Adding upstream version 1.16.10.upstream/1.16.10upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 755 insertions, 0 deletions

diff --git a/src/encoding/xml/read.go b/src/encoding/xml/read.go
new file mode 100644
index 0000000..ef5df3f
--- /dev/null
+++ b/src/encoding/xml/read.go
@@ -0,0 +1,755 @@
+// Copyright 2009 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 (
+	"bytes"
+	"encoding"
+	"errors"
+	"fmt"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+// BUG(rsc): Mapping between XML elements and data structures is inherently flawed:
+// an XML element is an order-dependent collection of anonymous
+// values, while a data structure is an order-independent collection
+// of named values.
+// See package json for a textual representation more suitable
+// to data structures.
+
+// Unmarshal parses the XML-encoded data and stores the result in
+// the value pointed to by v, which must be an arbitrary struct,
+// slice, or string. Well-formed data that does not fit into v is
+// discarded.
+//
+// Because Unmarshal uses the reflect package, it can only assign
+// to exported (upper case) fields. Unmarshal uses a case-sensitive
+// comparison to match XML element names to tag values and struct
+// field names.
+//
+// Unmarshal maps an XML element to a struct using the following rules.
+// In the rules, the tag of a field refers to the value associated with the
+// key 'xml' in the struct field's tag (see the example above).
+//
+//   * If the struct has a field of type []byte or string with tag
+//      ",innerxml", Unmarshal accumulates the raw XML nested inside the
+//      element in that field. The rest of the rules still apply.
+//
+//   * If the struct has a field named XMLName of type Name,
+//      Unmarshal records the element name in that field.
+//
+//   * If the XMLName field has an associated tag of the form
+//      "name" or "namespace-URL name", the XML element must have
+//      the given name (and, optionally, name space) or else Unmarshal
+//      returns an error.
+//
+//   * If the XML element has an attribute whose name matches a
+//      struct field name with an associated tag containing ",attr" or
+//      the explicit name in a struct field tag of the form "name,attr",
+//      Unmarshal records the attribute value in that field.
+//
+//   * If the XML element has an attribute not handled by the previous
+//      rule and the struct has a field with an associated tag containing
+//      ",any,attr", Unmarshal records the attribute value in the first
+//      such field.
+//
+//   * If the XML element contains character data, that data is
+//      accumulated in the first struct field that has tag ",chardata".
+//      The struct field may have type []byte or string.
+//      If there is no such field, the character data is discarded.
+//
+//   * If the XML element contains comments, they are accumulated in
+//      the first struct field that has tag ",comment".  The struct
+//      field may have type []byte or string. If there is no such
+//      field, the comments are discarded.
+//
+//   * If the XML element contains a sub-element whose name matches
+//      the prefix of a tag formatted as "a" or "a>b>c", unmarshal
+//      will descend into the XML structure looking for elements with the
+//      given names, and will map the innermost elements to that struct
+//      field. A tag starting with ">" is equivalent to one starting
+//      with the field name followed by ">".
+//
+//   * If the XML element contains a sub-element whose name matches
+//      a struct field's XMLName tag and the struct field has no
+//      explicit name tag as per the previous rule, unmarshal maps
+//      the sub-element to that struct field.
+//
+//   * If the XML element contains a sub-element whose name matches a
+//      field without any mode flags (",attr", ",chardata", etc), Unmarshal
+//      maps the sub-element to that struct field.
+//
+//   * If the XML element contains a sub-element that hasn't matched any
+//      of the above rules and the struct has a field with tag ",any",
+//      unmarshal maps the sub-element to that struct field.
+//
+//   * An anonymous struct field is handled as if the fields of its
+//      value were part of the outer struct.
+//
+//   * A struct field with tag "-" is never unmarshaled into.
+//
+// If Unmarshal encounters a field type that implements the Unmarshaler
+// interface, Unmarshal calls its UnmarshalXML method to produce the value from
+// the XML element.  Otherwise, if the value implements
+// encoding.TextUnmarshaler, Unmarshal calls that value's UnmarshalText method.
+//
+// Unmarshal maps an XML element to a string or []byte by saving the
+// concatenation of that element's character data in the string or
+// []byte. The saved []byte is never nil.
+//
+// Unmarshal maps an attribute value to a string or []byte by saving
+// the value in the string or slice.
+//
+// Unmarshal maps an attribute value to an Attr by saving the attribute,
+// including its name, in the Attr.
+//
+// Unmarshal maps an XML element or attribute value to a slice by
+// extending the length of the slice and mapping the element or attribute
+// to the newly created value.
+//
+// Unmarshal maps an XML element or attribute value to a bool by
+// setting it to the boolean value represented by the string. Whitespace
+// is trimmed and ignored.
+//
+// Unmarshal maps an XML element or attribute value to an integer or
+// floating-point field by setting the field to the result of
+// interpreting the string value in decimal. There is no check for
+// overflow. Whitespace is trimmed and ignored.
+//
+// Unmarshal maps an XML element to a Name by recording the element
+// name.
+//
+// Unmarshal maps an XML element to a pointer by setting the pointer
+// to a freshly allocated value and then mapping the element to that value.
+//
+// A missing element or empty attribute value will be unmarshaled as a zero value.
+// If the field is a slice, a zero value will be appended to the field. Otherwise, the
+// field will be set to its zero value.
+func Unmarshal(data []byte, v interface{}) error {
+	return NewDecoder(bytes.NewReader(data)).Decode(v)
+}
+
+// Decode works like Unmarshal, except it reads the decoder
+// stream to find the start element.
+func (d *Decoder) Decode(v interface{}) error {
+	return d.DecodeElement(v, nil)
+}
+
+// DecodeElement works like Unmarshal except that it takes
+// a pointer to the start XML element to decode into v.
+// It is useful when a client reads some raw XML tokens itself
+// but also wants to defer to Unmarshal for some elements.
+func (d *Decoder) DecodeElement(v interface{}, start *StartElement) error {
+	val := reflect.ValueOf(v)
+	if val.Kind() != reflect.Ptr {
+		return errors.New("non-pointer passed to Unmarshal")
+	}
+	return d.unmarshal(val.Elem(), start)
+}
+
+// An UnmarshalError represents an error in the unmarshaling process.
+type UnmarshalError string
+
+func (e UnmarshalError) Error() string { return string(e) }
+
+// Unmarshaler is the interface implemented by objects that can unmarshal
+// an XML element description of themselves.
+//
+// UnmarshalXML decodes a single XML element
+// beginning with the given start element.
+// If it returns an error, the outer call to Unmarshal stops and
+// returns that error.
+// UnmarshalXML must consume exactly one XML element.
+// One common implementation strategy is to unmarshal into
+// a separate value with a layout matching the expected XML
+// using d.DecodeElement, and then to copy the data from
+// that value into the receiver.
+// Another common strategy is to use d.Token to process the
+// XML object one token at a time.
+// UnmarshalXML may not use d.RawToken.
+type Unmarshaler interface {
+	UnmarshalXML(d *Decoder, start StartElement) error
+}
+
+// UnmarshalerAttr is the interface implemented by objects that can unmarshal
+// an XML attribute description of themselves.
+//
+// UnmarshalXMLAttr decodes a single XML attribute.
+// If it returns an error, the outer call to Unmarshal stops and
+// returns that error.
+// UnmarshalXMLAttr is used only for struct fields with the
+// "attr" option in the field tag.
+type UnmarshalerAttr interface {
+	UnmarshalXMLAttr(attr Attr) error
+}
+
+// receiverType returns the receiver type to use in an expression like "%s.MethodName".
+func receiverType(val interface{}) string {
+	t := reflect.TypeOf(val)
+	if t.Name() != "" {
+		return t.String()
+	}
+	return "(" + t.String() + ")"
+}
+
+// unmarshalInterface unmarshals a single XML element into val.
+// start is the opening tag of the element.
+func (d *Decoder) unmarshalInterface(val Unmarshaler, start *StartElement) error {
+	// Record that decoder must stop at end tag corresponding to start.
+	d.pushEOF()
+
+	d.unmarshalDepth++
+	err := val.UnmarshalXML(d, *start)
+	d.unmarshalDepth--
+	if err != nil {
+		d.popEOF()
+		return err
+	}
+
+	if !d.popEOF() {
+		return fmt.Errorf("xml: %s.UnmarshalXML did not consume entire <%s> element", receiverType(val), start.Name.Local)
+	}
+
+	return nil
+}
+
+// unmarshalTextInterface unmarshals a single XML element into val.
+// The chardata contained in the element (but not its children)
+// is passed to the text unmarshaler.
+func (d *Decoder) unmarshalTextInterface(val encoding.TextUnmarshaler) error {
+	var buf []byte
+	depth := 1
+	for depth > 0 {
+		t, err := d.Token()
+		if err != nil {
+			return err
+		}
+		switch t := t.(type) {
+		case CharData:
+			if depth == 1 {
+				buf = append(buf, t...)
+			}
+		case StartElement:
+			depth++
+		case EndElement:
+			depth--
+		}
+	}
+	return val.UnmarshalText(buf)
+}
+
+// unmarshalAttr unmarshals a single XML attribute into val.
+func (d *Decoder) unmarshalAttr(val reflect.Value, attr Attr) error {
+	if val.Kind() == reflect.Ptr {
+		if val.IsNil() {
+			val.Set(reflect.New(val.Type().Elem()))
+		}
+		val = val.Elem()
+	}
+	if val.CanInterface() && val.Type().Implements(unmarshalerAttrType) {
+		// This is an unmarshaler with a non-pointer receiver,
+		// so it's likely to be incorrect, but we do what we're told.
+		return val.Interface().(UnmarshalerAttr).UnmarshalXMLAttr(attr)
+	}
+	if val.CanAddr() {
+		pv := val.Addr()
+		if pv.CanInterface() && pv.Type().Implements(unmarshalerAttrType) {
+			return pv.Interface().(UnmarshalerAttr).UnmarshalXMLAttr(attr)
+		}
+	}
+
+	// Not an UnmarshalerAttr; try encoding.TextUnmarshaler.
+	if val.CanInterface() && val.Type().Implements(textUnmarshalerType) {
+		// This is an unmarshaler with a non-pointer receiver,
+		// so it's likely to be incorrect, but we do what we're told.
+		return val.Interface().(encoding.TextUnmarshaler).UnmarshalText([]byte(attr.Value))
+	}
+	if val.CanAddr() {
+		pv := val.Addr()
+		if pv.CanInterface() && pv.Type().Implements(textUnmarshalerType) {
+			return pv.Interface().(encoding.TextUnmarshaler).UnmarshalText([]byte(attr.Value))
+		}
+	}
+
+	if val.Type().Kind() == reflect.Slice && val.Type().Elem().Kind() != reflect.Uint8 {
+		// Slice of element values.
+		// Grow slice.
+		n := val.Len()
+		val.Set(reflect.Append(val, reflect.Zero(val.Type().Elem())))
+
+		// Recur to read element into slice.
+		if err := d.unmarshalAttr(val.Index(n), attr); err != nil {
+			val.SetLen(n)
+			return err
+		}
+		return nil
+	}
+
+	if val.Type() == attrType {
+		val.Set(reflect.ValueOf(attr))
+		return nil
+	}
+
+	return copyValue(val, []byte(attr.Value))
+}
+
+var (
+	attrType            = reflect.TypeOf(Attr{})
+	unmarshalerType     = reflect.TypeOf((*Unmarshaler)(nil)).Elem()
+	unmarshalerAttrType = reflect.TypeOf((*UnmarshalerAttr)(nil)).Elem()
+	textUnmarshalerType = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+)
+
+// Unmarshal a single XML element into val.
+func (d *Decoder) unmarshal(val reflect.Value, start *StartElement) error {
+	// Find start element if we need it.
+	if start == nil {
+		for {
+			tok, err := d.Token()
+			if err != nil {
+				return err
+			}
+			if t, ok := tok.(StartElement); ok {
+				start = &t
+				break
+			}
+		}
+	}
+
+	// Load value from interface, but only if the result will be
+	// usefully addressable.
+	if val.Kind() == reflect.Interface && !val.IsNil() {
+		e := val.Elem()
+		if e.Kind() == reflect.Ptr && !e.IsNil() {
+			val = e
+		}
+	}
+
+	if val.Kind() == reflect.Ptr {
+		if val.IsNil() {
+			val.Set(reflect.New(val.Type().Elem()))
+		}
+		val = val.Elem()
+	}
+
+	if val.CanInterface() && val.Type().Implements(unmarshalerType) {
+		// This is an unmarshaler with a non-pointer receiver,
+		// so it's likely to be incorrect, but we do what we're told.
+		return d.unmarshalInterface(val.Interface().(Unmarshaler), start)
+	}
+
+	if val.CanAddr() {
+		pv := val.Addr()
+		if pv.CanInterface() && pv.Type().Implements(unmarshalerType) {
+			return d.unmarshalInterface(pv.Interface().(Unmarshaler), start)
+		}
+	}
+
+	if val.CanInterface() && val.Type().Implements(textUnmarshalerType) {
+		return d.unmarshalTextInterface(val.Interface().(encoding.TextUnmarshaler))
+	}
+
+	if val.CanAddr() {
+		pv := val.Addr()
+		if pv.CanInterface() && pv.Type().Implements(textUnmarshalerType) {
+			return d.unmarshalTextInterface(pv.Interface().(encoding.TextUnmarshaler))
+		}
+	}
+
+	var (
+		data         []byte
+		saveData     reflect.Value
+		comment      []byte
+		saveComment  reflect.Value
+		saveXML      reflect.Value
+		saveXMLIndex int
+		saveXMLData  []byte
+		saveAny      reflect.Value
+		sv           reflect.Value
+		tinfo        *typeInfo
+		err          error
+	)
+
+	switch v := val; v.Kind() {
+	default:
+		return errors.New("unknown type " + v.Type().String())
+
+	case reflect.Interface:
+		// TODO: For now, simply ignore the field. In the near
+		//       future we may choose to unmarshal the start
+		//       element on it, if not nil.
+		return d.Skip()
+
+	case reflect.Slice:
+		typ := v.Type()
+		if typ.Elem().Kind() == reflect.Uint8 {
+			// []byte
+			saveData = v
+			break
+		}
+
+		// Slice of element values.
+		// Grow slice.
+		n := v.Len()
+		v.Set(reflect.Append(val, reflect.Zero(v.Type().Elem())))
+
+		// Recur to read element into slice.
+		if err := d.unmarshal(v.Index(n), start); err != nil {
+			v.SetLen(n)
+			return err
+		}
+		return nil
+
+	case reflect.Bool, reflect.Float32, reflect.Float64, reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64, reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr, reflect.String:
+		saveData = v
+
+	case reflect.Struct:
+		typ := v.Type()
+		if typ == nameType {
+			v.Set(reflect.ValueOf(start.Name))
+			break
+		}
+
+		sv = v
+		tinfo, err = getTypeInfo(typ)
+		if err != nil {
+			return err
+		}
+
+		// Validate and assign element name.
+		if tinfo.xmlname != nil {
+			finfo := tinfo.xmlname
+			if finfo.name != "" && finfo.name != start.Name.Local {
+				return UnmarshalError("expected element type <" + finfo.name + "> but have <" + start.Name.Local + ">")
+			}
+			if finfo.xmlns != "" && finfo.xmlns != start.Name.Space {
+				e := "expected element <" + finfo.name + "> in name space " + finfo.xmlns + " but have "
+				if start.Name.Space == "" {
+					e += "no name space"
+				} else {
+					e += start.Name.Space
+				}
+				return UnmarshalError(e)
+			}
+			fv := finfo.value(sv, initNilPointers)
+			if _, ok := fv.Interface().(Name); ok {
+				fv.Set(reflect.ValueOf(start.Name))
+			}
+		}
+
+		// Assign attributes.
+		for _, a := range start.Attr {
+			handled := false
+			any := -1
+			for i := range tinfo.fields {
+				finfo := &tinfo.fields[i]
+				switch finfo.flags & fMode {
+				case fAttr:
+					strv := finfo.value(sv, initNilPointers)
+					if a.Name.Local == finfo.name && (finfo.xmlns == "" || finfo.xmlns == a.Name.Space) {
+						if err := d.unmarshalAttr(strv, a); err != nil {
+							return err
+						}
+						handled = true
+					}
+
+				case fAny | fAttr:
+					if any == -1 {
+						any = i
+					}
+				}
+			}
+			if !handled && any >= 0 {
+				finfo := &tinfo.fields[any]
+				strv := finfo.value(sv, initNilPointers)
+				if err := d.unmarshalAttr(strv, a); err != nil {
+					return err
+				}
+			}
+		}
+
+		// Determine whether we need to save character data or comments.
+		for i := range tinfo.fields {
+			finfo := &tinfo.fields[i]
+			switch finfo.flags & fMode {
+			case fCDATA, fCharData:
+				if !saveData.IsValid() {
+					saveData = finfo.value(sv, initNilPointers)
+				}
+
+			case fComment:
+				if !saveComment.IsValid() {
+					saveComment = finfo.value(sv, initNilPointers)
+				}
+
+			case fAny, fAny | fElement:
+				if !saveAny.IsValid() {
+					saveAny = finfo.value(sv, initNilPointers)
+				}
+
+			case fInnerXML:
+				if !saveXML.IsValid() {
+					saveXML = finfo.value(sv, initNilPointers)
+					if d.saved == nil {
+						saveXMLIndex = 0
+						d.saved = new(bytes.Buffer)
+					} else {
+						saveXMLIndex = d.savedOffset()
+					}
+				}
+			}
+		}
+	}
+
+	// Find end element.
+	// Process sub-elements along the way.
+Loop:
+	for {
+		var savedOffset int
+		if saveXML.IsValid() {
+			savedOffset = d.savedOffset()
+		}
+		tok, err := d.Token()
+		if err != nil {
+			return err
+		}
+		switch t := tok.(type) {
+		case StartElement:
+			consumed := false
+			if sv.IsValid() {
+				consumed, err = d.unmarshalPath(tinfo, sv, nil, &t)
+				if err != nil {
+					return err
+				}
+				if !consumed && saveAny.IsValid() {
+					consumed = true
+					if err := d.unmarshal(saveAny, &t); err != nil {
+						return err
+					}
+				}
+			}
+			if !consumed {
+				if err := d.Skip(); err != nil {
+					return err
+				}
+			}
+
+		case EndElement:
+			if saveXML.IsValid() {
+				saveXMLData = d.saved.Bytes()[saveXMLIndex:savedOffset]
+				if saveXMLIndex == 0 {
+					d.saved = nil
+				}
+			}
+			break Loop
+
+		case CharData:
+			if saveData.IsValid() {
+				data = append(data, t...)
+			}
+
+		case Comment:
+			if saveComment.IsValid() {
+				comment = append(comment, t...)
+			}
+		}
+	}
+
+	if saveData.IsValid() && saveData.CanInterface() && saveData.Type().Implements(textUnmarshalerType) {
+		if err := saveData.Interface().(encoding.TextUnmarshaler).UnmarshalText(data); err != nil {
+			return err
+		}
+		saveData = reflect.Value{}
+	}
+
+	if saveData.IsValid() && saveData.CanAddr() {
+		pv := saveData.Addr()
+		if pv.CanInterface() && pv.Type().Implements(textUnmarshalerType) {
+			if err := pv.Interface().(encoding.TextUnmarshaler).UnmarshalText(data); err != nil {
+				return err
+			}
+			saveData = reflect.Value{}
+		}
+	}
+
+	if err := copyValue(saveData, data); err != nil {
+		return err
+	}
+
+	switch t := saveComment; t.Kind() {
+	case reflect.String:
+		t.SetString(string(comment))
+	case reflect.Slice:
+		t.Set(reflect.ValueOf(comment))
+	}
+
+	switch t := saveXML; t.Kind() {
+	case reflect.String:
+		t.SetString(string(saveXMLData))
+	case reflect.Slice:
+		if t.Type().Elem().Kind() == reflect.Uint8 {
+			t.Set(reflect.ValueOf(saveXMLData))
+		}
+	}
+
+	return nil
+}
+
+func copyValue(dst reflect.Value, src []byte) (err error) {
+	dst0 := dst
+
+	if dst.Kind() == reflect.Ptr {
+		if dst.IsNil() {
+			dst.Set(reflect.New(dst.Type().Elem()))
+		}
+		dst = dst.Elem()
+	}
+
+	// Save accumulated data.
+	switch dst.Kind() {
+	case reflect.Invalid:
+		// Probably a comment.
+	default:
+		return errors.New("cannot unmarshal into " + dst0.Type().String())
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		if len(src) == 0 {
+			dst.SetInt(0)
+			return nil
+		}
+		itmp, err := strconv.ParseInt(strings.TrimSpace(string(src)), 10, dst.Type().Bits())
+		if err != nil {
+			return err
+		}
+		dst.SetInt(itmp)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		if len(src) == 0 {
+			dst.SetUint(0)
+			return nil
+		}
+		utmp, err := strconv.ParseUint(strings.TrimSpace(string(src)), 10, dst.Type().Bits())
+		if err != nil {
+			return err
+		}
+		dst.SetUint(utmp)
+	case reflect.Float32, reflect.Float64:
+		if len(src) == 0 {
+			dst.SetFloat(0)
+			return nil
+		}
+		ftmp, err := strconv.ParseFloat(strings.TrimSpace(string(src)), dst.Type().Bits())
+		if err != nil {
+			return err
+		}
+		dst.SetFloat(ftmp)
+	case reflect.Bool:
+		if len(src) == 0 {
+			dst.SetBool(false)
+			return nil
+		}
+		value, err := strconv.ParseBool(strings.TrimSpace(string(src)))
+		if err != nil {
+			return err
+		}
+		dst.SetBool(value)
+	case reflect.String:
+		dst.SetString(string(src))
+	case reflect.Slice:
+		if len(src) == 0 {
+			// non-nil to flag presence
+			src = []byte{}
+		}
+		dst.SetBytes(src)
+	}
+	return nil
+}
+
+// unmarshalPath walks down an XML structure looking for wanted
+// paths, and calls unmarshal on them.
+// The consumed result tells whether XML elements have been consumed
+// from the Decoder until start's matching end element, or if it's
+// still untouched because start is uninteresting for sv's fields.
+func (d *Decoder) unmarshalPath(tinfo *typeInfo, sv reflect.Value, parents []string, start *StartElement) (consumed bool, err error) {
+	recurse := false
+Loop:
+	for i := range tinfo.fields {
+		finfo := &tinfo.fields[i]
+		if finfo.flags&fElement == 0 || len(finfo.parents) < len(parents) || finfo.xmlns != "" && finfo.xmlns != start.Name.Space {
+			continue
+		}
+		for j := range parents {
+			if parents[j] != finfo.parents[j] {
+				continue Loop
+			}
+		}
+		if len(finfo.parents) == len(parents) && finfo.name == start.Name.Local {
+			// It's a perfect match, unmarshal the field.
+			return true, d.unmarshal(finfo.value(sv, initNilPointers), start)
+		}
+		if len(finfo.parents) > len(parents) && finfo.parents[len(parents)] == start.Name.Local {
+			// It's a prefix for the field. Break and recurse
+			// since it's not ok for one field path to be itself
+			// the prefix for another field path.
+			recurse = true
+
+			// We can reuse the same slice as long as we
+			// don't try to append to it.
+			parents = finfo.parents[:len(parents)+1]
+			break
+		}
+	}
+	if !recurse {
+		// We have no business with this element.
+		return false, nil
+	}
+	// The element is not a perfect match for any field, but one
+	// or more fields have the path to this element as a parent
+	// prefix. Recurse and attempt to match these.
+	for {
+		var tok Token
+		tok, err = d.Token()
+		if err != nil {
+			return true, err
+		}
+		switch t := tok.(type) {
+		case StartElement:
+			consumed2, err := d.unmarshalPath(tinfo, sv, parents, &t)
+			if err != nil {
+				return true, err
+			}
+			if !consumed2 {
+				if err := d.Skip(); err != nil {
+					return true, err
+				}
+			}
+		case EndElement:
+			return true, nil
+		}
+	}
+}
+
+// Skip reads tokens until it has consumed the end element
+// matching the most recent start element already consumed.
+// It recurs if it encounters a start element, so it can be used to
+// skip nested structures.
+// It returns nil if it finds an end element matching the start
+// element; otherwise it returns an error describing the problem.
+func (d *Decoder) Skip() error {
+	for {
+		tok, err := d.Token()
+		if err != nil {
+			return err
+		}
+		switch tok.(type) {
+		case StartElement:
+			if err := d.Skip(); err != nil {
+				return err
+			}
+		case EndElement:
+			return nil
+		}
+	}
+}