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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"
"errors"
"internal/saferio"
"io"
"reflect"
"sync"
)
// tooBig provides a sanity check for sizes; used in several places. Upper limit
// of is 1GB on 32-bit systems, 8GB on 64-bit, allowing room to grow a little
// without overflow.
const tooBig = (1 << 30) << (^uint(0) >> 62)
// A Decoder manages the receipt of type and data information read from the
// remote side of a connection. It is safe for concurrent use by multiple
// goroutines.
//
// The Decoder does only basic sanity checking on decoded input sizes,
// and its limits are not configurable. Take caution when decoding gob data
// from untrusted sources.
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
var buf []byte
buf, dec.err = saferio.ReadData(dec.r, uint64(nbytes))
dec.buf.SetBytes(buf)
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 {
firstMessage := true
for dec.err == nil {
if dec.buf.Len() == 0 {
if !dec.recvMessage() {
// We can only return io.EOF if the input was empty.
// If we read one or more type spec messages,
// require a data item message to follow.
// If we hit an EOF before that, then give ErrUnexpectedEOF.
if !firstMessage && dec.err == io.EOF {
dec.err = io.ErrUnexpectedEOF
}
break
}
}
// Receive a type id.
id := typeId(dec.nextInt())
if id >= 0 {
// Value follows.
return id
}
// Type definition for (-id) follows.
dec.recvType(-id)
if dec.err != nil {
break
}
// 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()
}
firstMessage = false
}
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 any) 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.Pointer {
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.Pointer && !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)
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