// 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"	"errors"	"io"	"reflect"	"sync") // tooBig provides a sanity check for sizes; used in several places.// Upper limit of 1GB, allowing room to grow a little without overflow.// TODO: make this adjustable?const tooBig = 1 << 30 // 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          decBuffer                               // 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	err          error} // NewDecoder returns a new decoder that reads from the io.Reader.// If r does not also implement io.ByteReader, it will be wrapped in a// bufio.Reader.func NewDecoder(r io.Reader) *Decoder {	dec := new(Decoder)	// We use the ability to read bytes as a plausible surrogate for buffering.	if _, ok := r.(io.ByteReader); !ok {		r = bufio.NewReader(r)	}	dec.r = 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 = errors.New("gob: duplicate type received")		return	} 	// Type:	wire := new(wireType)	dec.decodeValue(tWireType, reflect.ValueOf(wire))	if dec.err != nil {		return	}	// Remember we've seen this type.	dec.wireType[id] = wire} var errBadCount = errors.New("invalid message length") // 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	}	if nbytes >= tooBig {		dec.err = errBadCount		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) {	if dec.buf.Len() != 0 {		// The buffer should always be empty now.		panic("non-empty decoder buffer")	}	// Read the data	dec.buf.Size(nbytes)	_, dec.err = io.ReadFull(dec.r, dec.buf.Bytes())	if dec.err != nil {		if dec.err == io.EOF {			dec.err = io.ErrUnexpectedEOF		}	}} // 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// 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 = errors.New("extra data in buffer")				break			}			dec.nextUint()		}	}	return -1} // Decode reads the next value from the input stream 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 be a pointer to the// correct type for the next data item received.// If the input is at EOF, Decode returns io.EOF and// does not modify e.func (dec *Decoder) Decode(e interface{}) error {	if e == nil {		return dec.DecodeValue(reflect.Value{})	}	value := reflect.ValueOf(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 = errors.New("gob: attempt to decode into a non-pointer")		return dec.err	}	return dec.DecodeValue(value)} // DecodeValue reads the next value from the input stream.// If v is the zero reflect.Value (v.Kind() == Invalid), DecodeValue discards the value.// Otherwise, it stores the value into v.  In that case, v must represent// a non-nil pointer to data or be an assignable reflect.Value (v.CanSet())// If the input is at EOF, DecodeValue returns io.EOF and// does not modify v.func (dec *Decoder) DecodeValue(v reflect.Value) error {	if v.IsValid() {		if v.Kind() == reflect.Ptr && !v.IsNil() {			// That's okay, we'll store through the pointer.		} else if !v.CanSet() {			return errors.New("gob: DecodeValue of unassignable value")		}	}	// 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, v)	}	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)