xml: add Marshal and MarshalIndent

I have written up a Marshal and MarshalIndent pair that should
closely reflect the way that Unmarshal works.  I would love feedback
on making this code more accessible and efficient... I haven't used
reflecton on this scale before, so there is probably a lot of work
that can be done on that.

Some potentially controversial things:
- All tag names are lower-cased by default.
- Zero-valued struct values are skipped.
- No namespace prefix (o:tag, etc) mechanism is supplied.
- You are allowed to marshal non-struct values (even though unmarshal
  cannot handle them).
- A tag for a non-XMLName struct field that isn't "attr", "chardata",
  or "innerxml" is used as the name of the tag.  This could wreak
  havoc if you try to marshal a protobuf struct.
- The "innerxml" and "chardata" are inserted verbatim.  If you try to
  marshal something straight from unmarshal, the results could be
  unexpected (remove "innerxml" support from Marshal would be one
  possible solution).

R=rsc
CC=golang-dev
https://golang.org/cl/4539082

src/pkg/xml/Makefile

@@ -7,6 +7,7 @@ include ../../Make.inc
 TARG=xml
 
 GOFILES=\
+	marshal.go\
 	read.go\
 	xml.go\
 

src/pkg/xml/atom_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xml
+
+var atomValue = &Feed{
+	Title:   "Example Feed",
+	Link:    []Link{{Href: "http://example.org/"}},
+	Updated: ParseTime("2003-12-13T18:30:02Z"),
+	Author:  Person{Name: "John Doe"},
+	Id:      "urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6",
+
+	Entry: []Entry{
+		{
+			Title:   "Atom-Powered Robots Run Amok",
+			Link:    []Link{{Href: "http://example.org/2003/12/13/atom03"}},
+			Id:      "urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a",
+			Updated: ParseTime("2003-12-13T18:30:02Z"),
+			Summary: NewText("Some text."),
+		},
+	},
+}
+
+var atomXml = `` +
+	`<feed xmlns="http://www.w3.org/2005/Atom">` +
+	`<Title>Example Feed</Title>` +
+	`<Id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</Id>` +
+	`<Link href="http://example.org/"></Link>` +
+	`<Updated>2003-12-13T18:30:02Z</Updated>` +
+	`<Author><Name>John Doe</Name><URI></URI><Email></Email></Author>` +
+	`<Entry>` +
+	`<Title>Atom-Powered Robots Run Amok</Title>` +
+	`<Id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</Id>` +
+	`<Link href="http://example.org/2003/12/13/atom03"></Link>` +
+	`<Updated>2003-12-13T18:30:02Z</Updated>` +
+	`<Author><Name></Name><URI></URI><Email></Email></Author>` +
+	`<Summary>Some text.</Summary>` +
+	`</Entry>` +
+	`</feed>`
+
+func ParseTime(str string) Time {
+	return Time(str)
+}
+
+func NewText(text string) Text {
+	return Text{
+		Body: text,
+	}
+}

src/pkg/xml/marshal.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xml
+
+import (
+	"bufio"
+	"io"
+	"os"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+const (
+	// A generic XML header suitable for use with the output of Marshal and MarshalIndent.
+	// This is not automatically added to any output of this package, it is provided as a
+	// convenience.
+	Header = `<?xml version="1.0" encoding="UTF-8">\n`
+)
+
+// A Marshaler can produce well-formatted XML representing its internal state.
+// It is used by both Marshal and MarshalIndent.
+type Marshaler interface {
+	MarshalXML() ([]byte, os.Error)
+}
+
+type printer struct {
+	*bufio.Writer
+}
+
+// Marshal writes an XML-formatted representation of v to w.
+//
+// If v implements Marshaler, then Marshal calls its MarshalXML method.
+// Otherwise, Marshal uses the following procedure to create the XML.
+//
+// Marshal handles an array or slice by marshalling each of the elements.
+// Marshal handles a pointer by marshalling the value it points at or, if the
+// pointer is nil, by writing nothing.  Marshal handles an interface value by
+// marshalling the value it contains or, if the interface value is nil, by
+// writing nothing.  Marshal handles all other data by writing a single XML
+// element containing the data.
+//
+// The name of that XML element is taken from, in order of preference:
+//     - the tag on an XMLName field, if the data is a struct
+//     - the value of an XMLName field of type xml.Name
+//     - the tag of the struct field used to obtain the data
+//     - the name of the struct field used to obtain the data
+//     - the name '???'.
+//
+// The XML element for a struct contains marshalled elements for each of the
+// exported fields of the struct, with these exceptions:
+//     - the XMLName field, described above, is omitted.
+//     - a field with tag "attr" becomes an attribute in the XML element.
+//     - a field with tag "chardata" is written as character data,
+//        not as an XML element.
+//     - a field with tag "innerxml" is written verbatim,
+//        not subject to the usual marshalling procedure.
+//
+// Marshal will return an error if asked to marshal a channel, function, or map.
+func Marshal(w io.Writer, v interface{}) (err os.Error) {
+	p := &printer{bufio.NewWriter(w)}
+	err = p.marshalValue(reflect.ValueOf(v), "???")
+	p.Flush()
+	return err
+}
+
+func (p *printer) marshalValue(val reflect.Value, name string) os.Error {
+	if !val.IsValid() {
+		return nil
+	}
+
+	kind := val.Kind()
+	typ := val.Type()
+
+	// Try Marshaler
+	if typ.NumMethod() > 0 {
+		if marshaler, ok := val.Interface().(Marshaler); ok {
+			bytes, err := marshaler.MarshalXML()
+			if err != nil {
+				return err
+			}
+			p.Write(bytes)
+			return nil
+		}
+	}
+
+	// Drill into pointers/interfaces
+	if kind == reflect.Ptr || kind == reflect.Interface {
+		if val.IsNil() {
+			return nil
+		}
+		return p.marshalValue(val.Elem(), name)
+	}
+
+	// Slices and arrays iterate over the elements. They do not have an enclosing tag.
+	if (kind == reflect.Slice || kind == reflect.Array) && typ.Elem().Kind() != reflect.Uint8 {
+		for i, n := 0, val.Len(); i < n; i++ {
+			if err := p.marshalValue(val.Index(i), name); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	// Find XML name
+	xmlns := ""
+	if kind == reflect.Struct {
+		if f, ok := typ.FieldByName("XMLName"); ok {
+			if tag := f.Tag; tag != "" {
+				if i := strings.Index(tag, " "); i >= 0 {
+					xmlns, name = tag[:i], tag[i+1:]
+				} else {
+					name = tag
+				}
+			} else if v, ok := val.FieldByIndex(f.Index).Interface().(Name); ok && v.Local != "" {
+				xmlns, name = v.Space, v.Local
+			}
+		}
+	}
+
+	p.WriteByte('<')
+	p.WriteString(name)
+
+	// Attributes
+	if kind == reflect.Struct {
+		if len(xmlns) > 0 {
+			p.WriteString(` xmlns="`)
+			Escape(p, []byte(xmlns))
+			p.WriteByte('"')
+		}
+
+		for i, n := 0, typ.NumField(); i < n; i++ {
+			if f := typ.Field(i); f.PkgPath == "" && f.Tag == "attr" {
+				if f.Type.Kind() == reflect.String {
+					if str := val.Field(i).String(); str != "" {
+						p.WriteByte(' ')
+						p.WriteString(strings.ToLower(f.Name))
+						p.WriteString(`="`)
+						Escape(p, []byte(str))
+						p.WriteByte('"')
+					}
+				}
+			}
+		}
+	}
+	p.WriteByte('>')
+
+	switch k := val.Kind(); k {
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		p.WriteString(strconv.Itoa64(val.Int()))
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		p.WriteString(strconv.Uitoa64(val.Uint()))
+	case reflect.Float32, reflect.Float64:
+		p.WriteString(strconv.Ftoa64(val.Float(), 'g', -1))
+	case reflect.String:
+		Escape(p, []byte(val.String()))
+	case reflect.Bool:
+		p.WriteString(strconv.Btoa(val.Bool()))
+	case reflect.Array:
+		// will be [...]byte
+		bytes := make([]byte, val.Len())
+		for i := range bytes {
+			bytes[i] = val.Index(i).Interface().(byte)
+		}
+		Escape(p, bytes)
+	case reflect.Slice:
+		// will be []byte
+		bytes := val.Interface().([]byte)
+		Escape(p, bytes)
+	case reflect.Struct:
+		for i, n := 0, val.NumField(); i < n; i++ {
+			if f := typ.Field(i); f.Name != "XMLName" && f.PkgPath == "" {
+				name := f.Name
+				switch tag := f.Tag; tag {
+				case "":
+				case "chardata":
+					if tk := f.Type.Kind(); tk == reflect.String {
+						p.Write([]byte(val.Field(i).String()))
+					} else if tk == reflect.Slice {
+						if elem, ok := val.Field(i).Interface().([]byte); ok {
+							Escape(p, elem)
+						}
+					}
+					continue
+				case "innerxml":
+					iface := val.Field(i).Interface()
+					switch raw := iface.(type) {
+					case []byte:
+						p.Write(raw)
+						continue
+					case string:
+						p.WriteString(raw)
+						continue
+					}
+				case "attr":
+					continue
+				default:
+					name = tag
+				}
+
+				if err := p.marshalValue(val.Field(i), name); err != nil {
+					return err
+				}
+			}
+		}
+	default:
+		return &UnsupportedTypeError{typ}
+	}
+
+	p.WriteByte('<')
+	p.WriteByte('/')
+	p.WriteString(name)
+	p.WriteByte('>')
+
+	return nil
+}
+
+// A MarshalXMLError is returned when Marshal or MarshalIndent encounter a type
+// that cannot be converted into XML.
+type UnsupportedTypeError struct {
+	Type reflect.Type
+}
+
+func (e *UnsupportedTypeError) String() string {
+	return "xml: unsupported type: " + e.Type.String()
+}

src/pkg/xml/marshal_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package xml
+
+import (
+	"reflect"
+	"testing"
+
+	"os"
+	"bytes"
+	"strings"
+	"strconv"
+)
+
+type DriveType int
+
+const (
+	HyperDrive DriveType = iota
+	ImprobabilityDrive
+)
+
+type Passenger struct {
+	Name   []string "name"
+	Weight float32  "weight"
+}
+
+type Ship struct {
+	XMLName Name "spaceship"
+
+	Name      string       "attr"
+	Pilot     string       "attr"
+	Drive     DriveType    "drive"
+	Age       uint         "age"
+	Passenger []*Passenger "passenger"
+	secret    string
+}
+
+type RawXML string
+
+func (rx RawXML) MarshalXML() ([]byte, os.Error) {
+	return []byte(rx), nil
+}
+
+type NamedType string
+
+type Port struct {
+	XMLName Name   "port"
+	Type    string "attr"
+	Number  string "chardata"
+}
+
+type Domain struct {
+	XMLName Name   "domain"
+	Country string "attr"
+	Name    []byte "chardata"
+}
+
+type SecretAgent struct {
+	XMLName   Name   "agent"
+	Handle    string "attr"
+	Identity  string
+	Obfuscate string "innerxml"
+}
+
+var nilStruct *Ship
+
+var marshalTests = []struct {
+	Value     interface{}
+	ExpectXML string
+}{
+	// Test nil marshals to nothing
+	{Value: nil, ExpectXML: ``},
+	{Value: nilStruct, ExpectXML: ``},
+
+	// Test value types (no tag name, so ???)
+	{Value: true, ExpectXML: `<???>true</???>`},
+	{Value: int(42), ExpectXML: `<???>42</???>`},
+	{Value: int8(42), ExpectXML: `<???>42</???>`},
+	{Value: int16(42), ExpectXML: `<???>42</???>`},
+	{Value: int32(42), ExpectXML: `<???>42</???>`},
+	{Value: uint(42), ExpectXML: `<???>42</???>`},
+	{Value: uint8(42), ExpectXML: `<???>42</???>`},
+	{Value: uint16(42), ExpectXML: `<???>42</???>`},
+	{Value: uint32(42), ExpectXML: `<???>42</???>`},
+	{Value: float32(1.25), ExpectXML: `<???>1.25</???>`},
+	{Value: float64(1.25), ExpectXML: `<???>1.25</???>`},
+	{Value: uintptr(0xFFDD), ExpectXML: `<???>65501</???>`},
+	{Value: "gopher", ExpectXML: `<???>gopher</???>`},
+	{Value: []byte("gopher"), ExpectXML: `<???>gopher</???>`},
+	{Value: "</>", ExpectXML: `<???>&lt;/&gt;</???>`},
+	{Value: []byte("</>"), ExpectXML: `<???>&lt;/&gt;</???>`},
+	{Value: [3]byte{'<', '/', '>'}, ExpectXML: `<???>&lt;/&gt;</???>`},
+	{Value: NamedType("potato"), ExpectXML: `<???>potato</???>`},
+	{Value: []int{1, 2, 3}, ExpectXML: `<???>1</???><???>2</???><???>3</???>`},
+	{Value: [3]int{1, 2, 3}, ExpectXML: `<???>1</???><???>2</???><???>3</???>`},
+
+	// Test innerxml
+	{Value: RawXML("</>"), ExpectXML: `</>`},
+	{
+		Value: &SecretAgent{
+			Handle:    "007",
+			Identity:  "James Bond",
+			Obfuscate: "<redacted/>",
+		},
+		//ExpectXML: `<agent handle="007"><redacted/></agent>`,
+		ExpectXML: `<agent handle="007"><Identity>James Bond</Identity><redacted/></agent>`,
+	},
+
+	// Test structs
+	{Value: &Port{Type: "ssl", Number: "443"}, ExpectXML: `<port type="ssl">443</port>`},
+	{Value: &Port{Number: "443"}, ExpectXML: `<port>443</port>`},
+	{Value: &Port{Type: "<unix>"}, ExpectXML: `<port type="&lt;unix&gt;"></port>`},
+	{Value: &Domain{Name: []byte("google.com&friends")}, ExpectXML: `<domain>google.com&amp;friends</domain>`},
+	{Value: atomValue, ExpectXML: atomXml},
+	{
+		Value: &Ship{
+			Name:  "Heart of Gold",
+			Pilot: "Computer",
+			Age:   1,
+			Drive: ImprobabilityDrive,
+			Passenger: []*Passenger{
+				&Passenger{
+					Name:   []string{"Zaphod", "Beeblebrox"},
+					Weight: 7.25,
+				},
+				&Passenger{
+					Name:   []string{"Trisha", "McMillen"},
+					Weight: 5.5,
+				},
+				&Passenger{
+					Name:   []string{"Ford", "Prefect"},
+					Weight: 7,
+				},
+				&Passenger{
+					Name:   []string{"Arthur", "Dent"},
+					Weight: 6.75,
+				},
+			},
+		},
+		ExpectXML: `<spaceship name="Heart of Gold" pilot="Computer">` +
+			`<drive>` + strconv.Itoa(int(ImprobabilityDrive)) + `</drive>` +
+			`<age>1</age>` +
+			`<passenger>` +
+			`<name>Zaphod</name>` +
+			`<name>Beeblebrox</name>` +
+			`<weight>7.25</weight>` +
+			`</passenger>` +
+			`<passenger>` +
+			`<name>Trisha</name>` +
+			`<name>McMillen</name>` +
+			`<weight>5.5</weight>` +
+			`</passenger>` +
+			`<passenger>` +
+			`<name>Ford</name>` +
+			`<name>Prefect</name>` +
+			`<weight>7</weight>` +
+			`</passenger>` +
+			`<passenger>` +
+			`<name>Arthur</name>` +
+			`<name>Dent</name>` +
+			`<weight>6.75</weight>` +
+			`</passenger>` +
+			`</spaceship>`,
+	},
+}
+
+func TestMarshal(t *testing.T) {
+	for idx, test := range marshalTests {
+		buf := bytes.NewBuffer(nil)
+		err := Marshal(buf, test.Value)
+		if err != nil {
+			t.Errorf("#%d: Error: %s", idx, err)
+			continue
+		}
+		if got, want := buf.String(), test.ExpectXML; got != want {
+			if strings.Contains(want, "\n") {
+				t.Errorf("#%d: marshal(%#v) - GOT:\n%s\nWANT:\n%s", idx, test.Value, got, want)
+			} else {
+				t.Errorf("#%d: marshal(%#v) = %#q want %#q", idx, test.Value, got, want)
+			}
+		}
+	}
+}
+
+var marshalErrorTests = []struct {
+	Value      interface{}
+	ExpectErr  string
+	ExpectKind reflect.Kind
+}{
+	{
+		Value:      make(chan bool),
+		ExpectErr:  "xml: unsupported type: chan bool",
+		ExpectKind: reflect.Chan,
+	},
+	{
+		Value: map[string]string{
+			"question": "What do you get when you multiply six by nine?",
+			"answer":   "42",
+		},
+		ExpectErr:  "xml: unsupported type: map[string] string",
+		ExpectKind: reflect.Map,
+	},
+	{
+		Value:      map[*Ship]bool{nil: false},
+		ExpectErr:  "xml: unsupported type: map[*xml.Ship] bool",
+		ExpectKind: reflect.Map,
+	},
+}
+
+func TestMarshalErrors(t *testing.T) {
+	for idx, test := range marshalErrorTests {
+		buf := bytes.NewBuffer(nil)
+		err := Marshal(buf, test.Value)
+		if got, want := err, test.ExpectErr; got == nil {
+			t.Errorf("#%d: want error %s", idx, want)
+			continue
+		} else if got.String() != want {
+			t.Errorf("#%d: marshal(%#v) = [error] %q, want %q", idx, test.Value, got, want)
+		}
+		if got, want := err.(*UnsupportedTypeError).Type.Kind(), test.ExpectKind; got != want {
+			t.Errorf("#%d: marshal(%#v) = [error kind] %s, want %s", idx, test.Value, got, want)
+		}
+	}
+}
+
+// Do invertibility testing on the various structures that we test
+func TestUnmarshal(t *testing.T) {
+	for i, test := range marshalTests {
+		// Skip the nil pointers
+		if i <= 1 {
+			continue
+		}
+
+		var dest interface{}
+
+		switch test.Value.(type) {
+		case *Ship, Ship:
+			dest = &Ship{}
+		case *Port, Port:
+			dest = &Port{}
+		case *Domain, Domain:
+			dest = &Domain{}
+		case *Feed, Feed:
+			dest = &Feed{}
+		default:
+			continue
+		}
+
+		buffer := bytes.NewBufferString(test.ExpectXML)
+		err := Unmarshal(buffer, dest)
+
+		// Don't compare XMLNames
+		switch fix := dest.(type) {
+		case *Ship:
+			fix.XMLName = Name{}
+		case *Port:
+			fix.XMLName = Name{}
+		case *Domain:
+			fix.XMLName = Name{}
+		case *Feed:
+			fix.XMLName = Name{}
+			fix.Author.InnerXML = ""
+			for i := range fix.Entry {
+				fix.Entry[i].Author.InnerXML = ""
+			}
+		}
+
+		if err != nil {
+			t.Errorf("#%d: unexpected error: %#v", i, err)
+		} else if got, want := dest, test.Value; !reflect.DeepEqual(got, want) {
+			t.Errorf("#%d: unmarshal(%#s) = %#v, want %#v", i, test.ExpectXML, got, want)
+		}
+	}
+}
+
+func BenchmarkMarshal(b *testing.B) {
+	idx := len(marshalTests) - 1
+	test := marshalTests[idx]
+
+	buf := bytes.NewBuffer(nil)
+	for i := 0; i < b.N; i++ {
+		Marshal(buf, test.Value)
+		buf.Truncate(0)
+	}
+}
+
+func BenchmarkUnmarshal(b *testing.B) {
+	idx := len(marshalTests) - 1
+	test := marshalTests[idx]
+	sm := &Ship{}
+	xml := []byte(test.ExpectXML)
+
+	for i := 0; i < b.N; i++ {
+		buffer := bytes.NewBuffer(xml)
+		Unmarshal(buffer, sm)
+	}
+}