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Source file src/encoding/gob/decoder.go

  // 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)
  

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