Source file src/encoding/gob/decode.go

     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  	"internal/saferio"
    13  	"io"
    14  	"math"
    15  	"math/bits"
    16  	"reflect"
    17  )
    18  
    19  var (
    20  	errBadUint = errors.New("gob: encoded unsigned integer out of range")
    21  	errBadType = errors.New("gob: unknown type id or corrupted data")
    22  	errRange   = errors.New("gob: bad data: field numbers out of bounds")
    23  )
    24  
    25  type decHelper func(state *decoderState, v reflect.Value, length int, ovfl error) bool
    26  
    27  // decoderState is the execution state of an instance of the decoder. A new state
    28  // is created for nested objects.
    29  type decoderState struct {
    30  	dec *Decoder
    31  	// The buffer is stored with an extra indirection because it may be replaced
    32  	// if we load a type during decode (when reading an interface value).
    33  	b        *decBuffer
    34  	fieldnum int           // the last field number read.
    35  	next     *decoderState // for free list
    36  }
    37  
    38  // decBuffer is an extremely simple, fast implementation of a read-only byte buffer.
    39  // It is initialized by calling Size and then copying the data into the slice returned by Bytes().
    40  type decBuffer struct {
    41  	data   []byte
    42  	offset int // Read offset.
    43  }
    44  
    45  func (d *decBuffer) Read(p []byte) (int, error) {
    46  	n := copy(p, d.data[d.offset:])
    47  	if n == 0 && len(p) != 0 {
    48  		return 0, io.EOF
    49  	}
    50  	d.offset += n
    51  	return n, nil
    52  }
    53  
    54  func (d *decBuffer) Drop(n int) {
    55  	if n > d.Len() {
    56  		panic("drop")
    57  	}
    58  	d.offset += n
    59  }
    60  
    61  func (d *decBuffer) ReadByte() (byte, error) {
    62  	if d.offset >= len(d.data) {
    63  		return 0, io.EOF
    64  	}
    65  	c := d.data[d.offset]
    66  	d.offset++
    67  	return c, nil
    68  }
    69  
    70  func (d *decBuffer) Len() int {
    71  	return len(d.data) - d.offset
    72  }
    73  
    74  func (d *decBuffer) Bytes() []byte {
    75  	return d.data[d.offset:]
    76  }
    77  
    78  // SetBytes sets the buffer to the bytes, discarding any existing data.
    79  func (d *decBuffer) SetBytes(data []byte) {
    80  	d.data = data
    81  	d.offset = 0
    82  }
    83  
    84  func (d *decBuffer) Reset() {
    85  	d.data = d.data[0:0]
    86  	d.offset = 0
    87  }
    88  
    89  // We pass the bytes.Buffer separately for easier testing of the infrastructure
    90  // without requiring a full Decoder.
    91  func (dec *Decoder) newDecoderState(buf *decBuffer) *decoderState {
    92  	d := dec.freeList
    93  	if d == nil {
    94  		d = new(decoderState)
    95  		d.dec = dec
    96  	} else {
    97  		dec.freeList = d.next
    98  	}
    99  	d.b = buf
   100  	return d
   101  }
   102  
   103  func (dec *Decoder) freeDecoderState(d *decoderState) {
   104  	d.next = dec.freeList
   105  	dec.freeList = d
   106  }
   107  
   108  func overflow(name string) error {
   109  	return errors.New(`value for "` + name + `" out of range`)
   110  }
   111  
   112  // decodeUintReader reads an encoded unsigned integer from an io.Reader.
   113  // Used only by the Decoder to read the message length.
   114  func decodeUintReader(r io.Reader, buf []byte) (x uint64, width int, err error) {
   115  	width = 1
   116  	n, err := io.ReadFull(r, buf[0:width])
   117  	if n == 0 {
   118  		return
   119  	}
   120  	b := buf[0]
   121  	if b <= 0x7f {
   122  		return uint64(b), width, nil
   123  	}
   124  	n = -int(int8(b))
   125  	if n > uint64Size {
   126  		err = errBadUint
   127  		return
   128  	}
   129  	width, err = io.ReadFull(r, buf[0:n])
   130  	if err != nil {
   131  		if err == io.EOF {
   132  			err = io.ErrUnexpectedEOF
   133  		}
   134  		return
   135  	}
   136  	// Could check that the high byte is zero but it's not worth it.
   137  	for _, b := range buf[0:width] {
   138  		x = x<<8 | uint64(b)
   139  	}
   140  	width++ // +1 for length byte
   141  	return
   142  }
   143  
   144  // decodeUint reads an encoded unsigned integer from state.r.
   145  // Does not check for overflow.
   146  func (state *decoderState) decodeUint() (x uint64) {
   147  	b, err := state.b.ReadByte()
   148  	if err != nil {
   149  		error_(err)
   150  	}
   151  	if b <= 0x7f {
   152  		return uint64(b)
   153  	}
   154  	n := -int(int8(b))
   155  	if n > uint64Size {
   156  		error_(errBadUint)
   157  	}
   158  	buf := state.b.Bytes()
   159  	if len(buf) < n {
   160  		errorf("invalid uint data length %d: exceeds input size %d", n, len(buf))
   161  	}
   162  	// Don't need to check error; it's safe to loop regardless.
   163  	// Could check that the high byte is zero but it's not worth it.
   164  	for _, b := range buf[0:n] {
   165  		x = x<<8 | uint64(b)
   166  	}
   167  	state.b.Drop(n)
   168  	return x
   169  }
   170  
   171  // decodeInt reads an encoded signed integer from state.r.
   172  // Does not check for overflow.
   173  func (state *decoderState) decodeInt() int64 {
   174  	x := state.decodeUint()
   175  	if x&1 != 0 {
   176  		return ^int64(x >> 1)
   177  	}
   178  	return int64(x >> 1)
   179  }
   180  
   181  // getLength decodes the next uint and makes sure it is a possible
   182  // size for a data item that follows, which means it must fit in a
   183  // non-negative int and fit in the buffer.
   184  func (state *decoderState) getLength() (int, bool) {
   185  	n := int(state.decodeUint())
   186  	if n < 0 || state.b.Len() < n || tooBig <= n {
   187  		return 0, false
   188  	}
   189  	return n, true
   190  }
   191  
   192  // decOp is the signature of a decoding operator for a given type.
   193  type decOp func(i *decInstr, state *decoderState, v reflect.Value)
   194  
   195  // The 'instructions' of the decoding machine
   196  type decInstr struct {
   197  	op    decOp
   198  	field int   // field number of the wire type
   199  	index []int // field access indices for destination type
   200  	ovfl  error // error message for overflow/underflow (for arrays, of the elements)
   201  }
   202  
   203  // ignoreUint discards a uint value with no destination.
   204  func ignoreUint(i *decInstr, state *decoderState, v reflect.Value) {
   205  	state.decodeUint()
   206  }
   207  
   208  // ignoreTwoUints discards a uint value with no destination. It's used to skip
   209  // complex values.
   210  func ignoreTwoUints(i *decInstr, state *decoderState, v reflect.Value) {
   211  	state.decodeUint()
   212  	state.decodeUint()
   213  }
   214  
   215  // Since the encoder writes no zeros, if we arrive at a decoder we have
   216  // a value to extract and store. The field number has already been read
   217  // (it's how we knew to call this decoder).
   218  // Each decoder is responsible for handling any indirections associated
   219  // with the data structure. If any pointer so reached is nil, allocation must
   220  // be done.
   221  
   222  // decAlloc takes a value and returns a settable value that can
   223  // be assigned to. If the value is a pointer, decAlloc guarantees it points to storage.
   224  // The callers to the individual decoders are expected to have used decAlloc.
   225  // The individual decoders don't need it.
   226  func decAlloc(v reflect.Value) reflect.Value {
   227  	for v.Kind() == reflect.Pointer {
   228  		if v.IsNil() {
   229  			v.Set(reflect.New(v.Type().Elem()))
   230  		}
   231  		v = v.Elem()
   232  	}
   233  	return v
   234  }
   235  
   236  // decBool decodes a uint and stores it as a boolean in value.
   237  func decBool(i *decInstr, state *decoderState, value reflect.Value) {
   238  	value.SetBool(state.decodeUint() != 0)
   239  }
   240  
   241  // decInt8 decodes an integer and stores it as an int8 in value.
   242  func decInt8(i *decInstr, state *decoderState, value reflect.Value) {
   243  	v := state.decodeInt()
   244  	if v < math.MinInt8 || math.MaxInt8 < v {
   245  		error_(i.ovfl)
   246  	}
   247  	value.SetInt(v)
   248  }
   249  
   250  // decUint8 decodes an unsigned integer and stores it as a uint8 in value.
   251  func decUint8(i *decInstr, state *decoderState, value reflect.Value) {
   252  	v := state.decodeUint()
   253  	if math.MaxUint8 < v {
   254  		error_(i.ovfl)
   255  	}
   256  	value.SetUint(v)
   257  }
   258  
   259  // decInt16 decodes an integer and stores it as an int16 in value.
   260  func decInt16(i *decInstr, state *decoderState, value reflect.Value) {
   261  	v := state.decodeInt()
   262  	if v < math.MinInt16 || math.MaxInt16 < v {
   263  		error_(i.ovfl)
   264  	}
   265  	value.SetInt(v)
   266  }
   267  
   268  // decUint16 decodes an unsigned integer and stores it as a uint16 in value.
   269  func decUint16(i *decInstr, state *decoderState, value reflect.Value) {
   270  	v := state.decodeUint()
   271  	if math.MaxUint16 < v {
   272  		error_(i.ovfl)
   273  	}
   274  	value.SetUint(v)
   275  }
   276  
   277  // decInt32 decodes an integer and stores it as an int32 in value.
   278  func decInt32(i *decInstr, state *decoderState, value reflect.Value) {
   279  	v := state.decodeInt()
   280  	if v < math.MinInt32 || math.MaxInt32 < v {
   281  		error_(i.ovfl)
   282  	}
   283  	value.SetInt(v)
   284  }
   285  
   286  // decUint32 decodes an unsigned integer and stores it as a uint32 in value.
   287  func decUint32(i *decInstr, state *decoderState, value reflect.Value) {
   288  	v := state.decodeUint()
   289  	if math.MaxUint32 < v {
   290  		error_(i.ovfl)
   291  	}
   292  	value.SetUint(v)
   293  }
   294  
   295  // decInt64 decodes an integer and stores it as an int64 in value.
   296  func decInt64(i *decInstr, state *decoderState, value reflect.Value) {
   297  	v := state.decodeInt()
   298  	value.SetInt(v)
   299  }
   300  
   301  // decUint64 decodes an unsigned integer and stores it as a uint64 in value.
   302  func decUint64(i *decInstr, state *decoderState, value reflect.Value) {
   303  	v := state.decodeUint()
   304  	value.SetUint(v)
   305  }
   306  
   307  // Floating-point numbers are transmitted as uint64s holding the bits
   308  // of the underlying representation. They are sent byte-reversed, with
   309  // the exponent end coming out first, so integer floating point numbers
   310  // (for example) transmit more compactly. This routine does the
   311  // unswizzling.
   312  func float64FromBits(u uint64) float64 {
   313  	v := bits.ReverseBytes64(u)
   314  	return math.Float64frombits(v)
   315  }
   316  
   317  // float32FromBits decodes an unsigned integer, treats it as a 32-bit floating-point
   318  // number, and returns it. It's a helper function for float32 and complex64.
   319  // It returns a float64 because that's what reflection needs, but its return
   320  // value is known to be accurately representable in a float32.
   321  func float32FromBits(u uint64, ovfl error) float64 {
   322  	v := float64FromBits(u)
   323  	av := v
   324  	if av < 0 {
   325  		av = -av
   326  	}
   327  	// +Inf is OK in both 32- and 64-bit floats. Underflow is always OK.
   328  	if math.MaxFloat32 < av && av <= math.MaxFloat64 {
   329  		error_(ovfl)
   330  	}
   331  	return v
   332  }
   333  
   334  // decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
   335  // number, and stores it in value.
   336  func decFloat32(i *decInstr, state *decoderState, value reflect.Value) {
   337  	value.SetFloat(float32FromBits(state.decodeUint(), i.ovfl))
   338  }
   339  
   340  // decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
   341  // number, and stores it in value.
   342  func decFloat64(i *decInstr, state *decoderState, value reflect.Value) {
   343  	value.SetFloat(float64FromBits(state.decodeUint()))
   344  }
   345  
   346  // decComplex64 decodes a pair of unsigned integers, treats them as a
   347  // pair of floating point numbers, and stores them as a complex64 in value.
   348  // The real part comes first.
   349  func decComplex64(i *decInstr, state *decoderState, value reflect.Value) {
   350  	real := float32FromBits(state.decodeUint(), i.ovfl)
   351  	imag := float32FromBits(state.decodeUint(), i.ovfl)
   352  	value.SetComplex(complex(real, imag))
   353  }
   354  
   355  // decComplex128 decodes a pair of unsigned integers, treats them as a
   356  // pair of floating point numbers, and stores them as a complex128 in value.
   357  // The real part comes first.
   358  func decComplex128(i *decInstr, state *decoderState, value reflect.Value) {
   359  	real := float64FromBits(state.decodeUint())
   360  	imag := float64FromBits(state.decodeUint())
   361  	value.SetComplex(complex(real, imag))
   362  }
   363  
   364  // decUint8Slice decodes a byte slice and stores in value a slice header
   365  // describing the data.
   366  // uint8 slices are encoded as an unsigned count followed by the raw bytes.
   367  func decUint8Slice(i *decInstr, state *decoderState, value reflect.Value) {
   368  	n, ok := state.getLength()
   369  	if !ok {
   370  		errorf("bad %s slice length: %d", value.Type(), n)
   371  	}
   372  	if value.Cap() < n {
   373  		safe := saferio.SliceCap[byte](uint64(n))
   374  		if safe < 0 {
   375  			errorf("%s slice too big: %d elements", value.Type(), n)
   376  		}
   377  		value.Set(reflect.MakeSlice(value.Type(), safe, safe))
   378  		ln := safe
   379  		i := 0
   380  		for i < n {
   381  			if i >= ln {
   382  				// We didn't allocate the entire slice,
   383  				// due to using saferio.SliceCap.
   384  				// Grow the slice for one more element.
   385  				// The slice is full, so this should
   386  				// bump up the capacity.
   387  				value.Grow(1)
   388  			}
   389  			// Copy into s up to the capacity or n,
   390  			// whichever is less.
   391  			ln = value.Cap()
   392  			if ln > n {
   393  				ln = n
   394  			}
   395  			value.SetLen(ln)
   396  			sub := value.Slice(i, ln)
   397  			if _, err := state.b.Read(sub.Bytes()); err != nil {
   398  				errorf("error decoding []byte at %d: %s", i, err)
   399  			}
   400  			i = ln
   401  		}
   402  	} else {
   403  		value.SetLen(n)
   404  		if _, err := state.b.Read(value.Bytes()); err != nil {
   405  			errorf("error decoding []byte: %s", err)
   406  		}
   407  	}
   408  }
   409  
   410  // decString decodes byte array and stores in value a string header
   411  // describing the data.
   412  // Strings are encoded as an unsigned count followed by the raw bytes.
   413  func decString(i *decInstr, state *decoderState, value reflect.Value) {
   414  	n, ok := state.getLength()
   415  	if !ok {
   416  		errorf("bad %s slice length: %d", value.Type(), n)
   417  	}
   418  	// Read the data.
   419  	data := state.b.Bytes()
   420  	if len(data) < n {
   421  		errorf("invalid string length %d: exceeds input size %d", n, len(data))
   422  	}
   423  	s := string(data[:n])
   424  	state.b.Drop(n)
   425  	value.SetString(s)
   426  }
   427  
   428  // ignoreUint8Array skips over the data for a byte slice value with no destination.
   429  func ignoreUint8Array(i *decInstr, state *decoderState, value reflect.Value) {
   430  	n, ok := state.getLength()
   431  	if !ok {
   432  		errorf("slice length too large")
   433  	}
   434  	bn := state.b.Len()
   435  	if bn < n {
   436  		errorf("invalid slice length %d: exceeds input size %d", n, bn)
   437  	}
   438  	state.b.Drop(n)
   439  }
   440  
   441  // Execution engine
   442  
   443  // The encoder engine is an array of instructions indexed by field number of the incoming
   444  // decoder. It is executed with random access according to field number.
   445  type decEngine struct {
   446  	instr    []decInstr
   447  	numInstr int // the number of active instructions
   448  }
   449  
   450  // decodeSingle decodes a top-level value that is not a struct and stores it in value.
   451  // Such values are preceded by a zero, making them have the memory layout of a
   452  // struct field (although with an illegal field number).
   453  func (dec *Decoder) decodeSingle(engine *decEngine, value reflect.Value) {
   454  	state := dec.newDecoderState(&dec.buf)
   455  	defer dec.freeDecoderState(state)
   456  	state.fieldnum = singletonField
   457  	if state.decodeUint() != 0 {
   458  		errorf("decode: corrupted data: non-zero delta for singleton")
   459  	}
   460  	instr := &engine.instr[singletonField]
   461  	instr.op(instr, state, value)
   462  }
   463  
   464  // decodeStruct decodes a top-level struct and stores it in value.
   465  // Indir is for the value, not the type. At the time of the call it may
   466  // differ from ut.indir, which was computed when the engine was built.
   467  // This state cannot arise for decodeSingle, which is called directly
   468  // from the user's value, not from the innards of an engine.
   469  func (dec *Decoder) decodeStruct(engine *decEngine, value reflect.Value) {
   470  	state := dec.newDecoderState(&dec.buf)
   471  	defer dec.freeDecoderState(state)
   472  	state.fieldnum = -1
   473  	for state.b.Len() > 0 {
   474  		delta := int(state.decodeUint())
   475  		if delta < 0 {
   476  			errorf("decode: corrupted data: negative delta")
   477  		}
   478  		if delta == 0 { // struct terminator is zero delta fieldnum
   479  			break
   480  		}
   481  		if state.fieldnum >= len(engine.instr)-delta { // subtract to compare without overflow
   482  			error_(errRange)
   483  		}
   484  		fieldnum := state.fieldnum + delta
   485  		instr := &engine.instr[fieldnum]
   486  		var field reflect.Value
   487  		if instr.index != nil {
   488  			// Otherwise the field is unknown to us and instr.op is an ignore op.
   489  			field = value.FieldByIndex(instr.index)
   490  			if field.Kind() == reflect.Pointer {
   491  				field = decAlloc(field)
   492  			}
   493  		}
   494  		instr.op(instr, state, field)
   495  		state.fieldnum = fieldnum
   496  	}
   497  }
   498  
   499  var noValue reflect.Value
   500  
   501  // ignoreStruct discards the data for a struct with no destination.
   502  func (dec *Decoder) ignoreStruct(engine *decEngine) {
   503  	state := dec.newDecoderState(&dec.buf)
   504  	defer dec.freeDecoderState(state)
   505  	state.fieldnum = -1
   506  	for state.b.Len() > 0 {
   507  		delta := int(state.decodeUint())
   508  		if delta < 0 {
   509  			errorf("ignore decode: corrupted data: negative delta")
   510  		}
   511  		if delta == 0 { // struct terminator is zero delta fieldnum
   512  			break
   513  		}
   514  		fieldnum := state.fieldnum + delta
   515  		if fieldnum >= len(engine.instr) {
   516  			error_(errRange)
   517  		}
   518  		instr := &engine.instr[fieldnum]
   519  		instr.op(instr, state, noValue)
   520  		state.fieldnum = fieldnum
   521  	}
   522  }
   523  
   524  // ignoreSingle discards the data for a top-level non-struct value with no
   525  // destination. It's used when calling Decode with a nil value.
   526  func (dec *Decoder) ignoreSingle(engine *decEngine) {
   527  	state := dec.newDecoderState(&dec.buf)
   528  	defer dec.freeDecoderState(state)
   529  	state.fieldnum = singletonField
   530  	delta := int(state.decodeUint())
   531  	if delta != 0 {
   532  		errorf("decode: corrupted data: non-zero delta for singleton")
   533  	}
   534  	instr := &engine.instr[singletonField]
   535  	instr.op(instr, state, noValue)
   536  }
   537  
   538  // decodeArrayHelper does the work for decoding arrays and slices.
   539  func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
   540  	if helper != nil && helper(state, value, length, ovfl) {
   541  		return
   542  	}
   543  	instr := &decInstr{elemOp, 0, nil, ovfl}
   544  	isPtr := value.Type().Elem().Kind() == reflect.Pointer
   545  	ln := value.Len()
   546  	for i := 0; i < length; i++ {
   547  		if state.b.Len() == 0 {
   548  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
   549  		}
   550  		if i >= ln {
   551  			// This is a slice that we only partially allocated.
   552  			// Grow it up to length.
   553  			value.Grow(1)
   554  			cp := value.Cap()
   555  			if cp > length {
   556  				cp = length
   557  			}
   558  			value.SetLen(cp)
   559  			ln = cp
   560  		}
   561  		v := value.Index(i)
   562  		if isPtr {
   563  			v = decAlloc(v)
   564  		}
   565  		elemOp(instr, state, v)
   566  	}
   567  }
   568  
   569  // decodeArray decodes an array and stores it in value.
   570  // The length is an unsigned integer preceding the elements. Even though the length is redundant
   571  // (it's part of the type), it's a useful check and is included in the encoding.
   572  func (dec *Decoder) decodeArray(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
   573  	if n := state.decodeUint(); n != uint64(length) {
   574  		errorf("length mismatch in decodeArray")
   575  	}
   576  	dec.decodeArrayHelper(state, value, elemOp, length, ovfl, helper)
   577  }
   578  
   579  // decodeIntoValue is a helper for map decoding.
   580  func decodeIntoValue(state *decoderState, op decOp, isPtr bool, value reflect.Value, instr *decInstr) reflect.Value {
   581  	v := value
   582  	if isPtr {
   583  		v = decAlloc(value)
   584  	}
   585  
   586  	op(instr, state, v)
   587  	return value
   588  }
   589  
   590  // decodeMap decodes a map and stores it in value.
   591  // Maps are encoded as a length followed by key:value pairs.
   592  // Because the internals of maps are not visible to us, we must
   593  // use reflection rather than pointer magic.
   594  func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, value reflect.Value, keyOp, elemOp decOp, ovfl error) {
   595  	n := int(state.decodeUint())
   596  	if value.IsNil() {
   597  		value.Set(reflect.MakeMapWithSize(mtyp, n))
   598  	}
   599  	keyIsPtr := mtyp.Key().Kind() == reflect.Pointer
   600  	elemIsPtr := mtyp.Elem().Kind() == reflect.Pointer
   601  	keyInstr := &decInstr{keyOp, 0, nil, ovfl}
   602  	elemInstr := &decInstr{elemOp, 0, nil, ovfl}
   603  	keyP := reflect.New(mtyp.Key())
   604  	elemP := reflect.New(mtyp.Elem())
   605  	for i := 0; i < n; i++ {
   606  		key := decodeIntoValue(state, keyOp, keyIsPtr, keyP.Elem(), keyInstr)
   607  		elem := decodeIntoValue(state, elemOp, elemIsPtr, elemP.Elem(), elemInstr)
   608  		value.SetMapIndex(key, elem)
   609  		keyP.Elem().SetZero()
   610  		elemP.Elem().SetZero()
   611  	}
   612  }
   613  
   614  // ignoreArrayHelper does the work for discarding arrays and slices.
   615  func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
   616  	instr := &decInstr{elemOp, 0, nil, errors.New("no error")}
   617  	for i := 0; i < length; i++ {
   618  		if state.b.Len() == 0 {
   619  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
   620  		}
   621  		elemOp(instr, state, noValue)
   622  	}
   623  }
   624  
   625  // ignoreArray discards the data for an array value with no destination.
   626  func (dec *Decoder) ignoreArray(state *decoderState, elemOp decOp, length int) {
   627  	if n := state.decodeUint(); n != uint64(length) {
   628  		errorf("length mismatch in ignoreArray")
   629  	}
   630  	dec.ignoreArrayHelper(state, elemOp, length)
   631  }
   632  
   633  // ignoreMap discards the data for a map value with no destination.
   634  func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
   635  	n := int(state.decodeUint())
   636  	keyInstr := &decInstr{keyOp, 0, nil, errors.New("no error")}
   637  	elemInstr := &decInstr{elemOp, 0, nil, errors.New("no error")}
   638  	for i := 0; i < n; i++ {
   639  		keyOp(keyInstr, state, noValue)
   640  		elemOp(elemInstr, state, noValue)
   641  	}
   642  }
   643  
   644  // decodeSlice decodes a slice and stores it in value.
   645  // Slices are encoded as an unsigned length followed by the elements.
   646  func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp decOp, ovfl error, helper decHelper) {
   647  	u := state.decodeUint()
   648  	typ := value.Type()
   649  	size := uint64(typ.Elem().Size())
   650  	nBytes := u * size
   651  	n := int(u)
   652  	// Take care with overflow in this calculation.
   653  	if n < 0 || uint64(n) != u || nBytes > tooBig || (size > 0 && nBytes/size != u) {
   654  		// We don't check n against buffer length here because if it's a slice
   655  		// of interfaces, there will be buffer reloads.
   656  		errorf("%s slice too big: %d elements of %d bytes", typ.Elem(), u, size)
   657  	}
   658  	if value.Cap() < n {
   659  		safe := saferio.SliceCapWithSize(size, uint64(n))
   660  		if safe < 0 {
   661  			errorf("%s slice too big: %d elements of %d bytes", typ.Elem(), u, size)
   662  		}
   663  		value.Set(reflect.MakeSlice(typ, safe, safe))
   664  	} else {
   665  		value.SetLen(n)
   666  	}
   667  	dec.decodeArrayHelper(state, value, elemOp, n, ovfl, helper)
   668  }
   669  
   670  // ignoreSlice skips over the data for a slice value with no destination.
   671  func (dec *Decoder) ignoreSlice(state *decoderState, elemOp decOp) {
   672  	dec.ignoreArrayHelper(state, elemOp, int(state.decodeUint()))
   673  }
   674  
   675  // decodeInterface decodes an interface value and stores it in value.
   676  // Interfaces are encoded as the name of a concrete type followed by a value.
   677  // If the name is empty, the value is nil and no value is sent.
   678  func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, value reflect.Value) {
   679  	// Read the name of the concrete type.
   680  	nr := state.decodeUint()
   681  	if nr > 1<<31 { // zero is permissible for anonymous types
   682  		errorf("invalid type name length %d", nr)
   683  	}
   684  	if nr > uint64(state.b.Len()) {
   685  		errorf("invalid type name length %d: exceeds input size", nr)
   686  	}
   687  	n := int(nr)
   688  	name := state.b.Bytes()[:n]
   689  	state.b.Drop(n)
   690  	// Allocate the destination interface value.
   691  	if len(name) == 0 {
   692  		// Copy the nil interface value to the target.
   693  		value.SetZero()
   694  		return
   695  	}
   696  	if len(name) > 1024 {
   697  		errorf("name too long (%d bytes): %.20q...", len(name), name)
   698  	}
   699  	// The concrete type must be registered.
   700  	typi, ok := nameToConcreteType.Load(string(name))
   701  	if !ok {
   702  		errorf("name not registered for interface: %q", name)
   703  	}
   704  	typ := typi.(reflect.Type)
   705  
   706  	// Read the type id of the concrete value.
   707  	concreteId := dec.decodeTypeSequence(true)
   708  	if concreteId < 0 {
   709  		error_(dec.err)
   710  	}
   711  	// Byte count of value is next; we don't care what it is (it's there
   712  	// in case we want to ignore the value by skipping it completely).
   713  	state.decodeUint()
   714  	// Read the concrete value.
   715  	v := allocValue(typ)
   716  	dec.decodeValue(concreteId, v)
   717  	if dec.err != nil {
   718  		error_(dec.err)
   719  	}
   720  	// Assign the concrete value to the interface.
   721  	// Tread carefully; it might not satisfy the interface.
   722  	if !typ.AssignableTo(ityp) {
   723  		errorf("%s is not assignable to type %s", typ, ityp)
   724  	}
   725  	// Copy the interface value to the target.
   726  	value.Set(v)
   727  }
   728  
   729  // ignoreInterface discards the data for an interface value with no destination.
   730  func (dec *Decoder) ignoreInterface(state *decoderState) {
   731  	// Read the name of the concrete type.
   732  	n, ok := state.getLength()
   733  	if !ok {
   734  		errorf("bad interface encoding: name too large for buffer")
   735  	}
   736  	bn := state.b.Len()
   737  	if bn < n {
   738  		errorf("invalid interface value length %d: exceeds input size %d", n, bn)
   739  	}
   740  	state.b.Drop(n)
   741  	id := dec.decodeTypeSequence(true)
   742  	if id < 0 {
   743  		error_(dec.err)
   744  	}
   745  	// At this point, the decoder buffer contains a delimited value. Just toss it.
   746  	n, ok = state.getLength()
   747  	if !ok {
   748  		errorf("bad interface encoding: data length too large for buffer")
   749  	}
   750  	state.b.Drop(n)
   751  }
   752  
   753  // decodeGobDecoder decodes something implementing the GobDecoder interface.
   754  // The data is encoded as a byte slice.
   755  func (dec *Decoder) decodeGobDecoder(ut *userTypeInfo, state *decoderState, value reflect.Value) {
   756  	// Read the bytes for the value.
   757  	n, ok := state.getLength()
   758  	if !ok {
   759  		errorf("GobDecoder: length too large for buffer")
   760  	}
   761  	b := state.b.Bytes()
   762  	if len(b) < n {
   763  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, len(b))
   764  	}
   765  	b = b[:n]
   766  	state.b.Drop(n)
   767  	var err error
   768  	// We know it's one of these.
   769  	switch ut.externalDec {
   770  	case xGob:
   771  		gobDecoder, _ := reflect.TypeAssert[GobDecoder](value)
   772  		err = gobDecoder.GobDecode(b)
   773  	case xBinary:
   774  		binaryUnmarshaler, _ := reflect.TypeAssert[encoding.BinaryUnmarshaler](value)
   775  		err = binaryUnmarshaler.UnmarshalBinary(b)
   776  	case xText:
   777  		textUnmarshaler, _ := reflect.TypeAssert[encoding.TextUnmarshaler](value)
   778  		err = textUnmarshaler.UnmarshalText(b)
   779  	}
   780  	if err != nil {
   781  		error_(err)
   782  	}
   783  }
   784  
   785  // ignoreGobDecoder discards the data for a GobDecoder value with no destination.
   786  func (dec *Decoder) ignoreGobDecoder(state *decoderState) {
   787  	// Read the bytes for the value.
   788  	n, ok := state.getLength()
   789  	if !ok {
   790  		errorf("GobDecoder: length too large for buffer")
   791  	}
   792  	bn := state.b.Len()
   793  	if bn < n {
   794  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, bn)
   795  	}
   796  	state.b.Drop(n)
   797  }
   798  
   799  // Index by Go types.
   800  var decOpTable = [...]decOp{
   801  	reflect.Bool:       decBool,
   802  	reflect.Int8:       decInt8,
   803  	reflect.Int16:      decInt16,
   804  	reflect.Int32:      decInt32,
   805  	reflect.Int64:      decInt64,
   806  	reflect.Uint8:      decUint8,
   807  	reflect.Uint16:     decUint16,
   808  	reflect.Uint32:     decUint32,
   809  	reflect.Uint64:     decUint64,
   810  	reflect.Float32:    decFloat32,
   811  	reflect.Float64:    decFloat64,
   812  	reflect.Complex64:  decComplex64,
   813  	reflect.Complex128: decComplex128,
   814  	reflect.String:     decString,
   815  }
   816  
   817  // Indexed by gob types.  tComplex will be added during type.init().
   818  var decIgnoreOpMap = map[typeId]decOp{
   819  	tBool:    ignoreUint,
   820  	tInt:     ignoreUint,
   821  	tUint:    ignoreUint,
   822  	tFloat:   ignoreUint,
   823  	tBytes:   ignoreUint8Array,
   824  	tString:  ignoreUint8Array,
   825  	tComplex: ignoreTwoUints,
   826  }
   827  
   828  // decOpFor returns the decoding op for the base type under rt and
   829  // the indirection count to reach it.
   830  func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProgress map[reflect.Type]*decOp) *decOp {
   831  	ut := userType(rt)
   832  	// If the type implements GobEncoder, we handle it without further processing.
   833  	if ut.externalDec != 0 {
   834  		return dec.gobDecodeOpFor(ut)
   835  	}
   836  
   837  	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   838  	// Return the pointer to the op we're already building.
   839  	if opPtr := inProgress[rt]; opPtr != nil {
   840  		return opPtr
   841  	}
   842  	typ := ut.base
   843  	var op decOp
   844  	k := typ.Kind()
   845  	if int(k) < len(decOpTable) {
   846  		op = decOpTable[k]
   847  	}
   848  	if op == nil {
   849  		inProgress[rt] = &op
   850  		// Special cases
   851  		switch t := typ; t.Kind() {
   852  		case reflect.Array:
   853  			name = "element of " + name
   854  			elemId := dec.wireType[wireId].ArrayT.Elem
   855  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   856  			ovfl := overflow(name)
   857  			helper := decArrayHelper[t.Elem().Kind()]
   858  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   859  				state.dec.decodeArray(state, value, *elemOp, t.Len(), ovfl, helper)
   860  			}
   861  
   862  		case reflect.Map:
   863  			keyId := dec.wireType[wireId].MapT.Key
   864  			elemId := dec.wireType[wireId].MapT.Elem
   865  			keyOp := dec.decOpFor(keyId, t.Key(), "key of "+name, inProgress)
   866  			elemOp := dec.decOpFor(elemId, t.Elem(), "element of "+name, inProgress)
   867  			ovfl := overflow(name)
   868  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   869  				state.dec.decodeMap(t, state, value, *keyOp, *elemOp, ovfl)
   870  			}
   871  
   872  		case reflect.Slice:
   873  			name = "element of " + name
   874  			if t.Elem().Kind() == reflect.Uint8 {
   875  				op = decUint8Slice
   876  				break
   877  			}
   878  			var elemId typeId
   879  			if tt := builtinIdToType(wireId); tt != nil {
   880  				elemId = tt.(*sliceType).Elem
   881  			} else {
   882  				elemId = dec.wireType[wireId].SliceT.Elem
   883  			}
   884  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   885  			ovfl := overflow(name)
   886  			helper := decSliceHelper[t.Elem().Kind()]
   887  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   888  				state.dec.decodeSlice(state, value, *elemOp, ovfl, helper)
   889  			}
   890  
   891  		case reflect.Struct:
   892  			// Generate a closure that calls out to the engine for the nested type.
   893  			ut := userType(typ)
   894  			enginePtr, err := dec.getDecEnginePtr(wireId, ut)
   895  			if err != nil {
   896  				error_(err)
   897  			}
   898  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   899  				// indirect through enginePtr to delay evaluation for recursive structs.
   900  				dec.decodeStruct(*enginePtr, value)
   901  			}
   902  		case reflect.Interface:
   903  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   904  				state.dec.decodeInterface(t, state, value)
   905  			}
   906  		}
   907  	}
   908  	if op == nil {
   909  		errorf("decode can't handle type %s", rt)
   910  	}
   911  	return &op
   912  }
   913  
   914  var maxIgnoreNestingDepth = 10000
   915  
   916  // decIgnoreOpFor returns the decoding op for a field that has no destination.
   917  func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp) *decOp {
   918  	// Track how deep we've recursed trying to skip nested ignored fields.
   919  	dec.ignoreDepth++
   920  	defer func() { dec.ignoreDepth-- }()
   921  	if dec.ignoreDepth > maxIgnoreNestingDepth {
   922  		error_(errors.New("invalid nesting depth"))
   923  	}
   924  	// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   925  	// Return the pointer to the op we're already building.
   926  	if opPtr := inProgress[wireId]; opPtr != nil {
   927  		return opPtr
   928  	}
   929  	op, ok := decIgnoreOpMap[wireId]
   930  	if !ok {
   931  		inProgress[wireId] = &op
   932  		if wireId == tInterface {
   933  			// Special case because it's a method: the ignored item might
   934  			// define types and we need to record their state in the decoder.
   935  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   936  				state.dec.ignoreInterface(state)
   937  			}
   938  			return &op
   939  		}
   940  		// Special cases
   941  		wire := dec.wireType[wireId]
   942  		switch {
   943  		case wire == nil:
   944  			errorf("bad data: undefined type %s", wireId.string())
   945  		case wire.ArrayT != nil:
   946  			elemId := wire.ArrayT.Elem
   947  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   948  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   949  				state.dec.ignoreArray(state, *elemOp, wire.ArrayT.Len)
   950  			}
   951  
   952  		case wire.MapT != nil:
   953  			keyId := dec.wireType[wireId].MapT.Key
   954  			elemId := dec.wireType[wireId].MapT.Elem
   955  			keyOp := dec.decIgnoreOpFor(keyId, inProgress)
   956  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   957  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   958  				state.dec.ignoreMap(state, *keyOp, *elemOp)
   959  			}
   960  
   961  		case wire.SliceT != nil:
   962  			elemId := wire.SliceT.Elem
   963  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
   964  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   965  				state.dec.ignoreSlice(state, *elemOp)
   966  			}
   967  
   968  		case wire.StructT != nil:
   969  			// Generate a closure that calls out to the engine for the nested type.
   970  			enginePtr, err := dec.getIgnoreEnginePtr(wireId)
   971  			if err != nil {
   972  				error_(err)
   973  			}
   974  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   975  				// indirect through enginePtr to delay evaluation for recursive structs
   976  				state.dec.ignoreStruct(*enginePtr)
   977  			}
   978  
   979  		case wire.GobEncoderT != nil, wire.BinaryMarshalerT != nil, wire.TextMarshalerT != nil:
   980  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
   981  				state.dec.ignoreGobDecoder(state)
   982  			}
   983  		}
   984  	}
   985  	if op == nil {
   986  		errorf("bad data: ignore can't handle type %s", wireId.string())
   987  	}
   988  	return &op
   989  }
   990  
   991  // gobDecodeOpFor returns the op for a type that is known to implement
   992  // GobDecoder.
   993  func (dec *Decoder) gobDecodeOpFor(ut *userTypeInfo) *decOp {
   994  	rcvrType := ut.user
   995  	if ut.decIndir == -1 {
   996  		rcvrType = reflect.PointerTo(rcvrType)
   997  	} else if ut.decIndir > 0 {
   998  		for i := int8(0); i < ut.decIndir; i++ {
   999  			rcvrType = rcvrType.Elem()
  1000  		}
  1001  	}
  1002  	var op decOp
  1003  	op = func(i *decInstr, state *decoderState, value reflect.Value) {
  1004  		// We now have the base type. We need its address if the receiver is a pointer.
  1005  		if value.Kind() != reflect.Pointer && rcvrType.Kind() == reflect.Pointer {
  1006  			value = value.Addr()
  1007  		}
  1008  		state.dec.decodeGobDecoder(ut, state, value)
  1009  	}
  1010  	return &op
  1011  }
  1012  
  1013  // compatibleType asks: Are these two gob Types compatible?
  1014  // Answers the question for basic types, arrays, maps and slices, plus
  1015  // GobEncoder/Decoder pairs.
  1016  // Structs are considered ok; fields will be checked later.
  1017  func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[reflect.Type]typeId) bool {
  1018  	if rhs, ok := inProgress[fr]; ok {
  1019  		return rhs == fw
  1020  	}
  1021  	inProgress[fr] = fw
  1022  	ut := userType(fr)
  1023  	wire, ok := dec.wireType[fw]
  1024  	// If wire was encoded with an encoding method, fr must have that method.
  1025  	// And if not, it must not.
  1026  	// At most one of the booleans in ut is set.
  1027  	// We could possibly relax this constraint in the future in order to
  1028  	// choose the decoding method using the data in the wireType.
  1029  	// The parentheses look odd but are correct.
  1030  	if (ut.externalDec == xGob) != (ok && wire.GobEncoderT != nil) ||
  1031  		(ut.externalDec == xBinary) != (ok && wire.BinaryMarshalerT != nil) ||
  1032  		(ut.externalDec == xText) != (ok && wire.TextMarshalerT != nil) {
  1033  		return false
  1034  	}
  1035  	if ut.externalDec != 0 { // This test trumps all others.
  1036  		return true
  1037  	}
  1038  	switch t := ut.base; t.Kind() {
  1039  	default:
  1040  		// chan, etc: cannot handle.
  1041  		return false
  1042  	case reflect.Bool:
  1043  		return fw == tBool
  1044  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1045  		return fw == tInt
  1046  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1047  		return fw == tUint
  1048  	case reflect.Float32, reflect.Float64:
  1049  		return fw == tFloat
  1050  	case reflect.Complex64, reflect.Complex128:
  1051  		return fw == tComplex
  1052  	case reflect.String:
  1053  		return fw == tString
  1054  	case reflect.Interface:
  1055  		return fw == tInterface
  1056  	case reflect.Array:
  1057  		if !ok || wire.ArrayT == nil {
  1058  			return false
  1059  		}
  1060  		array := wire.ArrayT
  1061  		return t.Len() == array.Len && dec.compatibleType(t.Elem(), array.Elem, inProgress)
  1062  	case reflect.Map:
  1063  		if !ok || wire.MapT == nil {
  1064  			return false
  1065  		}
  1066  		MapType := wire.MapT
  1067  		return dec.compatibleType(t.Key(), MapType.Key, inProgress) && dec.compatibleType(t.Elem(), MapType.Elem, inProgress)
  1068  	case reflect.Slice:
  1069  		// Is it an array of bytes?
  1070  		if t.Elem().Kind() == reflect.Uint8 {
  1071  			return fw == tBytes
  1072  		}
  1073  		// Extract and compare element types.
  1074  		var sw *sliceType
  1075  		if tt := builtinIdToType(fw); tt != nil {
  1076  			sw, _ = tt.(*sliceType)
  1077  		} else if wire != nil {
  1078  			sw = wire.SliceT
  1079  		}
  1080  		elem := userType(t.Elem()).base
  1081  		return sw != nil && dec.compatibleType(elem, sw.Elem, inProgress)
  1082  	case reflect.Struct:
  1083  		return true
  1084  	}
  1085  }
  1086  
  1087  // typeString returns a human-readable description of the type identified by remoteId.
  1088  func (dec *Decoder) typeString(remoteId typeId) string {
  1089  	typeLock.Lock()
  1090  	defer typeLock.Unlock()
  1091  	if t := idToType(remoteId); t != nil {
  1092  		// globally known type.
  1093  		return t.string()
  1094  	}
  1095  	return dec.wireType[remoteId].string()
  1096  }
  1097  
  1098  // compileSingle compiles the decoder engine for a non-struct top-level value, including
  1099  // GobDecoders.
  1100  func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1101  	rt := ut.user
  1102  	engine = new(decEngine)
  1103  	engine.instr = make([]decInstr, 1) // one item
  1104  	name := rt.String()                // best we can do
  1105  	if !dec.compatibleType(rt, remoteId, make(map[reflect.Type]typeId)) {
  1106  		remoteType := dec.typeString(remoteId)
  1107  		// Common confusing case: local interface type, remote concrete type.
  1108  		if ut.base.Kind() == reflect.Interface && remoteId != tInterface {
  1109  			return nil, errors.New("gob: local interface type " + name + " can only be decoded from remote interface type; received concrete type " + remoteType)
  1110  		}
  1111  		return nil, errors.New("gob: decoding into local type " + name + ", received remote type " + remoteType)
  1112  	}
  1113  	op := dec.decOpFor(remoteId, rt, name, make(map[reflect.Type]*decOp))
  1114  	ovfl := errors.New(`value for "` + name + `" out of range`)
  1115  	engine.instr[singletonField] = decInstr{*op, singletonField, nil, ovfl}
  1116  	engine.numInstr = 1
  1117  	return
  1118  }
  1119  
  1120  // compileIgnoreSingle compiles the decoder engine for a non-struct top-level value that will be discarded.
  1121  func (dec *Decoder) compileIgnoreSingle(remoteId typeId) *decEngine {
  1122  	engine := new(decEngine)
  1123  	engine.instr = make([]decInstr, 1) // one item
  1124  	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp))
  1125  	ovfl := overflow(dec.typeString(remoteId))
  1126  	engine.instr[0] = decInstr{*op, 0, nil, ovfl}
  1127  	engine.numInstr = 1
  1128  	return engine
  1129  }
  1130  
  1131  // compileDec compiles the decoder engine for a value. If the value is not a struct,
  1132  // it calls out to compileSingle.
  1133  func (dec *Decoder) compileDec(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1134  	defer catchError(&err)
  1135  	rt := ut.base
  1136  	srt := rt
  1137  	if srt.Kind() != reflect.Struct || ut.externalDec != 0 {
  1138  		return dec.compileSingle(remoteId, ut)
  1139  	}
  1140  	var wireStruct *structType
  1141  	// Builtin types can come from global pool; the rest must be defined by the decoder.
  1142  	// Also we know we're decoding a struct now, so the client must have sent one.
  1143  	if t := builtinIdToType(remoteId); t != nil {
  1144  		wireStruct, _ = t.(*structType)
  1145  	} else {
  1146  		wire := dec.wireType[remoteId]
  1147  		if wire == nil {
  1148  			error_(errBadType)
  1149  		}
  1150  		wireStruct = wire.StructT
  1151  	}
  1152  	if wireStruct == nil {
  1153  		errorf("type mismatch in decoder: want struct type %s; got non-struct", rt)
  1154  	}
  1155  	engine = new(decEngine)
  1156  	engine.instr = make([]decInstr, len(wireStruct.Field))
  1157  	seen := make(map[reflect.Type]*decOp)
  1158  	// Loop over the fields of the wire type.
  1159  	for fieldnum := 0; fieldnum < len(wireStruct.Field); fieldnum++ {
  1160  		wireField := wireStruct.Field[fieldnum]
  1161  		if wireField.Name == "" {
  1162  			errorf("empty name for remote field of type %s", wireStruct.Name)
  1163  		}
  1164  		ovfl := overflow(wireField.Name)
  1165  		// Find the field of the local type with the same name.
  1166  		localField, present := srt.FieldByName(wireField.Name)
  1167  		// TODO(r): anonymous names
  1168  		if !present || !isExported(wireField.Name) {
  1169  			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp))
  1170  			engine.instr[fieldnum] = decInstr{*op, fieldnum, nil, ovfl}
  1171  			continue
  1172  		}
  1173  		if !dec.compatibleType(localField.Type, wireField.Id, make(map[reflect.Type]typeId)) {
  1174  			errorf("wrong type (%s) for received field %s.%s", localField.Type, wireStruct.Name, wireField.Name)
  1175  		}
  1176  		op := dec.decOpFor(wireField.Id, localField.Type, localField.Name, seen)
  1177  		engine.instr[fieldnum] = decInstr{*op, fieldnum, localField.Index, ovfl}
  1178  		engine.numInstr++
  1179  	}
  1180  	return
  1181  }
  1182  
  1183  // getDecEnginePtr returns the engine for the specified type.
  1184  func (dec *Decoder) getDecEnginePtr(remoteId typeId, ut *userTypeInfo) (enginePtr **decEngine, err error) {
  1185  	rt := ut.user
  1186  	decoderMap, ok := dec.decoderCache[rt]
  1187  	if !ok {
  1188  		decoderMap = make(map[typeId]**decEngine)
  1189  		dec.decoderCache[rt] = decoderMap
  1190  	}
  1191  	if enginePtr, ok = decoderMap[remoteId]; !ok {
  1192  		// To handle recursive types, mark this engine as underway before compiling.
  1193  		enginePtr = new(*decEngine)
  1194  		decoderMap[remoteId] = enginePtr
  1195  		*enginePtr, err = dec.compileDec(remoteId, ut)
  1196  		if err != nil {
  1197  			delete(decoderMap, remoteId)
  1198  		}
  1199  	}
  1200  	return
  1201  }
  1202  
  1203  // emptyStruct is the type we compile into when ignoring a struct value.
  1204  type emptyStruct struct{}
  1205  
  1206  var emptyStructType = reflect.TypeFor[emptyStruct]()
  1207  
  1208  // getIgnoreEnginePtr returns the engine for the specified type when the value is to be discarded.
  1209  func (dec *Decoder) getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, err error) {
  1210  	var ok bool
  1211  	if enginePtr, ok = dec.ignorerCache[wireId]; !ok {
  1212  		// To handle recursive types, mark this engine as underway before compiling.
  1213  		enginePtr = new(*decEngine)
  1214  		dec.ignorerCache[wireId] = enginePtr
  1215  		wire := dec.wireType[wireId]
  1216  		if wire != nil && wire.StructT != nil {
  1217  			*enginePtr, err = dec.compileDec(wireId, userType(emptyStructType))
  1218  		} else {
  1219  			*enginePtr = dec.compileIgnoreSingle(wireId)
  1220  		}
  1221  		if err != nil {
  1222  			delete(dec.ignorerCache, wireId)
  1223  		}
  1224  	}
  1225  	return
  1226  }
  1227  
  1228  // decodeValue decodes the data stream representing a value and stores it in value.
  1229  func (dec *Decoder) decodeValue(wireId typeId, value reflect.Value) {
  1230  	defer catchError(&dec.err)
  1231  	// If the value is nil, it means we should just ignore this item.
  1232  	if !value.IsValid() {
  1233  		dec.decodeIgnoredValue(wireId)
  1234  		return
  1235  	}
  1236  	// Dereference down to the underlying type.
  1237  	ut := userType(value.Type())
  1238  	base := ut.base
  1239  	var enginePtr **decEngine
  1240  	enginePtr, dec.err = dec.getDecEnginePtr(wireId, ut)
  1241  	if dec.err != nil {
  1242  		return
  1243  	}
  1244  	value = decAlloc(value)
  1245  	engine := *enginePtr
  1246  	if st := base; st.Kind() == reflect.Struct && ut.externalDec == 0 {
  1247  		wt := dec.wireType[wireId]
  1248  		if engine.numInstr == 0 && st.NumField() > 0 &&
  1249  			wt != nil && len(wt.StructT.Field) > 0 {
  1250  			name := base.Name()
  1251  			errorf("type mismatch: no fields matched compiling decoder for %s", name)
  1252  		}
  1253  		dec.decodeStruct(engine, value)
  1254  	} else {
  1255  		dec.decodeSingle(engine, value)
  1256  	}
  1257  }
  1258  
  1259  // decodeIgnoredValue decodes the data stream representing a value of the specified type and discards it.
  1260  func (dec *Decoder) decodeIgnoredValue(wireId typeId) {
  1261  	var enginePtr **decEngine
  1262  	enginePtr, dec.err = dec.getIgnoreEnginePtr(wireId)
  1263  	if dec.err != nil {
  1264  		return
  1265  	}
  1266  	wire := dec.wireType[wireId]
  1267  	if wire != nil && wire.StructT != nil {
  1268  		dec.ignoreStruct(*enginePtr)
  1269  	} else {
  1270  		dec.ignoreSingle(*enginePtr)
  1271  	}
  1272  }
  1273  
  1274  const (
  1275  	intBits     = 32 << (^uint(0) >> 63)
  1276  	uintptrBits = 32 << (^uintptr(0) >> 63)
  1277  )
  1278  
  1279  func init() {
  1280  	var iop, uop decOp
  1281  	switch intBits {
  1282  	case 32:
  1283  		iop = decInt32
  1284  		uop = decUint32
  1285  	case 64:
  1286  		iop = decInt64
  1287  		uop = decUint64
  1288  	default:
  1289  		panic("gob: unknown size of int/uint")
  1290  	}
  1291  	decOpTable[reflect.Int] = iop
  1292  	decOpTable[reflect.Uint] = uop
  1293  
  1294  	// Finally uintptr
  1295  	switch uintptrBits {
  1296  	case 32:
  1297  		uop = decUint32
  1298  	case 64:
  1299  		uop = decUint64
  1300  	default:
  1301  		panic("gob: unknown size of uintptr")
  1302  	}
  1303  	decOpTable[reflect.Uintptr] = uop
  1304  }
  1305  
  1306  // Gob depends on being able to take the address
  1307  // of zeroed Values it creates, so use this wrapper instead
  1308  // of the standard reflect.Zero.
  1309  // Each call allocates once.
  1310  func allocValue(t reflect.Type) reflect.Value {
  1311  	return reflect.New(t).Elem()
  1312  }
  1313  

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