254 lines
7.4 KiB
Go
254 lines
7.4 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package gob
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import (
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"bytes"
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"errors"
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"io"
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"reflect"
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"sync"
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)
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// An Encoder manages the transmission of type and data information to the
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// other side of a connection.
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type Encoder struct {
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mutex sync.Mutex // each item must be sent atomically
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w []io.Writer // where to send the data
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sent map[reflect.Type]typeId // which types we've already sent
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countState *encoderState // stage for writing counts
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freeList *encoderState // list of free encoderStates; avoids reallocation
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byteBuf bytes.Buffer // buffer for top-level encoderState
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err error
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}
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// Before we encode a message, we reserve space at the head of the
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// buffer in which to encode its length. This means we can use the
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// buffer to assemble the message without another allocation.
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const maxLength = 9 // Maximum size of an encoded length.
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var spaceForLength = make([]byte, maxLength)
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// NewEncoder returns a new encoder that will transmit on the io.Writer.
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func NewEncoder(w io.Writer) *Encoder {
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enc := new(Encoder)
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enc.w = []io.Writer{w}
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enc.sent = make(map[reflect.Type]typeId)
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enc.countState = enc.newEncoderState(new(bytes.Buffer))
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return enc
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}
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// writer() returns the innermost writer the encoder is using
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func (enc *Encoder) writer() io.Writer {
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return enc.w[len(enc.w)-1]
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}
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// pushWriter adds a writer to the encoder.
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func (enc *Encoder) pushWriter(w io.Writer) {
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enc.w = append(enc.w, w)
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}
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// popWriter pops the innermost writer.
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func (enc *Encoder) popWriter() {
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enc.w = enc.w[0 : len(enc.w)-1]
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}
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func (enc *Encoder) badType(rt reflect.Type) {
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enc.setError(errors.New("gob: can't encode type " + rt.String()))
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}
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func (enc *Encoder) setError(err error) {
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if enc.err == nil { // remember the first.
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enc.err = err
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}
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}
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// writeMessage sends the data item preceded by a unsigned count of its length.
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func (enc *Encoder) writeMessage(w io.Writer, b *bytes.Buffer) {
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// Space has been reserved for the length at the head of the message.
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// This is a little dirty: we grab the slice from the bytes.Buffer and massage
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// it by hand.
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message := b.Bytes()
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messageLen := len(message) - maxLength
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// Encode the length.
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enc.countState.b.Reset()
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enc.countState.encodeUint(uint64(messageLen))
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// Copy the length to be a prefix of the message.
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offset := maxLength - enc.countState.b.Len()
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copy(message[offset:], enc.countState.b.Bytes())
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// Write the data.
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_, err := w.Write(message[offset:])
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// Drain the buffer and restore the space at the front for the count of the next message.
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b.Reset()
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b.Write(spaceForLength)
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if err != nil {
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enc.setError(err)
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}
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}
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// sendActualType sends the requested type, without further investigation, unless
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// it's been sent before.
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func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTypeInfo, actual reflect.Type) (sent bool) {
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if _, alreadySent := enc.sent[actual]; alreadySent {
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return false
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}
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typeLock.Lock()
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info, err := getTypeInfo(ut)
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typeLock.Unlock()
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if err != nil {
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enc.setError(err)
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return
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}
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// Send the pair (-id, type)
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// Id:
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state.encodeInt(-int64(info.id))
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// Type:
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enc.encode(state.b, reflect.ValueOf(info.wire), wireTypeUserInfo)
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enc.writeMessage(w, state.b)
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if enc.err != nil {
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return
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}
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// Remember we've sent this type, both what the user gave us and the base type.
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enc.sent[ut.base] = info.id
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if ut.user != ut.base {
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enc.sent[ut.user] = info.id
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}
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// Now send the inner types
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switch st := actual; st.Kind() {
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case reflect.Struct:
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for i := 0; i < st.NumField(); i++ {
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if isExported(st.Field(i).Name) {
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enc.sendType(w, state, st.Field(i).Type)
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}
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}
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case reflect.Array, reflect.Slice:
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enc.sendType(w, state, st.Elem())
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case reflect.Map:
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enc.sendType(w, state, st.Key())
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enc.sendType(w, state, st.Elem())
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}
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return true
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}
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// sendType sends the type info to the other side, if necessary.
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func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Type) (sent bool) {
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ut := userType(origt)
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if ut.isGobEncoder {
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// The rules are different: regardless of the underlying type's representation,
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// we need to tell the other side that this exact type is a GobEncoder.
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return enc.sendActualType(w, state, ut, ut.user)
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}
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// It's a concrete value, so drill down to the base type.
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switch rt := ut.base; rt.Kind() {
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default:
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// Basic types and interfaces do not need to be described.
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return
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case reflect.Slice:
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// If it's []uint8, don't send; it's considered basic.
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if rt.Elem().Kind() == reflect.Uint8 {
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return
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}
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// Otherwise we do send.
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break
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case reflect.Array:
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// arrays must be sent so we know their lengths and element types.
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break
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case reflect.Map:
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// maps must be sent so we know their lengths and key/value types.
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break
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case reflect.Struct:
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// structs must be sent so we know their fields.
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break
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case reflect.Chan, reflect.Func:
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// Probably a bad field in a struct.
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enc.badType(rt)
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return
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}
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return enc.sendActualType(w, state, ut, ut.base)
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}
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// Encode transmits the data item represented by the empty interface value,
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// guaranteeing that all necessary type information has been transmitted first.
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func (enc *Encoder) Encode(e interface{}) error {
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return enc.EncodeValue(reflect.ValueOf(e))
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}
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// sendTypeDescriptor makes sure the remote side knows about this type.
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// It will send a descriptor if this is the first time the type has been
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// sent.
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func (enc *Encoder) sendTypeDescriptor(w io.Writer, state *encoderState, ut *userTypeInfo) {
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// Make sure the type is known to the other side.
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// First, have we already sent this type?
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rt := ut.base
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if ut.isGobEncoder {
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rt = ut.user
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}
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if _, alreadySent := enc.sent[rt]; !alreadySent {
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// No, so send it.
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sent := enc.sendType(w, state, rt)
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if enc.err != nil {
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return
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}
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// If the type info has still not been transmitted, it means we have
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// a singleton basic type (int, []byte etc.) at top level. We don't
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// need to send the type info but we do need to update enc.sent.
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if !sent {
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typeLock.Lock()
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info, err := getTypeInfo(ut)
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typeLock.Unlock()
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if err != nil {
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enc.setError(err)
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return
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}
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enc.sent[rt] = info.id
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}
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}
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}
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// sendTypeId sends the id, which must have already been defined.
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func (enc *Encoder) sendTypeId(state *encoderState, ut *userTypeInfo) {
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// Identify the type of this top-level value.
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state.encodeInt(int64(enc.sent[ut.base]))
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}
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// EncodeValue transmits the data item represented by the reflection value,
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// guaranteeing that all necessary type information has been transmitted first.
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func (enc *Encoder) EncodeValue(value reflect.Value) error {
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// Make sure we're single-threaded through here, so multiple
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// goroutines can share an encoder.
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enc.mutex.Lock()
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defer enc.mutex.Unlock()
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// Remove any nested writers remaining due to previous errors.
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enc.w = enc.w[0:1]
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ut, err := validUserType(value.Type())
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if err != nil {
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return err
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}
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enc.err = nil
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enc.byteBuf.Reset()
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enc.byteBuf.Write(spaceForLength)
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state := enc.newEncoderState(&enc.byteBuf)
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enc.sendTypeDescriptor(enc.writer(), state, ut)
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enc.sendTypeId(state, ut)
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if enc.err != nil {
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return enc.err
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}
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// Encode the object.
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enc.encode(state.b, value, ut)
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if enc.err == nil {
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enc.writeMessage(enc.writer(), state.b)
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}
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enc.freeEncoderState(state)
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return enc.err
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}
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