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// 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 gob
import (
"bufio"
"bytes"
"io"
"os"
"reflect"
"sync"
)
// A Decoder manages the receipt of type and data information read from the
// remote side of a connection.
type Decoder struct {
mutex sync.Mutex // each item must be received atomically
r io.Reader // source of the data
buf bytes.Buffer // buffer for more efficient i/o from r
wireType map[typeId]*wireType // map from remote ID to local description
decoderCache map[reflect.Type]map[typeId]**decEngine // cache of compiled engines
ignorerCache map[typeId]**decEngine // ditto for ignored objects
freeList *decoderState // list of free decoderStates; avoids reallocation
countBuf []byte // used for decoding integers while parsing messages
tmp []byte // temporary storage for i/o; saves reallocating
err os.Error
}
// NewDecoder returns a new decoder that reads from the io.Reader.
func NewDecoder(r io.Reader) *Decoder {
dec := new(Decoder)
dec.r = bufio.NewReader(r)
dec.wireType = make(map[typeId]*wireType)
dec.decoderCache = make(map[reflect.Type]map[typeId]**decEngine)
dec.ignorerCache = make(map[typeId]**decEngine)
dec.countBuf = make([]byte, 9) // counts may be uint64s (unlikely!), require 9 bytes
return dec
}
// recvType loads the definition of a type.
func (dec *Decoder) recvType(id typeId) {
// Have we already seen this type? That's an error
if id < firstUserId || dec.wireType[id] != nil {
dec.err = os.ErrorString("gob: duplicate type received")
return
}
// Type:
wire := new(wireType)
dec.decodeValue(tWireType, reflect.NewValue(wire))
if dec.err != nil {
return
}
// Remember we've seen this type.
dec.wireType[id] = wire
}
// recvMessage reads the next count-delimited item from the input. It is the converse
// of Encoder.writeMessage. It returns false on EOF or other error reading the message.
func (dec *Decoder) recvMessage() bool {
// Read a count.
nbytes, _, err := decodeUintReader(dec.r, dec.countBuf)
if err != nil {
dec.err = err
return false
}
dec.readMessage(int(nbytes))
return dec.err == nil
}
// readMessage reads the next nbytes bytes from the input.
func (dec *Decoder) readMessage(nbytes int) {
// Allocate the buffer.
if cap(dec.tmp) < nbytes {
dec.tmp = make([]byte, nbytes+100) // room to grow
}
dec.tmp = dec.tmp[:nbytes]
// Read the data
_, dec.err = io.ReadFull(dec.r, dec.tmp)
if dec.err != nil {
if dec.err == os.EOF {
dec.err = io.ErrUnexpectedEOF
}
return
}
dec.buf.Write(dec.tmp)
}
// toInt turns an encoded uint64 into an int, according to the marshaling rules.
func toInt(x uint64) int64 {
i := int64(x >> 1)
if x&1 != 0 {
i = ^i
}
return i
}
func (dec *Decoder) nextInt() int64 {
n, _, err := decodeUintReader(&dec.buf, dec.countBuf)
if err != nil {
dec.err = err
}
return toInt(n)
}
func (dec *Decoder) nextUint() uint64 {
n, _, err := decodeUintReader(&dec.buf, dec.countBuf)
if err != nil {
dec.err = err
}
return n
}
// decodeTypeSequence parses:
// TypeSequence
// (TypeDefinition DelimitedTypeDefinition*)?
// and returns the type id of the next value. It returns -1 at
// EOF. Upon return, the remainder of dec.buf is the value to be
// decoded. If this is an interface value, it can be ignored by
// simply resetting that buffer.
func (dec *Decoder) decodeTypeSequence(isInterface bool) typeId {
for dec.err == nil {
if dec.buf.Len() == 0 {
if !dec.recvMessage() {
break
}
}
// Receive a type id.
id := typeId(dec.nextInt())
if id >= 0 {
// Value follows.
return id
}
// Type definition for (-id) follows.
dec.recvType(-id)
// When decoding an interface, after a type there may be a
// DelimitedValue still in the buffer. Skip its count.
// (Alternatively, the buffer is empty and the byte count
// will be absorbed by recvMessage.)
if dec.buf.Len() > 0 {
if !isInterface {
dec.err = os.ErrorString("extra data in buffer")
break
}
dec.nextUint()
}
}
return -1
}
// Decode reads the next value from the connection and stores
// it in the data represented by the empty interface value.
// If e is nil, the value will be discarded. Otherwise,
// the value underlying e must either be the correct type for the next
// data item received, and must be a pointer.
func (dec *Decoder) Decode(e interface{}) os.Error {
if e == nil {
return dec.DecodeValue(reflect.Value{})
}
value := reflect.NewValue(e)
// If e represents a value as opposed to a pointer, the answer won't
// get back to the caller. Make sure it's a pointer.
if value.Type().Kind() != reflect.Ptr {
dec.err = os.ErrorString("gob: attempt to decode into a non-pointer")
return dec.err
}
return dec.DecodeValue(value)
}
// DecodeValue reads the next value from the connection and stores
// it in the data represented by the reflection value.
// The value must be the correct type for the next
// data item received, or it may be nil, which means the
// value will be discarded.
func (dec *Decoder) DecodeValue(value reflect.Value) os.Error {
// Make sure we're single-threaded through here.
dec.mutex.Lock()
defer dec.mutex.Unlock()
dec.buf.Reset() // In case data lingers from previous invocation.
dec.err = nil
id := dec.decodeTypeSequence(false)
if dec.err == nil {
dec.decodeValue(id, value)
}
return dec.err
}
// If debug.go is compiled into the program , debugFunc prints a human-readable
// representation of the gob data read from r by calling that file's Debug function.
// Otherwise it is nil.
var debugFunc func(io.Reader)
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