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Source file src/encoding/xml/marshal.go

Documentation: encoding/xml

     1  // Copyright 2011 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package xml
     6  
     7  import (
     8  	"bufio"
     9  	"bytes"
    10  	"encoding"
    11  	"fmt"
    12  	"io"
    13  	"reflect"
    14  	"strconv"
    15  	"strings"
    16  )
    17  
    18  const (
    19  	// Header is a generic XML header suitable for use with the output of Marshal.
    20  	// This is not automatically added to any output of this package,
    21  	// it is provided as a convenience.
    22  	Header = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"
    23  )
    24  
    25  // Marshal returns the XML encoding of v.
    26  //
    27  // Marshal handles an array or slice by marshaling each of the elements.
    28  // Marshal handles a pointer by marshaling the value it points at or, if the
    29  // pointer is nil, by writing nothing. Marshal handles an interface value by
    30  // marshaling the value it contains or, if the interface value is nil, by
    31  // writing nothing. Marshal handles all other data by writing one or more XML
    32  // elements containing the data.
    33  //
    34  // The name for the XML elements is taken from, in order of preference:
    35  //     - the tag on the XMLName field, if the data is a struct
    36  //     - the value of the XMLName field of type Name
    37  //     - the tag of the struct field used to obtain the data
    38  //     - the name of the struct field used to obtain the data
    39  //     - the name of the marshaled type
    40  //
    41  // The XML element for a struct contains marshaled elements for each of the
    42  // exported fields of the struct, with these exceptions:
    43  //     - the XMLName field, described above, is omitted.
    44  //     - a field with tag "-" is omitted.
    45  //     - a field with tag "name,attr" becomes an attribute with
    46  //       the given name in the XML element.
    47  //     - a field with tag ",attr" becomes an attribute with the
    48  //       field name in the XML element.
    49  //     - a field with tag ",chardata" is written as character data,
    50  //       not as an XML element.
    51  //     - a field with tag ",cdata" is written as character data
    52  //       wrapped in one or more <![CDATA[ ... ]]> tags, not as an XML element.
    53  //     - a field with tag ",innerxml" is written verbatim, not subject
    54  //       to the usual marshaling procedure.
    55  //     - a field with tag ",comment" is written as an XML comment, not
    56  //       subject to the usual marshaling procedure. It must not contain
    57  //       the "--" string within it.
    58  //     - a field with a tag including the "omitempty" option is omitted
    59  //       if the field value is empty. The empty values are false, 0, any
    60  //       nil pointer or interface value, and any array, slice, map, or
    61  //       string of length zero.
    62  //     - an anonymous struct field is handled as if the fields of its
    63  //       value were part of the outer struct.
    64  //     - a field implementing Marshaler is written by calling its MarshalXML
    65  //       method.
    66  //     - a field implementing encoding.TextMarshaler is written by encoding the
    67  //       result of its MarshalText method as text.
    68  //
    69  // If a field uses a tag "a>b>c", then the element c will be nested inside
    70  // parent elements a and b. Fields that appear next to each other that name
    71  // the same parent will be enclosed in one XML element.
    72  //
    73  // If the XML name for a struct field is defined by both the field tag and the
    74  // struct's XMLName field, the names must match.
    75  //
    76  // See MarshalIndent for an example.
    77  //
    78  // Marshal will return an error if asked to marshal a channel, function, or map.
    79  func Marshal(v interface{}) ([]byte, error) {
    80  	var b bytes.Buffer
    81  	if err := NewEncoder(&b).Encode(v); err != nil {
    82  		return nil, err
    83  	}
    84  	return b.Bytes(), nil
    85  }
    86  
    87  // Marshaler is the interface implemented by objects that can marshal
    88  // themselves into valid XML elements.
    89  //
    90  // MarshalXML encodes the receiver as zero or more XML elements.
    91  // By convention, arrays or slices are typically encoded as a sequence
    92  // of elements, one per entry.
    93  // Using start as the element tag is not required, but doing so
    94  // will enable Unmarshal to match the XML elements to the correct
    95  // struct field.
    96  // One common implementation strategy is to construct a separate
    97  // value with a layout corresponding to the desired XML and then
    98  // to encode it using e.EncodeElement.
    99  // Another common strategy is to use repeated calls to e.EncodeToken
   100  // to generate the XML output one token at a time.
   101  // The sequence of encoded tokens must make up zero or more valid
   102  // XML elements.
   103  type Marshaler interface {
   104  	MarshalXML(e *Encoder, start StartElement) error
   105  }
   106  
   107  // MarshalerAttr is the interface implemented by objects that can marshal
   108  // themselves into valid XML attributes.
   109  //
   110  // MarshalXMLAttr returns an XML attribute with the encoded value of the receiver.
   111  // Using name as the attribute name is not required, but doing so
   112  // will enable Unmarshal to match the attribute to the correct
   113  // struct field.
   114  // If MarshalXMLAttr returns the zero attribute Attr{}, no attribute
   115  // will be generated in the output.
   116  // MarshalXMLAttr is used only for struct fields with the
   117  // "attr" option in the field tag.
   118  type MarshalerAttr interface {
   119  	MarshalXMLAttr(name Name) (Attr, error)
   120  }
   121  
   122  // MarshalIndent works like Marshal, but each XML element begins on a new
   123  // indented line that starts with prefix and is followed by one or more
   124  // copies of indent according to the nesting depth.
   125  func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
   126  	var b bytes.Buffer
   127  	enc := NewEncoder(&b)
   128  	enc.Indent(prefix, indent)
   129  	if err := enc.Encode(v); err != nil {
   130  		return nil, err
   131  	}
   132  	return b.Bytes(), nil
   133  }
   134  
   135  // An Encoder writes XML data to an output stream.
   136  type Encoder struct {
   137  	p printer
   138  }
   139  
   140  // NewEncoder returns a new encoder that writes to w.
   141  func NewEncoder(w io.Writer) *Encoder {
   142  	e := &Encoder{printer{Writer: bufio.NewWriter(w)}}
   143  	e.p.encoder = e
   144  	return e
   145  }
   146  
   147  // Indent sets the encoder to generate XML in which each element
   148  // begins on a new indented line that starts with prefix and is followed by
   149  // one or more copies of indent according to the nesting depth.
   150  func (enc *Encoder) Indent(prefix, indent string) {
   151  	enc.p.prefix = prefix
   152  	enc.p.indent = indent
   153  }
   154  
   155  // Encode writes the XML encoding of v to the stream.
   156  //
   157  // See the documentation for Marshal for details about the conversion
   158  // of Go values to XML.
   159  //
   160  // Encode calls Flush before returning.
   161  func (enc *Encoder) Encode(v interface{}) error {
   162  	err := enc.p.marshalValue(reflect.ValueOf(v), nil, nil)
   163  	if err != nil {
   164  		return err
   165  	}
   166  	return enc.p.Flush()
   167  }
   168  
   169  // EncodeElement writes the XML encoding of v to the stream,
   170  // using start as the outermost tag in the encoding.
   171  //
   172  // See the documentation for Marshal for details about the conversion
   173  // of Go values to XML.
   174  //
   175  // EncodeElement calls Flush before returning.
   176  func (enc *Encoder) EncodeElement(v interface{}, start StartElement) error {
   177  	err := enc.p.marshalValue(reflect.ValueOf(v), nil, &start)
   178  	if err != nil {
   179  		return err
   180  	}
   181  	return enc.p.Flush()
   182  }
   183  
   184  var (
   185  	begComment  = []byte("<!--")
   186  	endComment  = []byte("-->")
   187  	endProcInst = []byte("?>")
   188  )
   189  
   190  // EncodeToken writes the given XML token to the stream.
   191  // It returns an error if StartElement and EndElement tokens are not properly matched.
   192  //
   193  // EncodeToken does not call Flush, because usually it is part of a larger operation
   194  // such as Encode or EncodeElement (or a custom Marshaler's MarshalXML invoked
   195  // during those), and those will call Flush when finished.
   196  // Callers that create an Encoder and then invoke EncodeToken directly, without
   197  // using Encode or EncodeElement, need to call Flush when finished to ensure
   198  // that the XML is written to the underlying writer.
   199  //
   200  // EncodeToken allows writing a ProcInst with Target set to "xml" only as the first token
   201  // in the stream.
   202  func (enc *Encoder) EncodeToken(t Token) error {
   203  
   204  	p := &enc.p
   205  	switch t := t.(type) {
   206  	case StartElement:
   207  		if err := p.writeStart(&t); err != nil {
   208  			return err
   209  		}
   210  	case EndElement:
   211  		if err := p.writeEnd(t.Name); err != nil {
   212  			return err
   213  		}
   214  	case CharData:
   215  		escapeText(p, t, false)
   216  	case Comment:
   217  		if bytes.Contains(t, endComment) {
   218  			return fmt.Errorf("xml: EncodeToken of Comment containing --> marker")
   219  		}
   220  		p.WriteString("<!--")
   221  		p.Write(t)
   222  		p.WriteString("-->")
   223  		return p.cachedWriteError()
   224  	case ProcInst:
   225  		// First token to be encoded which is also a ProcInst with target of xml
   226  		// is the xml declaration. The only ProcInst where target of xml is allowed.
   227  		if t.Target == "xml" && p.Buffered() != 0 {
   228  			return fmt.Errorf("xml: EncodeToken of ProcInst xml target only valid for xml declaration, first token encoded")
   229  		}
   230  		if !isNameString(t.Target) {
   231  			return fmt.Errorf("xml: EncodeToken of ProcInst with invalid Target")
   232  		}
   233  		if bytes.Contains(t.Inst, endProcInst) {
   234  			return fmt.Errorf("xml: EncodeToken of ProcInst containing ?> marker")
   235  		}
   236  		p.WriteString("<?")
   237  		p.WriteString(t.Target)
   238  		if len(t.Inst) > 0 {
   239  			p.WriteByte(' ')
   240  			p.Write(t.Inst)
   241  		}
   242  		p.WriteString("?>")
   243  	case Directive:
   244  		if !isValidDirective(t) {
   245  			return fmt.Errorf("xml: EncodeToken of Directive containing wrong < or > markers")
   246  		}
   247  		p.WriteString("<!")
   248  		p.Write(t)
   249  		p.WriteString(">")
   250  	default:
   251  		return fmt.Errorf("xml: EncodeToken of invalid token type")
   252  
   253  	}
   254  	return p.cachedWriteError()
   255  }
   256  
   257  // isValidDirective reports whether dir is a valid directive text,
   258  // meaning angle brackets are matched, ignoring comments and strings.
   259  func isValidDirective(dir Directive) bool {
   260  	var (
   261  		depth     int
   262  		inquote   uint8
   263  		incomment bool
   264  	)
   265  	for i, c := range dir {
   266  		switch {
   267  		case incomment:
   268  			if c == '>' {
   269  				if n := 1 + i - len(endComment); n >= 0 && bytes.Equal(dir[n:i+1], endComment) {
   270  					incomment = false
   271  				}
   272  			}
   273  			// Just ignore anything in comment
   274  		case inquote != 0:
   275  			if c == inquote {
   276  				inquote = 0
   277  			}
   278  			// Just ignore anything within quotes
   279  		case c == '\'' || c == '"':
   280  			inquote = c
   281  		case c == '<':
   282  			if i+len(begComment) < len(dir) && bytes.Equal(dir[i:i+len(begComment)], begComment) {
   283  				incomment = true
   284  			} else {
   285  				depth++
   286  			}
   287  		case c == '>':
   288  			if depth == 0 {
   289  				return false
   290  			}
   291  			depth--
   292  		}
   293  	}
   294  	return depth == 0 && inquote == 0 && !incomment
   295  }
   296  
   297  // Flush flushes any buffered XML to the underlying writer.
   298  // See the EncodeToken documentation for details about when it is necessary.
   299  func (enc *Encoder) Flush() error {
   300  	return enc.p.Flush()
   301  }
   302  
   303  type printer struct {
   304  	*bufio.Writer
   305  	encoder    *Encoder
   306  	seq        int
   307  	indent     string
   308  	prefix     string
   309  	depth      int
   310  	indentedIn bool
   311  	putNewline bool
   312  	attrNS     map[string]string // map prefix -> name space
   313  	attrPrefix map[string]string // map name space -> prefix
   314  	prefixes   []string
   315  	tags       []Name
   316  }
   317  
   318  // createAttrPrefix finds the name space prefix attribute to use for the given name space,
   319  // defining a new prefix if necessary. It returns the prefix.
   320  func (p *printer) createAttrPrefix(url string) string {
   321  	if prefix := p.attrPrefix[url]; prefix != "" {
   322  		return prefix
   323  	}
   324  
   325  	// The "http://www.w3.org/XML/1998/namespace" name space is predefined as "xml"
   326  	// and must be referred to that way.
   327  	// (The "http://www.w3.org/2000/xmlns/" name space is also predefined as "xmlns",
   328  	// but users should not be trying to use that one directly - that's our job.)
   329  	if url == xmlURL {
   330  		return xmlPrefix
   331  	}
   332  
   333  	// Need to define a new name space.
   334  	if p.attrPrefix == nil {
   335  		p.attrPrefix = make(map[string]string)
   336  		p.attrNS = make(map[string]string)
   337  	}
   338  
   339  	// Pick a name. We try to use the final element of the path
   340  	// but fall back to _.
   341  	prefix := strings.TrimRight(url, "/")
   342  	if i := strings.LastIndex(prefix, "/"); i >= 0 {
   343  		prefix = prefix[i+1:]
   344  	}
   345  	if prefix == "" || !isName([]byte(prefix)) || strings.Contains(prefix, ":") {
   346  		prefix = "_"
   347  	}
   348  	if strings.HasPrefix(prefix, "xml") {
   349  		// xmlanything is reserved.
   350  		prefix = "_" + prefix
   351  	}
   352  	if p.attrNS[prefix] != "" {
   353  		// Name is taken. Find a better one.
   354  		for p.seq++; ; p.seq++ {
   355  			if id := prefix + "_" + strconv.Itoa(p.seq); p.attrNS[id] == "" {
   356  				prefix = id
   357  				break
   358  			}
   359  		}
   360  	}
   361  
   362  	p.attrPrefix[url] = prefix
   363  	p.attrNS[prefix] = url
   364  
   365  	p.WriteString(`xmlns:`)
   366  	p.WriteString(prefix)
   367  	p.WriteString(`="`)
   368  	EscapeText(p, []byte(url))
   369  	p.WriteString(`" `)
   370  
   371  	p.prefixes = append(p.prefixes, prefix)
   372  
   373  	return prefix
   374  }
   375  
   376  // deleteAttrPrefix removes an attribute name space prefix.
   377  func (p *printer) deleteAttrPrefix(prefix string) {
   378  	delete(p.attrPrefix, p.attrNS[prefix])
   379  	delete(p.attrNS, prefix)
   380  }
   381  
   382  func (p *printer) markPrefix() {
   383  	p.prefixes = append(p.prefixes, "")
   384  }
   385  
   386  func (p *printer) popPrefix() {
   387  	for len(p.prefixes) > 0 {
   388  		prefix := p.prefixes[len(p.prefixes)-1]
   389  		p.prefixes = p.prefixes[:len(p.prefixes)-1]
   390  		if prefix == "" {
   391  			break
   392  		}
   393  		p.deleteAttrPrefix(prefix)
   394  	}
   395  }
   396  
   397  var (
   398  	marshalerType     = reflect.TypeOf((*Marshaler)(nil)).Elem()
   399  	marshalerAttrType = reflect.TypeOf((*MarshalerAttr)(nil)).Elem()
   400  	textMarshalerType = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
   401  )
   402  
   403  // marshalValue writes one or more XML elements representing val.
   404  // If val was obtained from a struct field, finfo must have its details.
   405  func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplate *StartElement) error {
   406  	if startTemplate != nil && startTemplate.Name.Local == "" {
   407  		return fmt.Errorf("xml: EncodeElement of StartElement with missing name")
   408  	}
   409  
   410  	if !val.IsValid() {
   411  		return nil
   412  	}
   413  	if finfo != nil && finfo.flags&fOmitEmpty != 0 && isEmptyValue(val) {
   414  		return nil
   415  	}
   416  
   417  	// Drill into interfaces and pointers.
   418  	// This can turn into an infinite loop given a cyclic chain,
   419  	// but it matches the Go 1 behavior.
   420  	for val.Kind() == reflect.Interface || val.Kind() == reflect.Ptr {
   421  		if val.IsNil() {
   422  			return nil
   423  		}
   424  		val = val.Elem()
   425  	}
   426  
   427  	kind := val.Kind()
   428  	typ := val.Type()
   429  
   430  	// Check for marshaler.
   431  	if val.CanInterface() && typ.Implements(marshalerType) {
   432  		return p.marshalInterface(val.Interface().(Marshaler), defaultStart(typ, finfo, startTemplate))
   433  	}
   434  	if val.CanAddr() {
   435  		pv := val.Addr()
   436  		if pv.CanInterface() && pv.Type().Implements(marshalerType) {
   437  			return p.marshalInterface(pv.Interface().(Marshaler), defaultStart(pv.Type(), finfo, startTemplate))
   438  		}
   439  	}
   440  
   441  	// Check for text marshaler.
   442  	if val.CanInterface() && typ.Implements(textMarshalerType) {
   443  		return p.marshalTextInterface(val.Interface().(encoding.TextMarshaler), defaultStart(typ, finfo, startTemplate))
   444  	}
   445  	if val.CanAddr() {
   446  		pv := val.Addr()
   447  		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
   448  			return p.marshalTextInterface(pv.Interface().(encoding.TextMarshaler), defaultStart(pv.Type(), finfo, startTemplate))
   449  		}
   450  	}
   451  
   452  	// Slices and arrays iterate over the elements. They do not have an enclosing tag.
   453  	if (kind == reflect.Slice || kind == reflect.Array) && typ.Elem().Kind() != reflect.Uint8 {
   454  		for i, n := 0, val.Len(); i < n; i++ {
   455  			if err := p.marshalValue(val.Index(i), finfo, startTemplate); err != nil {
   456  				return err
   457  			}
   458  		}
   459  		return nil
   460  	}
   461  
   462  	tinfo, err := getTypeInfo(typ)
   463  	if err != nil {
   464  		return err
   465  	}
   466  
   467  	// Create start element.
   468  	// Precedence for the XML element name is:
   469  	// 0. startTemplate
   470  	// 1. XMLName field in underlying struct;
   471  	// 2. field name/tag in the struct field; and
   472  	// 3. type name
   473  	var start StartElement
   474  
   475  	if startTemplate != nil {
   476  		start.Name = startTemplate.Name
   477  		start.Attr = append(start.Attr, startTemplate.Attr...)
   478  	} else if tinfo.xmlname != nil {
   479  		xmlname := tinfo.xmlname
   480  		if xmlname.name != "" {
   481  			start.Name.Space, start.Name.Local = xmlname.xmlns, xmlname.name
   482  		} else {
   483  			fv := xmlname.value(val, dontInitNilPointers)
   484  			if v, ok := fv.Interface().(Name); ok && v.Local != "" {
   485  				start.Name = v
   486  			}
   487  		}
   488  	}
   489  	if start.Name.Local == "" && finfo != nil {
   490  		start.Name.Space, start.Name.Local = finfo.xmlns, finfo.name
   491  	}
   492  	if start.Name.Local == "" {
   493  		name := typ.Name()
   494  		if name == "" {
   495  			return &UnsupportedTypeError{typ}
   496  		}
   497  		start.Name.Local = name
   498  	}
   499  
   500  	// Attributes
   501  	for i := range tinfo.fields {
   502  		finfo := &tinfo.fields[i]
   503  		if finfo.flags&fAttr == 0 {
   504  			continue
   505  		}
   506  		fv := finfo.value(val, dontInitNilPointers)
   507  
   508  		if finfo.flags&fOmitEmpty != 0 && isEmptyValue(fv) {
   509  			continue
   510  		}
   511  
   512  		if fv.Kind() == reflect.Interface && fv.IsNil() {
   513  			continue
   514  		}
   515  
   516  		name := Name{Space: finfo.xmlns, Local: finfo.name}
   517  		if err := p.marshalAttr(&start, name, fv); err != nil {
   518  			return err
   519  		}
   520  	}
   521  
   522  	if err := p.writeStart(&start); err != nil {
   523  		return err
   524  	}
   525  
   526  	if val.Kind() == reflect.Struct {
   527  		err = p.marshalStruct(tinfo, val)
   528  	} else {
   529  		s, b, err1 := p.marshalSimple(typ, val)
   530  		if err1 != nil {
   531  			err = err1
   532  		} else if b != nil {
   533  			EscapeText(p, b)
   534  		} else {
   535  			p.EscapeString(s)
   536  		}
   537  	}
   538  	if err != nil {
   539  		return err
   540  	}
   541  
   542  	if err := p.writeEnd(start.Name); err != nil {
   543  		return err
   544  	}
   545  
   546  	return p.cachedWriteError()
   547  }
   548  
   549  // marshalAttr marshals an attribute with the given name and value, adding to start.Attr.
   550  func (p *printer) marshalAttr(start *StartElement, name Name, val reflect.Value) error {
   551  	if val.CanInterface() && val.Type().Implements(marshalerAttrType) {
   552  		attr, err := val.Interface().(MarshalerAttr).MarshalXMLAttr(name)
   553  		if err != nil {
   554  			return err
   555  		}
   556  		if attr.Name.Local != "" {
   557  			start.Attr = append(start.Attr, attr)
   558  		}
   559  		return nil
   560  	}
   561  
   562  	if val.CanAddr() {
   563  		pv := val.Addr()
   564  		if pv.CanInterface() && pv.Type().Implements(marshalerAttrType) {
   565  			attr, err := pv.Interface().(MarshalerAttr).MarshalXMLAttr(name)
   566  			if err != nil {
   567  				return err
   568  			}
   569  			if attr.Name.Local != "" {
   570  				start.Attr = append(start.Attr, attr)
   571  			}
   572  			return nil
   573  		}
   574  	}
   575  
   576  	if val.CanInterface() && val.Type().Implements(textMarshalerType) {
   577  		text, err := val.Interface().(encoding.TextMarshaler).MarshalText()
   578  		if err != nil {
   579  			return err
   580  		}
   581  		start.Attr = append(start.Attr, Attr{name, string(text)})
   582  		return nil
   583  	}
   584  
   585  	if val.CanAddr() {
   586  		pv := val.Addr()
   587  		if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
   588  			text, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
   589  			if err != nil {
   590  				return err
   591  			}
   592  			start.Attr = append(start.Attr, Attr{name, string(text)})
   593  			return nil
   594  		}
   595  	}
   596  
   597  	// Dereference or skip nil pointer, interface values.
   598  	switch val.Kind() {
   599  	case reflect.Ptr, reflect.Interface:
   600  		if val.IsNil() {
   601  			return nil
   602  		}
   603  		val = val.Elem()
   604  	}
   605  
   606  	// Walk slices.
   607  	if val.Kind() == reflect.Slice && val.Type().Elem().Kind() != reflect.Uint8 {
   608  		n := val.Len()
   609  		for i := 0; i < n; i++ {
   610  			if err := p.marshalAttr(start, name, val.Index(i)); err != nil {
   611  				return err
   612  			}
   613  		}
   614  		return nil
   615  	}
   616  
   617  	if val.Type() == attrType {
   618  		start.Attr = append(start.Attr, val.Interface().(Attr))
   619  		return nil
   620  	}
   621  
   622  	s, b, err := p.marshalSimple(val.Type(), val)
   623  	if err != nil {
   624  		return err
   625  	}
   626  	if b != nil {
   627  		s = string(b)
   628  	}
   629  	start.Attr = append(start.Attr, Attr{name, s})
   630  	return nil
   631  }
   632  
   633  // defaultStart returns the default start element to use,
   634  // given the reflect type, field info, and start template.
   635  func defaultStart(typ reflect.Type, finfo *fieldInfo, startTemplate *StartElement) StartElement {
   636  	var start StartElement
   637  	// Precedence for the XML element name is as above,
   638  	// except that we do not look inside structs for the first field.
   639  	if startTemplate != nil {
   640  		start.Name = startTemplate.Name
   641  		start.Attr = append(start.Attr, startTemplate.Attr...)
   642  	} else if finfo != nil && finfo.name != "" {
   643  		start.Name.Local = finfo.name
   644  		start.Name.Space = finfo.xmlns
   645  	} else if typ.Name() != "" {
   646  		start.Name.Local = typ.Name()
   647  	} else {
   648  		// Must be a pointer to a named type,
   649  		// since it has the Marshaler methods.
   650  		start.Name.Local = typ.Elem().Name()
   651  	}
   652  	return start
   653  }
   654  
   655  // marshalInterface marshals a Marshaler interface value.
   656  func (p *printer) marshalInterface(val Marshaler, start StartElement) error {
   657  	// Push a marker onto the tag stack so that MarshalXML
   658  	// cannot close the XML tags that it did not open.
   659  	p.tags = append(p.tags, Name{})
   660  	n := len(p.tags)
   661  
   662  	err := val.MarshalXML(p.encoder, start)
   663  	if err != nil {
   664  		return err
   665  	}
   666  
   667  	// Make sure MarshalXML closed all its tags. p.tags[n-1] is the mark.
   668  	if len(p.tags) > n {
   669  		return fmt.Errorf("xml: %s.MarshalXML wrote invalid XML: <%s> not closed", receiverType(val), p.tags[len(p.tags)-1].Local)
   670  	}
   671  	p.tags = p.tags[:n-1]
   672  	return nil
   673  }
   674  
   675  // marshalTextInterface marshals a TextMarshaler interface value.
   676  func (p *printer) marshalTextInterface(val encoding.TextMarshaler, start StartElement) error {
   677  	if err := p.writeStart(&start); err != nil {
   678  		return err
   679  	}
   680  	text, err := val.MarshalText()
   681  	if err != nil {
   682  		return err
   683  	}
   684  	EscapeText(p, text)
   685  	return p.writeEnd(start.Name)
   686  }
   687  
   688  // writeStart writes the given start element.
   689  func (p *printer) writeStart(start *StartElement) error {
   690  	if start.Name.Local == "" {
   691  		return fmt.Errorf("xml: start tag with no name")
   692  	}
   693  
   694  	p.tags = append(p.tags, start.Name)
   695  	p.markPrefix()
   696  
   697  	p.writeIndent(1)
   698  	p.WriteByte('<')
   699  	p.WriteString(start.Name.Local)
   700  
   701  	if start.Name.Space != "" {
   702  		p.WriteString(` xmlns="`)
   703  		p.EscapeString(start.Name.Space)
   704  		p.WriteByte('"')
   705  	}
   706  
   707  	// Attributes
   708  	for _, attr := range start.Attr {
   709  		name := attr.Name
   710  		if name.Local == "" {
   711  			continue
   712  		}
   713  		p.WriteByte(' ')
   714  		if name.Space != "" {
   715  			p.WriteString(p.createAttrPrefix(name.Space))
   716  			p.WriteByte(':')
   717  		}
   718  		p.WriteString(name.Local)
   719  		p.WriteString(`="`)
   720  		p.EscapeString(attr.Value)
   721  		p.WriteByte('"')
   722  	}
   723  	p.WriteByte('>')
   724  	return nil
   725  }
   726  
   727  func (p *printer) writeEnd(name Name) error {
   728  	if name.Local == "" {
   729  		return fmt.Errorf("xml: end tag with no name")
   730  	}
   731  	if len(p.tags) == 0 || p.tags[len(p.tags)-1].Local == "" {
   732  		return fmt.Errorf("xml: end tag </%s> without start tag", name.Local)
   733  	}
   734  	if top := p.tags[len(p.tags)-1]; top != name {
   735  		if top.Local != name.Local {
   736  			return fmt.Errorf("xml: end tag </%s> does not match start tag <%s>", name.Local, top.Local)
   737  		}
   738  		return fmt.Errorf("xml: end tag </%s> in namespace %s does not match start tag <%s> in namespace %s", name.Local, name.Space, top.Local, top.Space)
   739  	}
   740  	p.tags = p.tags[:len(p.tags)-1]
   741  
   742  	p.writeIndent(-1)
   743  	p.WriteByte('<')
   744  	p.WriteByte('/')
   745  	p.WriteString(name.Local)
   746  	p.WriteByte('>')
   747  	p.popPrefix()
   748  	return nil
   749  }
   750  
   751  func (p *printer) marshalSimple(typ reflect.Type, val reflect.Value) (string, []byte, error) {
   752  	switch val.Kind() {
   753  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   754  		return strconv.FormatInt(val.Int(), 10), nil, nil
   755  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   756  		return strconv.FormatUint(val.Uint(), 10), nil, nil
   757  	case reflect.Float32, reflect.Float64:
   758  		return strconv.FormatFloat(val.Float(), 'g', -1, val.Type().Bits()), nil, nil
   759  	case reflect.String:
   760  		return val.String(), nil, nil
   761  	case reflect.Bool:
   762  		return strconv.FormatBool(val.Bool()), nil, nil
   763  	case reflect.Array:
   764  		if typ.Elem().Kind() != reflect.Uint8 {
   765  			break
   766  		}
   767  		// [...]byte
   768  		var bytes []byte
   769  		if val.CanAddr() {
   770  			bytes = val.Slice(0, val.Len()).Bytes()
   771  		} else {
   772  			bytes = make([]byte, val.Len())
   773  			reflect.Copy(reflect.ValueOf(bytes), val)
   774  		}
   775  		return "", bytes, nil
   776  	case reflect.Slice:
   777  		if typ.Elem().Kind() != reflect.Uint8 {
   778  			break
   779  		}
   780  		// []byte
   781  		return "", val.Bytes(), nil
   782  	}
   783  	return "", nil, &UnsupportedTypeError{typ}
   784  }
   785  
   786  var ddBytes = []byte("--")
   787  
   788  // indirect drills into interfaces and pointers, returning the pointed-at value.
   789  // If it encounters a nil interface or pointer, indirect returns that nil value.
   790  // This can turn into an infinite loop given a cyclic chain,
   791  // but it matches the Go 1 behavior.
   792  func indirect(vf reflect.Value) reflect.Value {
   793  	for vf.Kind() == reflect.Interface || vf.Kind() == reflect.Ptr {
   794  		if vf.IsNil() {
   795  			return vf
   796  		}
   797  		vf = vf.Elem()
   798  	}
   799  	return vf
   800  }
   801  
   802  func (p *printer) marshalStruct(tinfo *typeInfo, val reflect.Value) error {
   803  	s := parentStack{p: p}
   804  	for i := range tinfo.fields {
   805  		finfo := &tinfo.fields[i]
   806  		if finfo.flags&fAttr != 0 {
   807  			continue
   808  		}
   809  		vf := finfo.value(val, dontInitNilPointers)
   810  		if !vf.IsValid() {
   811  			// The field is behind an anonymous struct field that's
   812  			// nil. Skip it.
   813  			continue
   814  		}
   815  
   816  		switch finfo.flags & fMode {
   817  		case fCDATA, fCharData:
   818  			emit := EscapeText
   819  			if finfo.flags&fMode == fCDATA {
   820  				emit = emitCDATA
   821  			}
   822  			if err := s.trim(finfo.parents); err != nil {
   823  				return err
   824  			}
   825  			if vf.CanInterface() && vf.Type().Implements(textMarshalerType) {
   826  				data, err := vf.Interface().(encoding.TextMarshaler).MarshalText()
   827  				if err != nil {
   828  					return err
   829  				}
   830  				if err := emit(p, data); err != nil {
   831  					return err
   832  				}
   833  				continue
   834  			}
   835  			if vf.CanAddr() {
   836  				pv := vf.Addr()
   837  				if pv.CanInterface() && pv.Type().Implements(textMarshalerType) {
   838  					data, err := pv.Interface().(encoding.TextMarshaler).MarshalText()
   839  					if err != nil {
   840  						return err
   841  					}
   842  					if err := emit(p, data); err != nil {
   843  						return err
   844  					}
   845  					continue
   846  				}
   847  			}
   848  
   849  			var scratch [64]byte
   850  			vf = indirect(vf)
   851  			switch vf.Kind() {
   852  			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   853  				if err := emit(p, strconv.AppendInt(scratch[:0], vf.Int(), 10)); err != nil {
   854  					return err
   855  				}
   856  			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   857  				if err := emit(p, strconv.AppendUint(scratch[:0], vf.Uint(), 10)); err != nil {
   858  					return err
   859  				}
   860  			case reflect.Float32, reflect.Float64:
   861  				if err := emit(p, strconv.AppendFloat(scratch[:0], vf.Float(), 'g', -1, vf.Type().Bits())); err != nil {
   862  					return err
   863  				}
   864  			case reflect.Bool:
   865  				if err := emit(p, strconv.AppendBool(scratch[:0], vf.Bool())); err != nil {
   866  					return err
   867  				}
   868  			case reflect.String:
   869  				if err := emit(p, []byte(vf.String())); err != nil {
   870  					return err
   871  				}
   872  			case reflect.Slice:
   873  				if elem, ok := vf.Interface().([]byte); ok {
   874  					if err := emit(p, elem); err != nil {
   875  						return err
   876  					}
   877  				}
   878  			}
   879  			continue
   880  
   881  		case fComment:
   882  			if err := s.trim(finfo.parents); err != nil {
   883  				return err
   884  			}
   885  			vf = indirect(vf)
   886  			k := vf.Kind()
   887  			if !(k == reflect.String || k == reflect.Slice && vf.Type().Elem().Kind() == reflect.Uint8) {
   888  				return fmt.Errorf("xml: bad type for comment field of %s", val.Type())
   889  			}
   890  			if vf.Len() == 0 {
   891  				continue
   892  			}
   893  			p.writeIndent(0)
   894  			p.WriteString("<!--")
   895  			dashDash := false
   896  			dashLast := false
   897  			switch k {
   898  			case reflect.String:
   899  				s := vf.String()
   900  				dashDash = strings.Contains(s, "--")
   901  				dashLast = s[len(s)-1] == '-'
   902  				if !dashDash {
   903  					p.WriteString(s)
   904  				}
   905  			case reflect.Slice:
   906  				b := vf.Bytes()
   907  				dashDash = bytes.Contains(b, ddBytes)
   908  				dashLast = b[len(b)-1] == '-'
   909  				if !dashDash {
   910  					p.Write(b)
   911  				}
   912  			default:
   913  				panic("can't happen")
   914  			}
   915  			if dashDash {
   916  				return fmt.Errorf(`xml: comments must not contain "--"`)
   917  			}
   918  			if dashLast {
   919  				// "--->" is invalid grammar. Make it "- -->"
   920  				p.WriteByte(' ')
   921  			}
   922  			p.WriteString("-->")
   923  			continue
   924  
   925  		case fInnerXML:
   926  			vf = indirect(vf)
   927  			iface := vf.Interface()
   928  			switch raw := iface.(type) {
   929  			case []byte:
   930  				p.Write(raw)
   931  				continue
   932  			case string:
   933  				p.WriteString(raw)
   934  				continue
   935  			}
   936  
   937  		case fElement, fElement | fAny:
   938  			if err := s.trim(finfo.parents); err != nil {
   939  				return err
   940  			}
   941  			if len(finfo.parents) > len(s.stack) {
   942  				if vf.Kind() != reflect.Ptr && vf.Kind() != reflect.Interface || !vf.IsNil() {
   943  					if err := s.push(finfo.parents[len(s.stack):]); err != nil {
   944  						return err
   945  					}
   946  				}
   947  			}
   948  		}
   949  		if err := p.marshalValue(vf, finfo, nil); err != nil {
   950  			return err
   951  		}
   952  	}
   953  	s.trim(nil)
   954  	return p.cachedWriteError()
   955  }
   956  
   957  // return the bufio Writer's cached write error
   958  func (p *printer) cachedWriteError() error {
   959  	_, err := p.Write(nil)
   960  	return err
   961  }
   962  
   963  func (p *printer) writeIndent(depthDelta int) {
   964  	if len(p.prefix) == 0 && len(p.indent) == 0 {
   965  		return
   966  	}
   967  	if depthDelta < 0 {
   968  		p.depth--
   969  		if p.indentedIn {
   970  			p.indentedIn = false
   971  			return
   972  		}
   973  		p.indentedIn = false
   974  	}
   975  	if p.putNewline {
   976  		p.WriteByte('\n')
   977  	} else {
   978  		p.putNewline = true
   979  	}
   980  	if len(p.prefix) > 0 {
   981  		p.WriteString(p.prefix)
   982  	}
   983  	if len(p.indent) > 0 {
   984  		for i := 0; i < p.depth; i++ {
   985  			p.WriteString(p.indent)
   986  		}
   987  	}
   988  	if depthDelta > 0 {
   989  		p.depth++
   990  		p.indentedIn = true
   991  	}
   992  }
   993  
   994  type parentStack struct {
   995  	p     *printer
   996  	stack []string
   997  }
   998  
   999  // trim updates the XML context to match the longest common prefix of the stack
  1000  // and the given parents. A closing tag will be written for every parent
  1001  // popped. Passing a zero slice or nil will close all the elements.
  1002  func (s *parentStack) trim(parents []string) error {
  1003  	split := 0
  1004  	for ; split < len(parents) && split < len(s.stack); split++ {
  1005  		if parents[split] != s.stack[split] {
  1006  			break
  1007  		}
  1008  	}
  1009  	for i := len(s.stack) - 1; i >= split; i-- {
  1010  		if err := s.p.writeEnd(Name{Local: s.stack[i]}); err != nil {
  1011  			return err
  1012  		}
  1013  	}
  1014  	s.stack = s.stack[:split]
  1015  	return nil
  1016  }
  1017  
  1018  // push adds parent elements to the stack and writes open tags.
  1019  func (s *parentStack) push(parents []string) error {
  1020  	for i := 0; i < len(parents); i++ {
  1021  		if err := s.p.writeStart(&StartElement{Name: Name{Local: parents[i]}}); err != nil {
  1022  			return err
  1023  		}
  1024  	}
  1025  	s.stack = append(s.stack, parents...)
  1026  	return nil
  1027  }
  1028  
  1029  // UnsupportedTypeError is returned when Marshal encounters a type
  1030  // that cannot be converted into XML.
  1031  type UnsupportedTypeError struct {
  1032  	Type reflect.Type
  1033  }
  1034  
  1035  func (e *UnsupportedTypeError) Error() string {
  1036  	return "xml: unsupported type: " + e.Type.String()
  1037  }
  1038  
  1039  func isEmptyValue(v reflect.Value) bool {
  1040  	switch v.Kind() {
  1041  	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
  1042  		return v.Len() == 0
  1043  	case reflect.Bool:
  1044  		return !v.Bool()
  1045  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1046  		return v.Int() == 0
  1047  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1048  		return v.Uint() == 0
  1049  	case reflect.Float32, reflect.Float64:
  1050  		return v.Float() == 0
  1051  	case reflect.Interface, reflect.Ptr:
  1052  		return v.IsNil()
  1053  	}
  1054  	return false
  1055  }
  1056  

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