex(v reflect.Value, index []int) reflect.Value {
+// fieldByIndex fetches (and allocates, if create is true) the field in v
+// indentified by index.
+func fieldByIndex(v reflect.Value, index []int, create bool) reflect.Value {
 	for _, i := range index {
 		if v.Kind() == reflect.Ptr {
 			if v.IsNil() {
-				return reflect.Value{}
+				if !create {
+					return reflect.Value{}
+				}
+				v.Set(reflect.New(v.Type().Elem()))
 			}
 			v = v.Elem()
 		}
@@ -926,6 +971,7 @@ type field struct {
 	typ       reflect.Type
 	omitEmpty bool
 	quoted    bool
+	overflow  bool
 }
 
 // byName sorts field by name, breaking ties with depth,
@@ -1027,8 +1073,15 @@ func typeFields(t reflect.Type) []field {
 					if name == "" {
 						name = sf.Name
 					}
-					fields = append(fields, field{name, tagged, index, ft,
-						opts.Contains("omitempty"), opts.Contains("string")})
+					fields = append(fields, field{
+						name:      name,
+						tag:       tagged,
+						index:     index,
+						typ:       ft,
+						omitEmpty: opts.Contains("omitempty"),
+						quoted:    opts.Contains("string"),
+						overflow:  opts.Contains("overflow")},
+					)
 					if count[f.typ] > 1 {
 						// If there were multiple instances, add a second,
 						// so that the annihilation code will see a duplicate.