Source file src/encoding/gob/decode.go

Documentation: encoding/gob

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:generate go run decgen.go -output dec_helpers.go
     6  
     7  package gob
     8  
     9  import (
    10  	"encoding"
    11  	"errors"
    12  	"io"
    13  	"math"
    14  	"math/bits"
    15  	"reflect"
    16  )
    17  
    18  var (
    19  	errBadUint = errors.New("gob: encoded unsigned integer out of range")
    20  	errBadType = errors.New("gob: unknown type id or corrupted data")
    21  	errRange   = errors.New("gob: bad data: field numbers out of bounds")
    22  )
    23  
    24  type decHelper func(state *decoderState, v reflect.Value, length int, ovfl error) bool
    25  
    26  // decoderState is the execution state of an instance of the decoder. A new state
    27  // is created for nested objects.
    28  type decoderState struct {
    29  	dec *Decoder
    30  	// The buffer is stored with an extra indirection because it may be replaced
    31  	// if we load a type during decode (when reading an interface value).
    32  	b        *decBuffer
    33  	fieldnum int           // the last field number read.
    34  	next     *decoderState // for free list
    35  }
    36  
    37  // decBuffer is an extremely simple, fast implementation of a read-only byte buffer.
    38  // It is initialized by calling Size and then copying the data into the slice returned by Bytes().
    39  type decBuffer struct {
    40  	data   []byte
    41  	offset int // Read offset.
    42  }
    43  
    44  func (d *decBuffer) Read(p []byte) (int, error) {
    45  	n := copy(p, d.data[d.offset:])
    46  	if n == 0 && len(p) != 0 {
    47  		return 0, io.EOF
    48  	}
    49  	d.offset += n
    50  	return n, nil
    51  }
    52  
    53  func (d *decBuffer) Drop(n int) {
    54  	if n > d.Len() {
    55  		panic("drop")
    56  	}
    57  	d.offset += n
    58  }
    59  
    60  // Size grows the buffer to exactly n bytes, so d.Bytes() will
    61  // return a slice of length n. Existing data is first discarded.
    62  func (d *decBuffer) Size(n int) {
    63  	d.Reset()
    64  	if cap(d.data) < n {
    65  		d.data = make([]byte, n)
    66  	} else {
    67  		d.data = d.data[0:n]
    68  	}
    69  }
    70  
    71  func (d *decBuffer) ReadByte() (byte, error) {
    72  	if d.offset >= len(d.data) {
    73  		return 0, io.EOF
    74  	}
    75  	c := d.data[d.offset]
    76  	d.offset++
    77  	return c, nil
    78  }
    79  
    80  func (d *decBuffer) Len() int {
    81  	return len(d.data) - d.offset
    82  }
    83  
    84  func (d *decBuffer) Bytes() []byte {
    85  	return d.data[d.offset:]
    86  }
    87  
    88  func (d *decBuffer) Reset() {
    89  	d.data = d.data[0:0]
    90  	d.offset = 0
    91  }
    92  
    93  // We pass the bytes.Buffer separately for easier testing of the infrastructure
    94  // without requiring a full Decoder.
    95  func (dec *Decoder) newDecoderState(buf *decBuffer) *decoderState {
    96  	d := dec.freeList
    97  	if d == nil {
    98  		d = new(decoderState)
    99  		d.dec = dec
   100  	} else {
   101  		dec.freeList = d.next
   102  	}
   103  	d.b = buf
   104  	return d
   105  }
   106  
   107  func (dec *Decoder) freeDecoderState(d *decoderState) {
   108  	d.next = dec.freeList
   109  	dec.freeList = d
   110  }
   111  
   112  func overflow(name string) error {
   113  	return errors.New(`value for "` + name + `" out of range`)
   114  }
   115  
   116  // decodeUintReader reads an encoded unsigned integer from an io.Reader.
   117  // Used only by the Decoder to read the message length.
   118  func decodeUintReader(r io.Reader, buf []byte) (x uint64, width int, err error) {
   119  	width = 1
   120  	n, err := io.ReadFull(r, buf[0:width])
   121  	if n == 0 {
   122  		return
   123  	}
   124  	b := buf[0]
   125  	if b <= 0x7f {
   126  		return uint64(b), width, nil
   127  	}
   128  	n = -int(int8(b))
   129  	if n > uint64Size {
   130  		err = errBadUint
   131  		return
   132  	}
   133  	width, err = io.ReadFull(r, buf[0:n])
   134  	if err != nil {
   135  		if err == io.EOF {
   136  			err = io.ErrUnexpectedEOF
   137  		}
   138  		return
   139  	}
   140  	// Could check that the high byte is zero but it's not worth it.
   141  	for _, b := range buf[0:width] {
   142  		x = x<<8 | uint64(b)
   143  	}
   144  	width++ // +1 for length byte
   145  	return
   146  }
   147  
   148  // decodeUint reads an encoded unsigned integer from state.r.
   149  // Does not check for overflow.
   150  func (state *decoderState) decodeUint() (x uint64) {
   151  	b, err := state.b.ReadByte()
   152  	if err != nil {
   153  		error_(err)
   154  	}
   155  	if b <= 0x7f {
   156  		return uint64(b)
   157  	}
   158  	n := -int(int8(b))
   159  	if n > uint64Size {
   160  		error_(errBadUint)
   161  	}
   162  	buf := state.b.Bytes()
   163  	if len(buf) < n {
   164  		errorf("invalid uint data length %d: exceeds input size %d", n, len(buf))
   165  	}
   166  	// Don't need to check error; it's safe to loop regardless.
   167  	// Could check that the high byte is zero but it's not worth it.
   168  	for _, b := range buf[0:n] {
   169  		x = x<<8 | uint64(b)
   170  	}
   171  	state.b.Drop(n)
   172  	return x
   173  }
   174  
   175  // decodeInt reads an encoded signed integer from state.r.
   176  // Does not check for overflow.
   177  func (state *decoderState) decodeInt() int64 {
   178  	x := state.decodeUint()
   179  	if x&1 != 0 {
   180  		return ^int64(x >> 1)
   181  	}
   182  	return int64(x >> 1)
   183  }
   184  
   185  // getLength decodes the next uint and makes sure it is a possible
   186  // size for a data item that follows, which means it must fit in a
   187  // non-negative int and fit in the buffer.
   188  func (state *decoderState) getLength() (int, bool) {
   189  	n := int(state.decodeUint())
   190  	if n < 0 || state.b.Len() < n || tooBig <= n {
   191  		return 0, false
   192  	}
   193  	return n, true
   194  }
   195  
   196  // decOp is the signature of a decoding operator for a given type.
   197  type decOp func(i *decInstr, state *decoderState, v reflect.Value)
   198  
   199  // The 'instructions' of the decoding machine
   200  type decInstr struct {
   201  	op    decOp
   202  	field int   // field number of the wire type
   203  	index []int // field access indices for destination type
   204  	ovfl  error // error message for overflow/underflow (for arrays, of the elements)
   205  }
   206  
   207  // ignoreUint discards a uint value with no destination.
   208  func ignoreUint(i *decInstr, state *decoderState, v reflect.Value) {
   209  	state.decodeUint()
   210  }
   211  
   212  // ignoreTwoUints discards a uint value with no destination. It's used to skip
   213  // complex values.
   214  func ignoreTwoUints(i *decInstr, state *decoderState, v reflect.Value) {
   215  	state.decodeUint()
   216  	state.decodeUint()
   217  }
   218  
   219  // Since the encoder writes no zeros, if we arrive at a decoder we have
   220  // a value to extract and store. The field number has already been read
   221  // (it's how we knew to call this decoder).
   222  // Each decoder is responsible for handling any indirections associated
   223  // with the data structure. If any pointer so reached is nil, allocation must
   224  // be done.
   225  
   226  // decAlloc takes a value and returns a settable value that can
   227  // be assigned to. If the value is a pointer, decAlloc guarantees it points to storage.
   228  // The callers to the individual decoders are expected to have used decAlloc.
   229  // The individual decoders don't need to it.
   230  func decAlloc(v reflect.Value) reflect.Value {
   231  	for v.Kind() == reflect.Ptr {
   232  		if v.IsNil() {
   233  			v.Set(reflect.New(v.Type().Elem()))
   234  		}
   235  		v = v.Elem()
   236  	}
   237  	return v
   238  }
   239  
   240  // decBool decodes a uint and stores it as a boolean in value.
   241  func decBool(i *decInstr, state *decoderState, value reflect.Value) {
   242  	value.SetBool(state.decodeUint() != 0)
   243  }
   244  
   245  // decInt8 decodes an integer and stores it as an int8 in value.
   246  func decInt8(i *decInstr, state *decoderState, value reflect.Value) {
   247  	v := state.decodeInt()
   248  	if v < math.MinInt8 || math.MaxInt8 < v {
   249  		error_(i.ovfl)
   250  	}
   251  	value.SetInt(v)
   252  }
   253  
   254  // decUint8 decodes an unsigned integer and stores it as a uint8 in value.
   255  func decUint8(i *decInstr, state *decoderState, value reflect.Value) {
   256  	v := state.decodeUint()
   257  	if math.MaxUint8 < v {
   258  		error_(i.ovfl)
   259  	}
   260  	value.SetUint(v)
   261  }
   262  
   263  // decInt16 decodes an integer and stores it as an int16 in value.
   264  func decInt16(i *decInstr, state *decoderState, value reflect.Value) {
   265  	v := state.decodeInt()
   266  	if v < math.MinInt16 || math.MaxInt16 < v {
   267  		error_(i.ovfl)
   268  	}
   269  	value.SetInt(v)
   270  }
   271  
   272  // decUint16 decodes an unsigned integer and stores it as a uint16 in value.
   273  func decUint16(i *decInstr, state *decoderState, value reflect.Value) {
   274  	v := state.decodeUint()
   275  	if math.MaxUint16 < v {
   276  		error_(i.ovfl)
   277  	}
   278  	value.SetUint(v)
   279  }
   280  
   281  // decInt32 decodes an integer and stores it as an int32 in value.
   282  func decInt32(i *decInstr, state *decoderState, value reflect.Value) {
   283  	v := state.decodeInt()
   284  	if v < math.MinInt32 || math.MaxInt32 < v {
   285  		error_(i.ovfl)
   286  	}
   287  	value.SetInt(v)
   288  }
   289  
   290  // decUint32 decodes an unsigned integer and stores it as a uint32 in value.
   291  func decUint32(i *decInstr, state *decoderState, value reflect.Value) {
   292  	v := state.decodeUint()
   293  	if math.MaxUint32 < v {
   294  		error_(i.ovfl)
   295  	}
   296  	value.SetUint(v)
   297  }
   298  
   299  // decInt64 decodes an integer and stores it as an int64 in value.
   300  func decInt64(i *decInstr, state *decoderState, value reflect.Value) {
   301  	v := state.decodeInt()
   302  	value.SetInt(v)
   303  }
   304  
   305  // decUint64 decodes an unsigned integer and stores it as a uint64 in value.
   306  func decUint64(i *decInstr, state *decoderState, value reflect.Value) {
   307  	v := state.decodeUint()
   308  	value.SetUint(v)
   309  }
   310  
   311  // Floating-point numbers are transmitted as uint64s holding the bits
   312  // of the underlying representation. They are sent byte-reversed, with
   313  // the exponent end coming out first, so integer floating point numbers
   314  // (for example) transmit more compactly. This routine does the
   315  // unswizzling.
   316  func float64FromBits(u uint64) float64 {
   317  	v := bits.ReverseBytes64(u)
   318  	return math.Float64frombits(v)
   319  }
   320  
   321  // float32FromBits decodes an unsigned integer, treats it as a 32-bit floating-point
   322  // number, and returns it. It's a helper function for float32 and complex64.
   323  // It returns a float64 because that's what reflection needs, but its return
   324  // value is known to be accurately representable in a float32.
   325  func float32FromBits(u uint64, ovfl error) float64 {
   326  	v := float64FromBits(u)
   327  	av := v
   328  	if av < 0 {
   329  		av = -av
   330  	}
   331  	// +Inf is OK in both 32- and 64-bit floats. Underflow is always OK.
   332  	if math.MaxFloat32 < av && av <= math.MaxFloat64 {
   333  		error_(ovfl)
   334  	}
   335  	return v
   336  }
   337  
   338  // decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
   339  // number, and stores it in value.
   340  func decFloat32(i *decInstr, state *decoderState, value reflect.Value) {
   341  	value.SetFloat(float32FromBits(state.decodeUint(), i.ovfl))
   342  }
   343  
   344  // decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
   345  // number, and stores it in value.
   346  func decFloat64(i *decInstr, state *decoderState, value reflect.Value) {
   347  	value.SetFloat(float64FromBits(state.decodeUint()))
   348  }
   349  
   350  // decComplex64 decodes a pair of unsigned integers, treats them as a
   351  // pair of floating point numbers, and stores them as a complex64 in value.
   352  // The real part comes first.
   353  func decComplex64(i *decInstr, state *decoderState, value reflect.Value) {
   354  	real := float32FromBits(state.decodeUint(), i.ovfl)
   355  	imag := float32FromBits(state.decodeUint(), i.ovfl)
   356  	value.SetComplex(complex(real, imag))
   357  }
   358  
   359  // decComplex128 decodes a pair of unsigned integers, treats them as a
   360  // pair of floating point numbers, and stores them as a complex128 in value.
   361  // The real part comes first.
   362  func decComplex128(i *decInstr, state *decoderState, value reflect.Value) {
   363  	real := float64FromBits(state.decodeUint())
   364  	imag := float64FromBits(state.decodeUint())
   365  	value.SetComplex(complex(real, imag))
   366  }
   367  
   368  // decUint8Slice decodes a byte slice and stores in value a slice header
   369  // describing the data.
   370  // uint8 slices are encoded as an unsigned count followed by the raw bytes.
   371  func decUint8Slice(i *decInstr, state *decoderState, value reflect.Value) {
   372  	n, ok := state.getLength()
   373  	if !ok {
   374  		errorf("bad %s slice length: %d", value.Type(), n)
   375  	}
   376  	if value.Cap() < n {
   377  		value.Set(reflect.MakeSlice(value.Type(), n, n))
   378  	} else {
   379  		value.Set(value.Slice(0, n))
   380  	}
   381  	if _, err := state.b.Read(value.Bytes()); err != nil {
   382  		errorf("error decoding []byte: %s", err)
   383  	}
   384  }
   385  
   386  // decString decodes byte array and stores in value a string header
   387  // describing the data.
   388  // Strings are encoded as an unsigned count followed by the raw bytes.
   389  func decString(i *decInstr, state *decoderState, value reflect.Value) {
   390  	n, ok := state.getLength()
   391  	if !ok {
   392  		errorf("bad %s slice length: %d", value.Type(), n)
   393  	}
   394  	// Read the data.
   395  	data := state.b.Bytes()
   396  	if len(data) < n {
   397  		errorf("invalid string length %d: exceeds input size %d", n, len(data))
   398  	}
   399  	s := string(data[:n])
   400  	state.b.Drop(n)
   401  	value.SetString(s)
   402  }
   403  
   404  // ignoreUint8Array skips over the data for a byte slice value with no destination.
   405  func ignoreUint8Array(i *decInstr, state *decoderState, value reflect.Value) {
   406  	n, ok := state.getLength()
   407  	if !ok {
   408  		errorf("slice length too large")
   409  	}
   410  	bn := state.b.Len()
   411  	if bn < n {
   412  		errorf("invalid slice length %d: exceeds input size %d", n, bn)
   413  	}
   414  	state.b.Drop(n)
   415  }
   416  
   417  // Execution engine
   418  
   419  // The encoder engine is an array of instructions indexed by field number of the incoming
   420  // decoder. It is executed with random access according to field number.
   421  type decEngine struct {
   422  	instr    []decInstr
   423  	numInstr int // the number of active instructions
   424  }
   425  
   426  // decodeSingle decodes a top-level value that is not a struct and stores it in value.
   427  // Such values are preceded by a zero, making them have the memory layout of a
   428  // struct field (although with an illegal field number).
   429  func (dec *Decoder) decodeSingle(engine *decEngine, value reflect.Value) {
   430  	state := dec.newDecoderState(&dec.buf)
   431  	defer dec.freeDecoderState(state)
   432  	state.fieldnum = singletonField
   433  	if state.decodeUint() != 0 {
   434  		errorf("decode: corrupted data: non-zero delta for singleton")
   435  	}
   436  	instr := &engine.instr[singletonField]
   437  	instr.op(instr, state, value)
   438  }
   439  
   440  // decodeStruct decodes a top-level struct and stores it in value.
   441  // Indir is for the value, not the type. At the time of the call it may
   442  // differ from ut.indir, which was computed when the engine was built.
   443  // This state cannot arise for decodeSingle, which is called directly
   444  // from the user's value, not from the innards of an engine.
   445  func (dec *Decoder) decodeStruct(engine *decEngine, value reflect.Value) {
   446  	state := dec.newDecoderState(&dec.buf)
   447  	defer dec.freeDecoderState(state)
   448  	state.fieldnum = -1
   449  	for state.b.Len() > 0 {
   450  		delta := int(state.decodeUint())
   451  		if delta < 0 {
   452  			errorf("decode: corrupted data: negative delta")
   453  		}
   454  		if delta == 0 { // struct terminator is zero delta fieldnum
   455  			break
   456  		}
   457  		fieldnum := state.fieldnum + delta
   458  		if fieldnum >= len(engine.instr) {
   459  			error_(errRange)
   460  			break
   461  		}
   462  		instr := &engine.instr[fieldnum]
   463  		var field reflect.Value
   464  		if instr.index != nil {
   465  			// Otherwise the field is unknown to us and instr.op is an ignore op.
   466  			field = value.FieldByIndex(instr.index)
   467  			if field.Kind() == reflect.Ptr {
   468  				field = decAlloc(field)
   469  			}
   470  		}
   471  		instr.op(instr, state, field)
   472  		state.fieldnum = fieldnum
   473  	}
   474  }
   475  
   476  var noValue reflect.Value
   477  
   478  // ignoreStruct discards the data for a struct with no destination.
   479  func (dec *Decoder) ignoreStruct(engine *decEngine) {
   480  	state := dec.newDecoderState(&dec.buf)
   481  	defer dec.freeDecoderState(state)
   482  	state.fieldnum = -1
   483  	for state.b.Len() > 0 {
   484  		delta := int(state.decodeUint())
   485  		if delta < 0 {
   486  			errorf("ignore decode: corrupted data: negative delta")
   487  		}
   488  		if delta == 0 { // struct terminator is zero delta fieldnum
   489  			break
   490  		}
   491  		fieldnum := state.fieldnum + delta
   492  		if fieldnum >= len(engine.instr) {
   493  			error_(errRange)
   494  		}
   495  		instr := &engine.instr[fieldnum]
   496  		instr.op(instr, state, noValue)
   497  		state.fieldnum = fieldnum
   498  	}
   499  }
   500  
   501  // ignoreSingle discards the data for a top-level non-struct value with no
   502  // destination. It's used when calling Decode with a nil value.
   503  func (dec *Decoder) ignoreSingle(engine *decEngine) {
   504  	state := dec.newDecoderState(&dec.buf)
   505  	defer dec.freeDecoderState(state)
   506  	state.fieldnum = singletonField
   507  	delta := int(state.decodeUint())
   508  	if delta != 0 {
   509  		errorf("decode: corrupted data: non-zero delta for singleton")
   510  	}
   511  	instr := &engine.instr[singletonField]
   512  	instr.op(instr, state, noValue)
   513  }
   514  
   515  // decodeArrayHelper does the work for decoding arrays and slices.
   516  func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
   517  	if helper != nil && helper(state, value, length, ovfl) {
   518  		return
   519  	}
   520  	instr := &decInstr{elemOp, 0, nil, ovfl}
   521  	isPtr := value.Type().Elem().Kind() == reflect.Ptr
   522  	for i := 0; i < length; i++ {
   523  		if state.b.Len() == 0 {
   524  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
   525  		}
   526  		v := value.Index(i)
   527  		if isPtr {
   528  			v = decAlloc(v)
   529  		}
   530  		elemOp(instr, state, v)
   531  	}
   532  }
   533  
   534  // decodeArray decodes an array and stores it in value.
   535  // The length is an unsigned integer preceding the elements. Even though the length is redundant
   536  // (it's part of the type), it's a useful check and is included in the encoding.
   537  func (dec *Decoder) decodeArray(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
   538  	if n := state.decodeUint(); n != uint64(length) {
   539  		errorf("length mismatch in decodeArray")
   540  	}
   541  	dec.decodeArrayHelper(state, value, elemOp, length, ovfl, helper)
   542  }
   543  
   544  // decodeIntoValue is a helper for map decoding.
   545  func decodeIntoValue(state *decoderState, op decOp, isPtr bool, value reflect.Value, instr *decInstr) reflect.Value {
   546  	v := value
   547  	if isPtr {
   548  		v = decAlloc(value)
   549  	}
   550  
   551  	op(instr, state, v)
   552  	return value
   553  }
   554  
   555  // decodeMap decodes a map and stores it in value.
   556  // Maps are encoded as a length followed by key:value pairs.
   557  // Because the internals of maps are not visible to us, we must
   558  // use reflection rather than pointer magic.
   559  func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, value reflect.Value, keyOp, elemOp decOp, ovfl error) {
   560  	n := int(state.decodeUint())
   561  	if value.IsNil() {
   562  		value.Set(reflect.MakeMapWithSize(mtyp, n))
   563  	}
   564  	keyIsPtr := mtyp.Key().Kind() == reflect.Ptr
   565  	elemIsPtr := mtyp.Elem().Kind() == reflect.Ptr
   566  	keyInstr := &decInstr{keyOp, 0, nil, ovfl}
   567  	elemInstr := &decInstr{elemOp, 0, nil, ovfl}
   568  	keyP := reflect.New(mtyp.Key())
   569  	keyZ := reflect.Zero(mtyp.Key())
   570  	elemP := reflect.New(mtyp.Elem())
   571  	elemZ := reflect.Zero(mtyp.Elem())
   572  	for i := 0; i < n; i++ {
   573  		key := decodeIntoValue(state, keyOp, keyIsPtr, keyP.Elem(), keyInstr)
   574  		elem := decodeIntoValue(state, elemOp, elemIsPtr, elemP.Elem(), elemInstr)
   575  		value.SetMapIndex(key, elem)
   576  		keyP.Elem().Set(keyZ)
   577  		elemP.Elem().Set(elemZ)
   578  	}
   579  }
   580  
   581  // ignoreArrayHelper does the work for discarding arrays and slices.
   582  func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
   583  	instr := &decInstr{elemOp, 0, nil, errors.New("no error")}
   584  	for i := 0; i < length; i++ {
   585  		if state.b.Len() == 0 {
   586  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
   587  		}
   588  		elemOp(instr, state, noValue)
   589  	}
   590  }
   591  
   592  // ignoreArray discards the data for an array value with no destination.
   593  func (dec *Decoder) ignoreArray(state *decoderState, elemOp decOp, length int) {
   594  	if n := state.decodeUint(); n != uint64(length) {
   595  		errorf("length mismatch in ignoreArray")
   596  	}
   597  	dec.ignoreArrayHelper(state, elemOp, length)
   598  }
   599  
   600  // ignoreMap discards the data for a map value with no destination.
   601  func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
   602  	n := int(state.decodeUint())
   603  	keyInstr := &decInstr{keyOp, 0, nil, errors.New("no error")}
   604  	elemInstr := &decInstr{elemOp, 0, nil, errors.New("no error")}
   605  	for i := 0; i < n; i++ {
   606  		keyOp(keyInstr, state, noValue)
   607  		elemOp(elemInstr, state, noValue)
   608  	}
   609  }
   610  
   611  // decodeSlice decodes a slice and stores it in value.
   612  // Slices are encoded as an unsigned length followed by the elements.
   613  func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp decOp, ovfl error, helper decHelper) {
   614  	u := state.decodeUint()
   615  	typ := value.Type()
   616  	size := uint64(typ.Elem().Size())
   617  	nBytes := u * size
   618  	n := int(u)
   619  	// Take care with overflow in this calculation.
   620  	if n < 0 || uint64(n) != u || nBytes > tooBig || (size > 0 && nBytes/size != u) {
   621  		// We don't check n against buffer length here because if it's a slice
   622  		// of interfaces, there will be buffer reloads.
   623  		errorf("%s slice too big: %d elements of %d bytes", typ.Elem(), u, size)
   624  	}
   625  	if value.Cap() < n {
   626  		value.Set(reflect.MakeSlice(typ, n, n))
   627  	} else {
   628  		value.Set(value.Slice(0, n))
   629  	}
   630  	dec.decodeArrayHelper(state, value, elemOp, n, ovfl, helper)
   631  }
   632  
   633  // ignoreSlice skips over the data for a slice value with no destination.
   634  func (dec *Decoder) ignoreSlice(state *decoderState, elemOp decOp) {
   635  	dec.ignoreArrayHelper(state, elemOp, int(state.decodeUint()))
   636  }
   637  
   638  // decodeInterface decodes an interface value and stores it in value.
   639  // Interfaces are encoded as the name of a concrete type followed by a value.
   640  // If the name is empty, the value is nil and no value is sent.
   641  func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, value reflect.Value) {
   642  	// Read the name of the concrete type.
   643  	nr := state.decodeUint()
   644  	if nr > 1<<31 { // zero is permissible for anonymous types
   645  		errorf("invalid type name length %d", nr)
   646  	}
   647  	if nr > uint64(state.b.Len()) {
   648  		errorf("invalid type name length %d: exceeds input size", nr)
   649  	}
   650  	n := int(nr)
   651  	name := state.b.Bytes()[:n]
   652  	state.b.Drop(n)
   653  	// Allocate the destination interface value.
   654  	if len(name) == 0 {
   655  		// Copy the nil interface value to the target.
   656  		value.Set(reflect.Zero(value.Type()))
   657  		return
   658  	}
   659  	if len(name) > 1024 {
   660  		errorf("name too long (%d bytes): %.20q...", len(name), name)
   661  	}
   662  	// The concrete type must be registered.
   663  	typi, ok := nameToConcreteType.Load(string(name))
   664  	if !ok {
   665  		errorf("name not registered for interface: %q", name)
   666  	}
   667  	typ := typi.(reflect.Type)
   668  
   669  	// Read the type id of the concrete value.
   670  	concreteId := dec.decodeTypeSequence(true)
   671  	if concreteId < 0 {
   672  		error_(dec.err)
   673  	}
   674  	// Byte count of value is next; we don't care what it is (it's there
   675  	// in case we want to ignore the value by skipping it completely).
   676  	state.decodeUint()
   677  	// Read the concrete value.
   678  	v := allocValue(typ)
   679  	dec.decodeValue(concreteId, v)
   680  	if dec.err != nil {
   681  		error_(dec.err)
   682  	}
   683  	// Assign the concrete value to the interface.
   684  	// Tread carefully; it might not satisfy the interface.
   685  	if !typ.AssignableTo(ityp) {
   686  		errorf("%s is not assignable to type %s", typ, ityp)
   687  	}
   688  	// Copy the interface value to the target.
   689  	value.Set(v)
   690  }
   691  
   692  // ignoreInterface discards the data for an interface value with no destination.
   693  func (dec *Decoder) ignoreInterface(state *decoderState) {
   694  	// Read the name of the concrete type.
   695  	n, ok := state.getLength()
   696  	if !ok {
   697  		errorf("bad interface encoding: name too large for buffer")
   698  	}
   699  	bn := state.b.Len()
   700  	if bn < n {
   701  		errorf("invalid interface value length %d: exceeds input size %d", n, bn)
   702  	}
   703  	state.b.Drop(n)
   704  	id := dec.decodeTypeSequence(true)
   705  	if id < 0 {
   706  		error_(dec.err)
   707  	}
   708  	// At this point, the decoder buffer contains a delimited value. Just toss it.
   709  	n, ok = state.getLength()
   710  	if !ok {
   711  		errorf("bad interface encoding: data length too large for buffer")
   712  	}
   713  	state.b.Drop(n)
   714  }
   715  
   716  // decodeGobDecoder decodes something implementing the GobDecoder interface.
   717  // The data is encoded as a byte slice.
   718  func (dec *Decoder) decodeGobDecoder(ut *userTypeInfo, state *decoderState, value reflect.Value) {
   719  	// Read the bytes for the value.
   720  	n, ok := state.getLength()
   721  	if !ok {
   722  		errorf("GobDecoder: length too large for buffer")
   723  	}
   724  	b := state.b.Bytes()
   725  	if len(b) < n {
   726  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, len(b))
   727  	}
   728  	b = b[:n]
   729  	state.b.Drop(n)
   730  	var err error
   731  	// We know it's one of these.
   732  	switch ut.externalDec {
   733  	case xGob:
   734  		err = value.Interface().(GobDecoder).GobDecode(b)
   735  	case xBinary:
   736  		err = value.Interface().(encoding.BinaryUnmarshaler).UnmarshalBinary(b)
   737  	case xText:
   738  		err = value.Interface().(encoding.TextUnmarshaler).UnmarshalText(b)
   739  	}
   740  	if err != nil {
   741  		error_(err)
   742  	}
   743  }
   744  
   745  // ignoreGobDecoder discards the data for a GobDecoder value with no destination.
   746  func (dec *Decoder) ignoreGobDecoder(state *decoderState) {
   747  	// Read the bytes for the value.
   748  	n, ok := state.getLength()
   749  	if !ok {
   750  		errorf("GobDecoder: length too large for buffer")
   751  	}
   752  	bn := state.b.Len()
   753  	if bn < n {
   754  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, bn)
   755  	}
   756  	state.b.Drop(n)
   757  }
   758  
   759  // Index by Go types.
   760  var decOpTable = [...]decOp{
   761  	reflect.Bool:       decBool,
   762  	reflect.Int8:       decInt8,
   763  	reflect.Int16:      decInt16,
   764  	reflect.Int32:      decInt32,
   765  	reflect.Int64:      decInt64,
   766  	reflect.Uint8:      decUint8,
   767  	reflect.Uint16:     decUint16,
   768  	reflect.Uint32:     decUint32,
   769  	reflect.Uint64:     decUint64,
   770  	reflect.Float32:    decFloat32,
   771  	reflect.Float64:    decFloat64,
   772  	reflect.Complex64:  decComplex64,
   773  	reflect.Complex128: decComplex128,
   774  	reflect.String:     decString,
   775  }
   776  
   777  // Indexed by gob types.  tComplex will be added during type.init().
   778  var decIgnoreOpMap = map[typeId]decOp{
   779  	tBool:    ignoreUint,
   780  	tInt:     ignoreUint,
   781  	tUint:    ignoreUint,
   782  	tFloat:   ignoreUint,
   783  	tBytes:   ignoreUint8Array,
   784  	tString:  ignoreUint8Array,
   785  	tComplex: ignoreTwoUints,
   786  }
   787  
   788  // decOpFor returns the decoding op for the base type under rt and
   789  // the indirection count to reach it.
   790  func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProgress map[reflect.Type]*decOp) *decOp {
   791  	ut := userType(rt)
   792  	// If the type implements GobEncoder, we handle it without further processing.
   793  	if ut.externalDec != 0 {
   794  		return dec.gobDecodeOpFor(ut)
   795  	}
   796  
   797  	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   798  	// Return the pointer to the op we're already building.
   799  	if opPtr := inProgress[rt]; opPtr != nil {
   800  		return opPtr
   801  	}
   802  	typ := ut.base
   803  	var op decOp
   804  	k := typ.Kind()
   805  	if int(k) < len(decOpTable) {
   806  		op = decOpTable[k]
   807  	}
   808  	if op == nil {
   809  		inProgress[rt] = &op
   810  		// Special cases
   811  		switch t := typ; t.Kind() {
   812  		case reflect.Array:
   813  			name = "element of " + name
   814  			elemId := dec.wireType[wireId].ArrayT.Elem
   815  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   816  			ovfl := overflow(name)
   817  			helper := decArrayHelper[t.Elem().Kind()]
   818  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   819  				state.dec.decodeArray(state, value, *elemOp, t.Len(), ovfl, helper)
   820  			}
   821  
   822  		case reflect.Map:
   823  			keyId := dec.wireType[wireId].MapT.Key
   824  			elemId := dec.wireType[wireId].MapT.Elem
   825  			keyOp := dec.decOpFor(keyId, t.Key(), "key of "+name, inProgress)
   826  			elemOp := dec.decOpFor(elemId, t.Elem(), "element of "+name, inProgress)
   827  			ovfl := overflow(name)
   828  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   829  				state.dec.decodeMap(t, state, value, *keyOp, *elemOp, ovfl)
   830  			}
   831  
   832  		case reflect.Slice:
   833  			name = "element of " + name
   834  			if t.Elem().Kind() == reflect.Uint8 {
   835  				op = decUint8Slice
   836  				break
   837  			}
   838  			var elemId typeId
   839  			if tt, ok := builtinIdToType[wireId]; ok {
   840  				elemId = tt.(*sliceType).Elem
   841  			} else {
   842  				elemId = dec.wireType[wireId].SliceT.Elem
   843  			}
   844  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   845  			ovfl := overflow(name)
   846  			helper := decSliceHelper[t.Elem().Kind()]
   847  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   848  				state.dec.decodeSlice(state, value, *elemOp, ovfl, helper)
   849  			}
   850  
   851  		case reflect.Struct:
   852  			// Generate a closure that calls out to the engine for the nested type.
   853  			ut := userType(typ)
   854  			enginePtr, err := dec.getDecEnginePtr(wireId, ut)
   855  			if err != nil {
   856  				error_(err)
   857  			}
   858  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   859  				// indirect through enginePtr to delay evaluation for recursive structs.
   860  				dec.decodeStruct(*enginePtr, value)
   861  			}
   862  		case reflect.Interface:
   863  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   864  				state.dec.decodeInterface(t, state, value)
   865  			}
   866  		}
   867  	}
   868  	if op == nil {
   869  		errorf("decode can't handle type %s", rt)
   870  	}
   871  	return &op
   872  }
   873  
   874  // decIgnoreOpFor returns the decoding op for a field that has no destination.
   875  func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp) *decOp {
   876  	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   877  	// Return the pointer to the op we're already building.
   878  	if opPtr := inProgress[wireId]; opPtr != nil {
   879  		return opPtr
   880  	}
   881  	op, ok := decIgnoreOpMap[wireId]
   882  	if !ok {
   883  		inProgress[wireId] = &op
   884  		if wireId == tInterface {
   885  			// Special case because it's a method: the ignored item might
   886  			// define types and we need to record their state in the decoder.
   887  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   888  				state.dec.ignoreInterface(state)
   889  			}
   890  			return &op
   891  		}
   892  		// Special cases
   893  		wire := dec.wireType[wireId]
   894  		switch {
   895  		case wire == nil:
   896  			errorf("bad data: undefined type %s", wireId.string())
   897  		case wire.ArrayT != nil:
   898  			elemId := wire.ArrayT.Elem
   899  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   900  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   901  				state.dec.ignoreArray(state, *elemOp, wire.ArrayT.Len)
   902  			}
   903  
   904  		case wire.MapT != nil:
   905  			keyId := dec.wireType[wireId].MapT.Key
   906  			elemId := dec.wireType[wireId].MapT.Elem
   907  			keyOp := dec.decIgnoreOpFor(keyId, inProgress)
   908  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   909  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   910  				state.dec.ignoreMap(state, *keyOp, *elemOp)
   911  			}
   912  
   913  		case wire.SliceT != nil:
   914  			elemId := wire.SliceT.Elem
   915  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   916  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   917  				state.dec.ignoreSlice(state, *elemOp)
   918  			}
   919  
   920  		case wire.StructT != nil:
   921  			// Generate a closure that calls out to the engine for the nested type.
   922  			enginePtr, err := dec.getIgnoreEnginePtr(wireId)
   923  			if err != nil {
   924  				error_(err)
   925  			}
   926  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   927  				// indirect through enginePtr to delay evaluation for recursive structs
   928  				state.dec.ignoreStruct(*enginePtr)
   929  			}
   930  
   931  		case wire.GobEncoderT != nil, wire.BinaryMarshalerT != nil, wire.TextMarshalerT != nil:
   932  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   933  				state.dec.ignoreGobDecoder(state)
   934  			}
   935  		}
   936  	}
   937  	if op == nil {
   938  		errorf("bad data: ignore can't handle type %s", wireId.string())
   939  	}
   940  	return &op
   941  }
   942  
   943  // gobDecodeOpFor returns the op for a type that is known to implement
   944  // GobDecoder.
   945  func (dec *Decoder) gobDecodeOpFor(ut *userTypeInfo) *decOp {
   946  	rcvrType := ut.user
   947  	if ut.decIndir == -1 {
   948  		rcvrType = reflect.PtrTo(rcvrType)
   949  	} else if ut.decIndir > 0 {
   950  		for i := int8(0); i < ut.decIndir; i++ {
   951  			rcvrType = rcvrType.Elem()
   952  		}
   953  	}
   954  	var op decOp
   955  	op = func(i *decInstr, state *decoderState, value reflect.Value) {
   956  		// We now have the base type. We need its address if the receiver is a pointer.
   957  		if value.Kind() != reflect.Ptr && rcvrType.Kind() == reflect.Ptr {
   958  			value = value.Addr()
   959  		}
   960  		state.dec.decodeGobDecoder(ut, state, value)
   961  	}
   962  	return &op
   963  }
   964  
   965  // compatibleType asks: Are these two gob Types compatible?
   966  // Answers the question for basic types, arrays, maps and slices, plus
   967  // GobEncoder/Decoder pairs.
   968  // Structs are considered ok; fields will be checked later.
   969  func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[reflect.Type]typeId) bool {
   970  	if rhs, ok := inProgress[fr]; ok {
   971  		return rhs == fw
   972  	}
   973  	inProgress[fr] = fw
   974  	ut := userType(fr)
   975  	wire, ok := dec.wireType[fw]
   976  	// If wire was encoded with an encoding method, fr must have that method.
   977  	// And if not, it must not.
   978  	// At most one of the booleans in ut is set.
   979  	// We could possibly relax this constraint in the future in order to
   980  	// choose the decoding method using the data in the wireType.
   981  	// The parentheses look odd but are correct.
   982  	if (ut.externalDec == xGob) != (ok && wire.GobEncoderT != nil) ||
   983  		(ut.externalDec == xBinary) != (ok && wire.BinaryMarshalerT != nil) ||
   984  		(ut.externalDec == xText) != (ok && wire.TextMarshalerT != nil) {
   985  		return false
   986  	}
   987  	if ut.externalDec != 0 { // This test trumps all others.
   988  		return true
   989  	}
   990  	switch t := ut.base; t.Kind() {
   991  	default:
   992  		// chan, etc: cannot handle.
   993  		return false
   994  	case reflect.Bool:
   995  		return fw == tBool
   996  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   997  		return fw == tInt
   998  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   999  		return fw == tUint
  1000  	case reflect.Float32, reflect.Float64:
  1001  		return fw == tFloat
  1002  	case reflect.Complex64, reflect.Complex128:
  1003  		return fw == tComplex
  1004  	case reflect.String:
  1005  		return fw == tString
  1006  	case reflect.Interface:
  1007  		return fw == tInterface
  1008  	case reflect.Array:
  1009  		if !ok || wire.ArrayT == nil {
  1010  			return false
  1011  		}
  1012  		array := wire.ArrayT
  1013  		return t.Len() == array.Len && dec.compatibleType(t.Elem(), array.Elem, inProgress)
  1014  	case reflect.Map:
  1015  		if !ok || wire.MapT == nil {
  1016  			return false
  1017  		}
  1018  		MapType := wire.MapT
  1019  		return dec.compatibleType(t.Key(), MapType.Key, inProgress) && dec.compatibleType(t.Elem(), MapType.Elem, inProgress)
  1020  	case reflect.Slice:
  1021  		// Is it an array of bytes?
  1022  		if t.Elem().Kind() == reflect.Uint8 {
  1023  			return fw == tBytes
  1024  		}
  1025  		// Extract and compare element types.
  1026  		var sw *sliceType
  1027  		if tt, ok := builtinIdToType[fw]; ok {
  1028  			sw, _ = tt.(*sliceType)
  1029  		} else if wire != nil {
  1030  			sw = wire.SliceT
  1031  		}
  1032  		elem := userType(t.Elem()).base
  1033  		return sw != nil && dec.compatibleType(elem, sw.Elem, inProgress)
  1034  	case reflect.Struct:
  1035  		return true
  1036  	}
  1037  }
  1038  
  1039  // typeString returns a human-readable description of the type identified by remoteId.
  1040  func (dec *Decoder) typeString(remoteId typeId) string {
  1041  	typeLock.Lock()
  1042  	defer typeLock.Unlock()
  1043  	if t := idToType[remoteId]; t != nil {
  1044  		// globally known type.
  1045  		return t.string()
  1046  	}
  1047  	return dec.wireType[remoteId].string()
  1048  }
  1049  
  1050  // compileSingle compiles the decoder engine for a non-struct top-level value, including
  1051  // GobDecoders.
  1052  func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1053  	rt := ut.user
  1054  	engine = new(decEngine)
  1055  	engine.instr = make([]decInstr, 1) // one item
  1056  	name := rt.String()                // best we can do
  1057  	if !dec.compatibleType(rt, remoteId, make(map[reflect.Type]typeId)) {
  1058  		remoteType := dec.typeString(remoteId)
  1059  		// Common confusing case: local interface type, remote concrete type.
  1060  		if ut.base.Kind() == reflect.Interface && remoteId != tInterface {
  1061  			return nil, errors.New("gob: local interface type " + name + " can only be decoded from remote interface type; received concrete type " + remoteType)
  1062  		}
  1063  		return nil, errors.New("gob: decoding into local type " + name + ", received remote type " + remoteType)
  1064  	}
  1065  	op := dec.decOpFor(remoteId, rt, name, make(map[reflect.Type]*decOp))
  1066  	ovfl := errors.New(`value for "` + name + `" out of range`)
  1067  	engine.instr[singletonField] = decInstr{*op, singletonField, nil, ovfl}
  1068  	engine.numInstr = 1
  1069  	return
  1070  }
  1071  
  1072  // compileIgnoreSingle compiles the decoder engine for a non-struct top-level value that will be discarded.
  1073  func (dec *Decoder) compileIgnoreSingle(remoteId typeId) *decEngine {
  1074  	engine := new(decEngine)
  1075  	engine.instr = make([]decInstr, 1) // one item
  1076  	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp))
  1077  	ovfl := overflow(dec.typeString(remoteId))
  1078  	engine.instr[0] = decInstr{*op, 0, nil, ovfl}
  1079  	engine.numInstr = 1
  1080  	return engine
  1081  }
  1082  
  1083  // compileDec compiles the decoder engine for a value. If the value is not a struct,
  1084  // it calls out to compileSingle.
  1085  func (dec *Decoder) compileDec(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1086  	defer catchError(&err)
  1087  	rt := ut.base
  1088  	srt := rt
  1089  	if srt.Kind() != reflect.Struct || ut.externalDec != 0 {
  1090  		return dec.compileSingle(remoteId, ut)
  1091  	}
  1092  	var wireStruct *structType
  1093  	// Builtin types can come from global pool; the rest must be defined by the decoder.
  1094  	// Also we know we're decoding a struct now, so the client must have sent one.
  1095  	if t, ok := builtinIdToType[remoteId]; ok {
  1096  		wireStruct, _ = t.(*structType)
  1097  	} else {
  1098  		wire := dec.wireType[remoteId]
  1099  		if wire == nil {
  1100  			error_(errBadType)
  1101  		}
  1102  		wireStruct = wire.StructT
  1103  	}
  1104  	if wireStruct == nil {
  1105  		errorf("type mismatch in decoder: want struct type %s; got non-struct", rt)
  1106  	}
  1107  	engine = new(decEngine)
  1108  	engine.instr = make([]decInstr, len(wireStruct.Field))
  1109  	seen := make(map[reflect.Type]*decOp)
  1110  	// Loop over the fields of the wire type.
  1111  	for fieldnum := 0; fieldnum < len(wireStruct.Field); fieldnum++ {
  1112  		wireField := wireStruct.Field[fieldnum]
  1113  		if wireField.Name == "" {
  1114  			errorf("empty name for remote field of type %s", wireStruct.Name)
  1115  		}
  1116  		ovfl := overflow(wireField.Name)
  1117  		// Find the field of the local type with the same name.
  1118  		localField, present := srt.FieldByName(wireField.Name)
  1119  		// TODO(r): anonymous names
  1120  		if !present || !isExported(wireField.Name) {
  1121  			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp))
  1122  			engine.instr[fieldnum] = decInstr{*op, fieldnum, nil, ovfl}
  1123  			continue
  1124  		}
  1125  		if !dec.compatibleType(localField.Type, wireField.Id, make(map[reflect.Type]typeId)) {
  1126  			errorf("wrong type (%s) for received field %s.%s", localField.Type, wireStruct.Name, wireField.Name)
  1127  		}
  1128  		op := dec.decOpFor(wireField.Id, localField.Type, localField.Name, seen)
  1129  		engine.instr[fieldnum] = decInstr{*op, fieldnum, localField.Index, ovfl}
  1130  		engine.numInstr++
  1131  	}
  1132  	return
  1133  }
  1134  
  1135  // getDecEnginePtr returns the engine for the specified type.
  1136  func (dec *Decoder) getDecEnginePtr(remoteId typeId, ut *userTypeInfo) (enginePtr **decEngine, err error) {
  1137  	rt := ut.user
  1138  	decoderMap, ok := dec.decoderCache[rt]
  1139  	if !ok {
  1140  		decoderMap = make(map[typeId]**decEngine)
  1141  		dec.decoderCache[rt] = decoderMap
  1142  	}
  1143  	if enginePtr, ok = decoderMap[remoteId]; !ok {
  1144  		// To handle recursive types, mark this engine as underway before compiling.
  1145  		enginePtr = new(*decEngine)
  1146  		decoderMap[remoteId] = enginePtr
  1147  		*enginePtr, err = dec.compileDec(remoteId, ut)
  1148  		if err != nil {
  1149  			delete(decoderMap, remoteId)
  1150  		}
  1151  	}
  1152  	return
  1153  }
  1154  
  1155  // emptyStruct is the type we compile into when ignoring a struct value.
  1156  type emptyStruct struct{}
  1157  
  1158  var emptyStructType = reflect.TypeOf(emptyStruct{})
  1159  
  1160  // getIgnoreEnginePtr returns the engine for the specified type when the value is to be discarded.
  1161  func (dec *Decoder) getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, err error) {
  1162  	var ok bool
  1163  	if enginePtr, ok = dec.ignorerCache[wireId]; !ok {
  1164  		// To handle recursive types, mark this engine as underway before compiling.
  1165  		enginePtr = new(*decEngine)
  1166  		dec.ignorerCache[wireId] = enginePtr
  1167  		wire := dec.wireType[wireId]
  1168  		if wire != nil && wire.StructT != nil {
  1169  			*enginePtr, err = dec.compileDec(wireId, userType(emptyStructType))
  1170  		} else {
  1171  			*enginePtr = dec.compileIgnoreSingle(wireId)
  1172  		}
  1173  		if err != nil {
  1174  			delete(dec.ignorerCache, wireId)
  1175  		}
  1176  	}
  1177  	return
  1178  }
  1179  
  1180  // decodeValue decodes the data stream representing a value and stores it in value.
  1181  func (dec *Decoder) decodeValue(wireId typeId, value reflect.Value) {
  1182  	defer catchError(&dec.err)
  1183  	// If the value is nil, it means we should just ignore this item.
  1184  	if !value.IsValid() {
  1185  		dec.decodeIgnoredValue(wireId)
  1186  		return
  1187  	}
  1188  	// Dereference down to the underlying type.
  1189  	ut := userType(value.Type())
  1190  	base := ut.base
  1191  	var enginePtr **decEngine
  1192  	enginePtr, dec.err = dec.getDecEnginePtr(wireId, ut)
  1193  	if dec.err != nil {
  1194  		return
  1195  	}
  1196  	value = decAlloc(value)
  1197  	engine := *enginePtr
  1198  	if st := base; st.Kind() == reflect.Struct && ut.externalDec == 0 {
  1199  		wt := dec.wireType[wireId]
  1200  		if engine.numInstr == 0 && st.NumField() > 0 &&
  1201  			wt != nil && len(wt.StructT.Field) > 0 {
  1202  			name := base.Name()
  1203  			errorf("type mismatch: no fields matched compiling decoder for %s", name)
  1204  		}
  1205  		dec.decodeStruct(engine, value)
  1206  	} else {
  1207  		dec.decodeSingle(engine, value)
  1208  	}
  1209  }
  1210  
  1211  // decodeIgnoredValue decodes the data stream representing a value of the specified type and discards it.
  1212  func (dec *Decoder) decodeIgnoredValue(wireId typeId) {
  1213  	var enginePtr **decEngine
  1214  	enginePtr, dec.err = dec.getIgnoreEnginePtr(wireId)
  1215  	if dec.err != nil {
  1216  		return
  1217  	}
  1218  	wire := dec.wireType[wireId]
  1219  	if wire != nil && wire.StructT != nil {
  1220  		dec.ignoreStruct(*enginePtr)
  1221  	} else {
  1222  		dec.ignoreSingle(*enginePtr)
  1223  	}
  1224  }
  1225  
  1226  func init() {
  1227  	var iop, uop decOp
  1228  	switch reflect.TypeOf(int(0)).Bits() {
  1229  	case 32:
  1230  		iop = decInt32
  1231  		uop = decUint32
  1232  	case 64:
  1233  		iop = decInt64
  1234  		uop = decUint64
  1235  	default:
  1236  		panic("gob: unknown size of int/uint")
  1237  	}
  1238  	decOpTable[reflect.Int] = iop
  1239  	decOpTable[reflect.Uint] = uop
  1240  
  1241  	// Finally uintptr
  1242  	switch reflect.TypeOf(uintptr(0)).Bits() {
  1243  	case 32:
  1244  		uop = decUint32
  1245  	case 64:
  1246  		uop = decUint64
  1247  	default:
  1248  		panic("gob: unknown size of uintptr")
  1249  	}
  1250  	decOpTable[reflect.Uintptr] = uop
  1251  }
  1252  
  1253  // Gob depends on being able to take the address
  1254  // of zeroed Values it creates, so use this wrapper instead
  1255  // of the standard reflect.Zero.
  1256  // Each call allocates once.
  1257  func allocValue(t reflect.Type) reflect.Value {
  1258  	return reflect.New(t).Elem()
  1259  }
  1260  

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