go/src/github.com/golang/protobuf/proto/extensions.go - third_party - Git at Google

 // Go support for Protocol Buffers - Google's data interchange format
 //
 // Copyright 2010 The Go Authors.  All rights reserved.
 // https://github.com/golang/protobuf
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
 // in the documentation and/or other materials provided with the
 // distribution.
 //     * Neither the name of Google Inc. nor the names of its
 // contributors may be used to endorse or promote products derived from
 // this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 package proto

 /*
  * Types and routines for supporting protocol buffer extensions.
  */

 import (
 	"errors"
 	"fmt"
 	"reflect"
 	"strconv"
 	"sync"
 )

 // ErrMissingExtension is the error returned by GetExtension if the named extension is not in the message.
 var ErrMissingExtension = errors.New("proto: missing extension")

 // ExtensionRange represents a range of message extensions for a protocol buffer.
 // Used in code generated by the protocol compiler.
 type ExtensionRange struct {
 	Start, End int32 // both inclusive
 }

 // extendableProto is an interface implemented by any protocol buffer that may be extended.
 type extendableProto interface {
 	Message
 	ExtensionRangeArray() []ExtensionRange
 	ExtensionMap() map[int32]Extension
 }

 var extendableProtoType = reflect.TypeOf((*extendableProto)(nil)).Elem()

 // ExtensionDesc represents an extension specification.
 // Used in generated code from the protocol compiler.
 type ExtensionDesc struct {
 	ExtendedType  Message     // nil pointer to the type that is being extended
 	ExtensionType interface{} // nil pointer to the extension type
 	Field         int32       // field number
 	Name          string      // fully-qualified name of extension, for text formatting
 	Tag           string      // protobuf tag style
 }

 func (ed *ExtensionDesc) repeated() bool {
 	t := reflect.TypeOf(ed.ExtensionType)
 	return t.Kind() == reflect.Slice && t.Elem().Kind() != reflect.Uint8
 }

 // Extension represents an extension in a message.
 type Extension struct {
 	// When an extension is stored in a message using SetExtension
 	// only desc and value are set. When the message is marshaled
 	// enc will be set to the encoded form of the message.
 	//
 	// When a message is unmarshaled and contains extensions, each
 	// extension will have only enc set. When such an extension is
 	// accessed using GetExtension (or GetExtensions) desc and value
 	// will be set.
 	desc  *ExtensionDesc
 	value interface{}
 	enc   []byte
 }

 // SetRawExtension is for testing only.
 func SetRawExtension(base extendableProto, id int32, b []byte) {
 	base.ExtensionMap()[id] = Extension{enc: b}
 }

 // isExtensionField returns true iff the given field number is in an extension range.
 func isExtensionField(pb extendableProto, field int32) bool {
 	for _, er := range pb.ExtensionRangeArray() {
 		if er.Start <= field && field <= er.End {
 			return true
 		}
 	}
 	return false
 }

 // checkExtensionTypes checks that the given extension is valid for pb.
 func checkExtensionTypes(pb extendableProto, extension *ExtensionDesc) error {
 	// Check the extended type.
 	if a, b := reflect.TypeOf(pb), reflect.TypeOf(extension.ExtendedType); a != b {
 		return errors.New("proto: bad extended type; " + b.String() + " does not extend " + a.String())
 	}
 	// Check the range.
 	if !isExtensionField(pb, extension.Field) {
 		return errors.New("proto: bad extension number; not in declared ranges")
 	}
 	return nil
 }

 // extPropKey is sufficient to uniquely identify an extension.
 type extPropKey struct {
 	base  reflect.Type
 	field int32
 }

 var extProp = struct {
 	sync.RWMutex
 	m map[extPropKey]*Properties
 }{
 	m: make(map[extPropKey]*Properties),
 }

 func extensionProperties(ed *ExtensionDesc) *Properties {
 	key := extPropKey{base: reflect.TypeOf(ed.ExtendedType), field: ed.Field}

 	extProp.RLock()
 	if prop, ok := extProp.m[key]; ok {
 		extProp.RUnlock()
 		return prop
 	}
 	extProp.RUnlock()

 	extProp.Lock()
 	defer extProp.Unlock()
 	// Check again.
 	if prop, ok := extProp.m[key]; ok {
 		return prop
 	}

 	prop := new(Properties)
 	prop.Init(reflect.TypeOf(ed.ExtensionType), "unknown_name", ed.Tag, nil)
 	extProp.m[key] = prop
 	return prop
 }

 // encodeExtensionMap encodes any unmarshaled (unencoded) extensions in m.
 func encodeExtensionMap(m map[int32]Extension) error {
 	for k, e := range m {
 		if e.value == nil || e.desc == nil {
 			// Extension is only in its encoded form.
 			continue
 		}

 		// We don't skip extensions that have an encoded form set,
 		// because the extension value may have been mutated after
 		// the last time this function was called.

 		et := reflect.TypeOf(e.desc.ExtensionType)
 		props := extensionProperties(e.desc)

 		p := NewBuffer(nil)
 		// If e.value has type T, the encoder expects a *struct{ X T }.
 		// Pass a *T with a zero field and hope it all works out.
 		x := reflect.New(et)
 		x.Elem().Set(reflect.ValueOf(e.value))
 		if err := props.enc(p, props, toStructPointer(x)); err != nil {
 			return err
 		}
 		e.enc = p.buf
 		m[k] = e
 	}
 	return nil
 }

 func sizeExtensionMap(m map[int32]Extension) (n int) {
 	for _, e := range m {
 		if e.value == nil || e.desc == nil {
 			// Extension is only in its encoded form.
 			n += len(e.enc)
 			continue
 		}

 		// We don't skip extensions that have an encoded form set,
 		// because the extension value may have been mutated after
 		// the last time this function was called.

 		et := reflect.TypeOf(e.desc.ExtensionType)
 		props := extensionProperties(e.desc)

 		// If e.value has type T, the encoder expects a *struct{ X T }.
 		// Pass a *T with a zero field and hope it all works out.
 		x := reflect.New(et)
 		x.Elem().Set(reflect.ValueOf(e.value))
 		n += props.size(props, toStructPointer(x))
 	}
 	return
 }

 // HasExtension returns whether the given extension is present in pb.
 func HasExtension(pb extendableProto, extension *ExtensionDesc) bool {
 	// TODO: Check types, field numbers, etc.?
 	_, ok := pb.ExtensionMap()[extension.Field]
 	return ok
 }

 // ClearExtension removes the given extension from pb.
 func ClearExtension(pb extendableProto, extension *ExtensionDesc) {
 	// TODO: Check types, field numbers, etc.?
 	delete(pb.ExtensionMap(), extension.Field)
 }

 // GetExtension parses and returns the given extension of pb.
 // If the extension is not present and has no default value it returns ErrMissingExtension.
 func GetExtension(pb extendableProto, extension *ExtensionDesc) (interface{}, error) {
 	if err := checkExtensionTypes(pb, extension); err != nil {
 		return nil, err
 	}

 	emap := pb.ExtensionMap()
 	e, ok := emap[extension.Field]
 	if !ok {
 		// defaultExtensionValue returns the default value or
 		// ErrMissingExtension if there is no default.
 		return defaultExtensionValue(extension)
 	}

 	if e.value != nil {
 		// Already decoded. Check the descriptor, though.
 		if e.desc != extension {
 			// This shouldn't happen. If it does, it means that
 			// GetExtension was called twice with two different
 			// descriptors with the same field number.
 			return nil, errors.New("proto: descriptor conflict")
 		}
 		return e.value, nil
 	}

 	v, err := decodeExtension(e.enc, extension)
 	if err != nil {
 		return nil, err
 	}

 	// Remember the decoded version and drop the encoded version.
 	// That way it is safe to mutate what we return.
 	e.value = v
 	e.desc = extension
 	e.enc = nil
 	emap[extension.Field] = e
 	return e.value, nil
 }

 // defaultExtensionValue returns the default value for extension.
 // If no default for an extension is defined ErrMissingExtension is returned.
 func defaultExtensionValue(extension *ExtensionDesc) (interface{}, error) {
 	t := reflect.TypeOf(extension.ExtensionType)
 	props := extensionProperties(extension)

 	sf, _, err := fieldDefault(t, props)
 	if err != nil {
 		return nil, err
 	}

 	if sf == nil || sf.value == nil {
 		// There is no default value.
 		return nil, ErrMissingExtension
 	}

 	if t.Kind() != reflect.Ptr {
 		// We do not need to return a Ptr, we can directly return sf.value.
 		return sf.value, nil
 	}

 	// We need to return an interface{} that is a pointer to sf.value.
 	value := reflect.New(t).Elem()
 	value.Set(reflect.New(value.Type().Elem()))
 	if sf.kind == reflect.Int32 {
 		// We may have an int32 or an enum, but the underlying data is int32.
 		// Since we can't set an int32 into a non int32 reflect.value directly
 		// set it as a int32.
 		value.Elem().SetInt(int64(sf.value.(int32)))
 	} else {
 		value.Elem().Set(reflect.ValueOf(sf.value))
 	}
 	return value.Interface(), nil
 }

 // decodeExtension decodes an extension encoded in b.
 func decodeExtension(b []byte, extension *ExtensionDesc) (interface{}, error) {
 	o := NewBuffer(b)

 	t := reflect.TypeOf(extension.ExtensionType)

 	props := extensionProperties(extension)

 	// t is a pointer to a struct, pointer to basic type or a slice.
 	// Allocate a "field" to store the pointer/slice itself; the
 	// pointer/slice will be stored here. We pass
 	// the address of this field to props.dec.
 	// This passes a zero field and a *t and lets props.dec
 	// interpret it as a *struct{ x t }.
 	value := reflect.New(t).Elem()

 	for {
 		// Discard wire type and field number varint. It isn't needed.
 		if _, err := o.DecodeVarint(); err != nil {
 			return nil, err
 		}

 		if err := props.dec(o, props, toStructPointer(value.Addr())); err != nil {
 			return nil, err
 		}

 		if o.index >= len(o.buf) {
 			break
 		}
 	}
 	return value.Interface(), nil
 }

 // GetExtensions returns a slice of the extensions present in pb that are also listed in es.
 // The returned slice has the same length as es; missing extensions will appear as nil elements.
 func GetExtensions(pb Message, es []*ExtensionDesc) (extensions []interface{}, err error) {
 	epb, ok := pb.(extendableProto)
 	if !ok {
 		err = errors.New("proto: not an extendable proto")
 		return
 	}
 	extensions = make([]interface{}, len(es))
 	for i, e := range es {
 		extensions[i], err = GetExtension(epb, e)
 		if err == ErrMissingExtension {
 			err = nil
 		}
 		if err != nil {
 			return
 		}
 	}
 	return
 }

 // SetExtension sets the specified extension of pb to the specified value.
 func SetExtension(pb extendableProto, extension *ExtensionDesc, value interface{}) error {
 	if err := checkExtensionTypes(pb, extension); err != nil {
 		return err
 	}
 	typ := reflect.TypeOf(extension.ExtensionType)
 	if typ != reflect.TypeOf(value) {
 		return errors.New("proto: bad extension value type")
 	}
 	// nil extension values need to be caught early, because the
 	// encoder can't distinguish an ErrNil due to a nil extension
 	// from an ErrNil due to a missing field. Extensions are
 	// always optional, so the encoder would just swallow the error
 	// and drop all the extensions from the encoded message.
 	if reflect.ValueOf(value).IsNil() {
 		return fmt.Errorf("proto: SetExtension called with nil value of type %T", value)
 	}

 	pb.ExtensionMap()[extension.Field] = Extension{desc: extension, value: value}
 	return nil
 }

 // A global registry of extensions.
 // The generated code will register the generated descriptors by calling RegisterExtension.

 var extensionMaps = make(map[reflect.Type]map[int32]*ExtensionDesc)

 // RegisterExtension is called from the generated code.
 func RegisterExtension(desc *ExtensionDesc) {
 	st := reflect.TypeOf(desc.ExtendedType).Elem()
 	m := extensionMaps[st]
 	if m == nil {
 		m = make(map[int32]*ExtensionDesc)
 		extensionMaps[st] = m
 	}
 	if _, ok := m[desc.Field]; ok {
 		panic("proto: duplicate extension registered: " + st.String() + " " + strconv.Itoa(int(desc.Field)))
 	}
 	m[desc.Field] = desc
 }

 // RegisteredExtensions returns a map of the registered extensions of a
 // protocol buffer struct, indexed by the extension number.
 // The argument pb should be a nil pointer to the struct type.
 func RegisteredExtensions(pb Message) map[int32]*ExtensionDesc {
 	return extensionMaps[reflect.TypeOf(pb).Elem()]
 }
	// Go support for Protocol Buffers - Google's data interchange format
	//
	// Copyright 2010 The Go Authors. All rights reserved.
	// https://github.com/golang/protobuf
	//
	// Redistribution and use in source and binary forms, with or without
	// modification, are permitted provided that the following conditions are
	// met:
	//
	// * Redistributions of source code must retain the above copyright
	// notice, this list of conditions and the following disclaimer.
	// * Redistributions in binary form must reproduce the above
	// copyright notice, this list of conditions and the following disclaimer
	// in the documentation and/or other materials provided with the
	// distribution.
	// * Neither the name of Google Inc. nor the names of its
	// contributors may be used to endorse or promote products derived from
	// this software without specific prior written permission.
	//
	// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
	// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
	// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
	// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
	// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
	// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
	// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
	// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

	package proto

	/*
	* Types and routines for supporting protocol buffer extensions.
	*/

	import (
	"errors"
	"fmt"
	"reflect"
	"strconv"
	"sync"
	)

	// ErrMissingExtension is the error returned by GetExtension if the named extension is not in the message.
	var ErrMissingExtension = errors.New("proto: missing extension")

	// ExtensionRange represents a range of message extensions for a protocol buffer.
	// Used in code generated by the protocol compiler.
	type ExtensionRange struct {
	Start, End int32 // both inclusive
	}

	// extendableProto is an interface implemented by any protocol buffer that may be extended.
	type extendableProto interface {
	Message
	ExtensionRangeArray() []ExtensionRange
	ExtensionMap() map[int32]Extension
	}

	var extendableProtoType = reflect.TypeOf((*extendableProto)(nil)).Elem()

	// ExtensionDesc represents an extension specification.
	// Used in generated code from the protocol compiler.
	type ExtensionDesc struct {
	ExtendedType Message // nil pointer to the type that is being extended
	ExtensionType interface{} // nil pointer to the extension type
	Field int32 // field number
	Name string // fully-qualified name of extension, for text formatting
	Tag string // protobuf tag style
	}

	func (ed *ExtensionDesc) repeated() bool {
	t := reflect.TypeOf(ed.ExtensionType)
	return t.Kind() == reflect.Slice && t.Elem().Kind() != reflect.Uint8
	}

	// Extension represents an extension in a message.
	type Extension struct {
	// When an extension is stored in a message using SetExtension
	// only desc and value are set. When the message is marshaled
	// enc will be set to the encoded form of the message.
	//
	// When a message is unmarshaled and contains extensions, each
	// extension will have only enc set. When such an extension is
	// accessed using GetExtension (or GetExtensions) desc and value
	// will be set.
	desc *ExtensionDesc
	value interface{}
	enc []byte
	}

	// SetRawExtension is for testing only.
	func SetRawExtension(base extendableProto, id int32, b []byte) {
	base.ExtensionMap()[id] = Extension{enc: b}
	}

	// isExtensionField returns true iff the given field number is in an extension range.
	func isExtensionField(pb extendableProto, field int32) bool {
	for _, er := range pb.ExtensionRangeArray() {
	if er.Start <= field && field <= er.End {
	return true
	}
	}
	return false
	}

	// checkExtensionTypes checks that the given extension is valid for pb.
	func checkExtensionTypes(pb extendableProto, extension *ExtensionDesc) error {
	// Check the extended type.
	if a, b := reflect.TypeOf(pb), reflect.TypeOf(extension.ExtendedType); a != b {
	return errors.New("proto: bad extended type; " + b.String() + " does not extend " + a.String())
	}
	// Check the range.
	if !isExtensionField(pb, extension.Field) {
	return errors.New("proto: bad extension number; not in declared ranges")
	}
	return nil
	}

	// extPropKey is sufficient to uniquely identify an extension.
	type extPropKey struct {
	base reflect.Type
	field int32
	}

	var extProp = struct {
	sync.RWMutex
	m map[extPropKey]*Properties
	}{
	m: make(map[extPropKey]*Properties),
	}

	func extensionProperties(ed ExtensionDesc) Properties {
	key := extPropKey{base: reflect.TypeOf(ed.ExtendedType), field: ed.Field}

	extProp.RLock()
	if prop, ok := extProp.m[key]; ok {
	extProp.RUnlock()
	return prop
	}
	extProp.RUnlock()

	extProp.Lock()
	defer extProp.Unlock()
	// Check again.
	if prop, ok := extProp.m[key]; ok {
	return prop
	}

	prop := new(Properties)
	prop.Init(reflect.TypeOf(ed.ExtensionType), "unknown_name", ed.Tag, nil)
	extProp.m[key] = prop
	return prop
	}

	// encodeExtensionMap encodes any unmarshaled (unencoded) extensions in m.
	func encodeExtensionMap(m map[int32]Extension) error {
	for k, e := range m {
	if e.value == nil \|\| e.desc == nil {
	// Extension is only in its encoded form.
	continue
	}

	// We don't skip extensions that have an encoded form set,
	// because the extension value may have been mutated after
	// the last time this function was called.

	et := reflect.TypeOf(e.desc.ExtensionType)
	props := extensionProperties(e.desc)

	p := NewBuffer(nil)
	// If e.value has type T, the encoder expects a *struct{ X T }.
	// Pass a *T with a zero field and hope it all works out.
	x := reflect.New(et)
	x.Elem().Set(reflect.ValueOf(e.value))
	if err := props.enc(p, props, toStructPointer(x)); err != nil {
	return err
	}
	e.enc = p.buf
	m[k] = e
	}
	return nil
	}

	func sizeExtensionMap(m map[int32]Extension) (n int) {
	for _, e := range m {
	if e.value == nil \|\| e.desc == nil {
	// Extension is only in its encoded form.
	n += len(e.enc)
	continue
	}

	// We don't skip extensions that have an encoded form set,
	// because the extension value may have been mutated after
	// the last time this function was called.

	et := reflect.TypeOf(e.desc.ExtensionType)
	props := extensionProperties(e.desc)

	// If e.value has type T, the encoder expects a *struct{ X T }.
	// Pass a *T with a zero field and hope it all works out.
	x := reflect.New(et)
	x.Elem().Set(reflect.ValueOf(e.value))
	n += props.size(props, toStructPointer(x))
	}
	return
	}

	// HasExtension returns whether the given extension is present in pb.
	func HasExtension(pb extendableProto, extension *ExtensionDesc) bool {
	// TODO: Check types, field numbers, etc.?
	_, ok := pb.ExtensionMap()[extension.Field]
	return ok
	}

	// ClearExtension removes the given extension from pb.
	func ClearExtension(pb extendableProto, extension *ExtensionDesc) {
	// TODO: Check types, field numbers, etc.?
	delete(pb.ExtensionMap(), extension.Field)
	}

	// GetExtension parses and returns the given extension of pb.
	// If the extension is not present and has no default value it returns ErrMissingExtension.
	func GetExtension(pb extendableProto, extension *ExtensionDesc) (interface{}, error) {
	if err := checkExtensionTypes(pb, extension); err != nil {
	return nil, err
	}

	emap := pb.ExtensionMap()
	e, ok := emap[extension.Field]
	if !ok {
	// defaultExtensionValue returns the default value or
	// ErrMissingExtension if there is no default.
	return defaultExtensionValue(extension)
	}

	if e.value != nil {
	// Already decoded. Check the descriptor, though.
	if e.desc != extension {
	// This shouldn't happen. If it does, it means that
	// GetExtension was called twice with two different
	// descriptors with the same field number.
	return nil, errors.New("proto: descriptor conflict")
	}
	return e.value, nil
	}

	v, err := decodeExtension(e.enc, extension)
	if err != nil {
	return nil, err
	}

	// Remember the decoded version and drop the encoded version.
	// That way it is safe to mutate what we return.
	e.value = v
	e.desc = extension
	e.enc = nil
	emap[extension.Field] = e
	return e.value, nil
	}

	// defaultExtensionValue returns the default value for extension.
	// If no default for an extension is defined ErrMissingExtension is returned.
	func defaultExtensionValue(extension *ExtensionDesc) (interface{}, error) {
	t := reflect.TypeOf(extension.ExtensionType)
	props := extensionProperties(extension)

	sf, _, err := fieldDefault(t, props)
	if err != nil {
	return nil, err
	}

	if sf == nil \|\| sf.value == nil {
	// There is no default value.
	return nil, ErrMissingExtension
	}

	if t.Kind() != reflect.Ptr {
	// We do not need to return a Ptr, we can directly return sf.value.
	return sf.value, nil
	}

	// We need to return an interface{} that is a pointer to sf.value.
	value := reflect.New(t).Elem()
	value.Set(reflect.New(value.Type().Elem()))
	if sf.kind == reflect.Int32 {
	// We may have an int32 or an enum, but the underlying data is int32.
	// Since we can't set an int32 into a non int32 reflect.value directly
	// set it as a int32.
	value.Elem().SetInt(int64(sf.value.(int32)))
	} else {
	value.Elem().Set(reflect.ValueOf(sf.value))
	}
	return value.Interface(), nil
	}

	// decodeExtension decodes an extension encoded in b.
	func decodeExtension(b []byte, extension *ExtensionDesc) (interface{}, error) {
	o := NewBuffer(b)

	t := reflect.TypeOf(extension.ExtensionType)

	props := extensionProperties(extension)

	// t is a pointer to a struct, pointer to basic type or a slice.
	// Allocate a "field" to store the pointer/slice itself; the
	// pointer/slice will be stored here. We pass
	// the address of this field to props.dec.
	// This passes a zero field and a *t and lets props.dec
	// interpret it as a *struct{ x t }.
	value := reflect.New(t).Elem()

	for {
	// Discard wire type and field number varint. It isn't needed.
	if _, err := o.DecodeVarint(); err != nil {
	return nil, err
	}

	if err := props.dec(o, props, toStructPointer(value.Addr())); err != nil {
	return nil, err
	}

	if o.index >= len(o.buf) {
	break
	}
	}
	return value.Interface(), nil
	}

	// GetExtensions returns a slice of the extensions present in pb that are also listed in es.
	// The returned slice has the same length as es; missing extensions will appear as nil elements.
	func GetExtensions(pb Message, es []*ExtensionDesc) (extensions []interface{}, err error) {
	epb, ok := pb.(extendableProto)
	if !ok {
	err = errors.New("proto: not an extendable proto")
	return
	}
	extensions = make([]interface{}, len(es))
	for i, e := range es {
	extensions[i], err = GetExtension(epb, e)
	if err == ErrMissingExtension {
	err = nil
	}
	if err != nil {
	return
	}
	}
	return
	}

	// SetExtension sets the specified extension of pb to the specified value.
	func SetExtension(pb extendableProto, extension *ExtensionDesc, value interface{}) error {
	if err := checkExtensionTypes(pb, extension); err != nil {
	return err
	}
	typ := reflect.TypeOf(extension.ExtensionType)
	if typ != reflect.TypeOf(value) {
	return errors.New("proto: bad extension value type")
	}
	// nil extension values need to be caught early, because the
	// encoder can't distinguish an ErrNil due to a nil extension
	// from an ErrNil due to a missing field. Extensions are
	// always optional, so the encoder would just swallow the error
	// and drop all the extensions from the encoded message.
	if reflect.ValueOf(value).IsNil() {
	return fmt.Errorf("proto: SetExtension called with nil value of type %T", value)
	}

	pb.ExtensionMap()[extension.Field] = Extension{desc: extension, value: value}
	return nil
	}

	// A global registry of extensions.
	// The generated code will register the generated descriptors by calling RegisterExtension.

	var extensionMaps = make(map[reflect.Type]map[int32]*ExtensionDesc)

	// RegisterExtension is called from the generated code.
	func RegisterExtension(desc *ExtensionDesc) {
	st := reflect.TypeOf(desc.ExtendedType).Elem()
	m := extensionMaps[st]
	if m == nil {
	m = make(map[int32]*ExtensionDesc)
	extensionMaps[st] = m
	}
	if _, ok := m[desc.Field]; ok {
	panic("proto: duplicate extension registered: " + st.String() + " " + strconv.Itoa(int(desc.Field)))
	}
	m[desc.Field] = desc
	}

	// RegisteredExtensions returns a map of the registered extensions of a
	// protocol buffer struct, indexed by the extension number.
	// The argument pb should be a nil pointer to the struct type.
	func RegisteredExtensions(pb Message) map[int32]*ExtensionDesc {
	return extensionMaps[reflect.TypeOf(pb).Elem()]
	}