Source file src/pkg/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	package gob
     6	
     7	// TODO(rsc): When garbage collector changes, revisit
     8	// the allocations in this file that use unsafe.Pointer.
     9	
    10	import (
    11		"bytes"
    12		"errors"
    13		"io"
    14		"math"
    15		"reflect"
    16		"unsafe"
    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	// decoderState is the execution state of an instance of the decoder. A new state
    26	// is created for nested objects.
    27	type decoderState struct {
    28		dec *Decoder
    29		// The buffer is stored with an extra indirection because it may be replaced
    30		// if we load a type during decode (when reading an interface value).
    31		b        *bytes.Buffer
    32		fieldnum int // the last field number read.
    33		buf      []byte
    34		next     *decoderState // for free list
    35	}
    36	
    37	// We pass the bytes.Buffer separately for easier testing of the infrastructure
    38	// without requiring a full Decoder.
    39	func (dec *Decoder) newDecoderState(buf *bytes.Buffer) *decoderState {
    40		d := dec.freeList
    41		if d == nil {
    42			d = new(decoderState)
    43			d.dec = dec
    44			d.buf = make([]byte, uint64Size)
    45		} else {
    46			dec.freeList = d.next
    47		}
    48		d.b = buf
    49		return d
    50	}
    51	
    52	func (dec *Decoder) freeDecoderState(d *decoderState) {
    53		d.next = dec.freeList
    54		dec.freeList = d
    55	}
    56	
    57	func overflow(name string) error {
    58		return errors.New(`value for "` + name + `" out of range`)
    59	}
    60	
    61	// decodeUintReader reads an encoded unsigned integer from an io.Reader.
    62	// Used only by the Decoder to read the message length.
    63	func decodeUintReader(r io.Reader, buf []byte) (x uint64, width int, err error) {
    64		width = 1
    65		_, err = r.Read(buf[0:width])
    66		if err != nil {
    67			return
    68		}
    69		b := buf[0]
    70		if b <= 0x7f {
    71			return uint64(b), width, nil
    72		}
    73		n := -int(int8(b))
    74		if n > uint64Size {
    75			err = errBadUint
    76			return
    77		}
    78		width, err = io.ReadFull(r, buf[0:n])
    79		if err != nil {
    80			if err == io.EOF {
    81				err = io.ErrUnexpectedEOF
    82			}
    83			return
    84		}
    85		// Could check that the high byte is zero but it's not worth it.
    86		for _, b := range buf[0:width] {
    87			x = x<<8 | uint64(b)
    88		}
    89		width++ // +1 for length byte
    90		return
    91	}
    92	
    93	// decodeUint reads an encoded unsigned integer from state.r.
    94	// Does not check for overflow.
    95	func (state *decoderState) decodeUint() (x uint64) {
    96		b, err := state.b.ReadByte()
    97		if err != nil {
    98			error_(err)
    99		}
   100		if b <= 0x7f {
   101			return uint64(b)
   102		}
   103		n := -int(int8(b))
   104		if n > uint64Size {
   105			error_(errBadUint)
   106		}
   107		width, err := state.b.Read(state.buf[0:n])
   108		if err != nil {
   109			error_(err)
   110		}
   111		// Don't need to check error; it's safe to loop regardless.
   112		// Could check that the high byte is zero but it's not worth it.
   113		for _, b := range state.buf[0:width] {
   114			x = x<<8 | uint64(b)
   115		}
   116		return x
   117	}
   118	
   119	// decodeInt reads an encoded signed integer from state.r.
   120	// Does not check for overflow.
   121	func (state *decoderState) decodeInt() int64 {
   122		x := state.decodeUint()
   123		if x&1 != 0 {
   124			return ^int64(x >> 1)
   125		}
   126		return int64(x >> 1)
   127	}
   128	
   129	// decOp is the signature of a decoding operator for a given type.
   130	type decOp func(i *decInstr, state *decoderState, p unsafe.Pointer)
   131	
   132	// The 'instructions' of the decoding machine
   133	type decInstr struct {
   134		op     decOp
   135		field  int     // field number of the wire type
   136		indir  int     // how many pointer indirections to reach the value in the struct
   137		offset uintptr // offset in the structure of the field to encode
   138		ovfl   error   // error message for overflow/underflow (for arrays, of the elements)
   139	}
   140	
   141	// Since the encoder writes no zeros, if we arrive at a decoder we have
   142	// a value to extract and store.  The field number has already been read
   143	// (it's how we knew to call this decoder).
   144	// Each decoder is responsible for handling any indirections associated
   145	// with the data structure.  If any pointer so reached is nil, allocation must
   146	// be done.
   147	
   148	// Walk the pointer hierarchy, allocating if we find a nil.  Stop one before the end.
   149	func decIndirect(p unsafe.Pointer, indir int) unsafe.Pointer {
   150		for ; indir > 1; indir-- {
   151			if *(*unsafe.Pointer)(p) == nil {
   152				// Allocation required
   153				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(unsafe.Pointer))
   154			}
   155			p = *(*unsafe.Pointer)(p)
   156		}
   157		return p
   158	}
   159	
   160	// ignoreUint discards a uint value with no destination.
   161	func ignoreUint(i *decInstr, state *decoderState, p unsafe.Pointer) {
   162		state.decodeUint()
   163	}
   164	
   165	// ignoreTwoUints discards a uint value with no destination. It's used to skip
   166	// complex values.
   167	func ignoreTwoUints(i *decInstr, state *decoderState, p unsafe.Pointer) {
   168		state.decodeUint()
   169		state.decodeUint()
   170	}
   171	
   172	// decBool decodes a uint and stores it as a boolean through p.
   173	func decBool(i *decInstr, state *decoderState, p unsafe.Pointer) {
   174		if i.indir > 0 {
   175			if *(*unsafe.Pointer)(p) == nil {
   176				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(bool))
   177			}
   178			p = *(*unsafe.Pointer)(p)
   179		}
   180		*(*bool)(p) = state.decodeUint() != 0
   181	}
   182	
   183	// decInt8 decodes an integer and stores it as an int8 through p.
   184	func decInt8(i *decInstr, state *decoderState, p unsafe.Pointer) {
   185		if i.indir > 0 {
   186			if *(*unsafe.Pointer)(p) == nil {
   187				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int8))
   188			}
   189			p = *(*unsafe.Pointer)(p)
   190		}
   191		v := state.decodeInt()
   192		if v < math.MinInt8 || math.MaxInt8 < v {
   193			error_(i.ovfl)
   194		} else {
   195			*(*int8)(p) = int8(v)
   196		}
   197	}
   198	
   199	// decUint8 decodes an unsigned integer and stores it as a uint8 through p.
   200	func decUint8(i *decInstr, state *decoderState, p unsafe.Pointer) {
   201		if i.indir > 0 {
   202			if *(*unsafe.Pointer)(p) == nil {
   203				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint8))
   204			}
   205			p = *(*unsafe.Pointer)(p)
   206		}
   207		v := state.decodeUint()
   208		if math.MaxUint8 < v {
   209			error_(i.ovfl)
   210		} else {
   211			*(*uint8)(p) = uint8(v)
   212		}
   213	}
   214	
   215	// decInt16 decodes an integer and stores it as an int16 through p.
   216	func decInt16(i *decInstr, state *decoderState, p unsafe.Pointer) {
   217		if i.indir > 0 {
   218			if *(*unsafe.Pointer)(p) == nil {
   219				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int16))
   220			}
   221			p = *(*unsafe.Pointer)(p)
   222		}
   223		v := state.decodeInt()
   224		if v < math.MinInt16 || math.MaxInt16 < v {
   225			error_(i.ovfl)
   226		} else {
   227			*(*int16)(p) = int16(v)
   228		}
   229	}
   230	
   231	// decUint16 decodes an unsigned integer and stores it as a uint16 through p.
   232	func decUint16(i *decInstr, state *decoderState, p unsafe.Pointer) {
   233		if i.indir > 0 {
   234			if *(*unsafe.Pointer)(p) == nil {
   235				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint16))
   236			}
   237			p = *(*unsafe.Pointer)(p)
   238		}
   239		v := state.decodeUint()
   240		if math.MaxUint16 < v {
   241			error_(i.ovfl)
   242		} else {
   243			*(*uint16)(p) = uint16(v)
   244		}
   245	}
   246	
   247	// decInt32 decodes an integer and stores it as an int32 through p.
   248	func decInt32(i *decInstr, state *decoderState, p unsafe.Pointer) {
   249		if i.indir > 0 {
   250			if *(*unsafe.Pointer)(p) == nil {
   251				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int32))
   252			}
   253			p = *(*unsafe.Pointer)(p)
   254		}
   255		v := state.decodeInt()
   256		if v < math.MinInt32 || math.MaxInt32 < v {
   257			error_(i.ovfl)
   258		} else {
   259			*(*int32)(p) = int32(v)
   260		}
   261	}
   262	
   263	// decUint32 decodes an unsigned integer and stores it as a uint32 through p.
   264	func decUint32(i *decInstr, state *decoderState, p unsafe.Pointer) {
   265		if i.indir > 0 {
   266			if *(*unsafe.Pointer)(p) == nil {
   267				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint32))
   268			}
   269			p = *(*unsafe.Pointer)(p)
   270		}
   271		v := state.decodeUint()
   272		if math.MaxUint32 < v {
   273			error_(i.ovfl)
   274		} else {
   275			*(*uint32)(p) = uint32(v)
   276		}
   277	}
   278	
   279	// decInt64 decodes an integer and stores it as an int64 through p.
   280	func decInt64(i *decInstr, state *decoderState, p unsafe.Pointer) {
   281		if i.indir > 0 {
   282			if *(*unsafe.Pointer)(p) == nil {
   283				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(int64))
   284			}
   285			p = *(*unsafe.Pointer)(p)
   286		}
   287		*(*int64)(p) = int64(state.decodeInt())
   288	}
   289	
   290	// decUint64 decodes an unsigned integer and stores it as a uint64 through p.
   291	func decUint64(i *decInstr, state *decoderState, p unsafe.Pointer) {
   292		if i.indir > 0 {
   293			if *(*unsafe.Pointer)(p) == nil {
   294				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint64))
   295			}
   296			p = *(*unsafe.Pointer)(p)
   297		}
   298		*(*uint64)(p) = uint64(state.decodeUint())
   299	}
   300	
   301	// Floating-point numbers are transmitted as uint64s holding the bits
   302	// of the underlying representation.  They are sent byte-reversed, with
   303	// the exponent end coming out first, so integer floating point numbers
   304	// (for example) transmit more compactly.  This routine does the
   305	// unswizzling.
   306	func floatFromBits(u uint64) float64 {
   307		var v uint64
   308		for i := 0; i < 8; i++ {
   309			v <<= 8
   310			v |= u & 0xFF
   311			u >>= 8
   312		}
   313		return math.Float64frombits(v)
   314	}
   315	
   316	// storeFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
   317	// number, and stores it through p. It's a helper function for float32 and complex64.
   318	func storeFloat32(i *decInstr, state *decoderState, p unsafe.Pointer) {
   319		v := floatFromBits(state.decodeUint())
   320		av := v
   321		if av < 0 {
   322			av = -av
   323		}
   324		// +Inf is OK in both 32- and 64-bit floats.  Underflow is always OK.
   325		if math.MaxFloat32 < av && av <= math.MaxFloat64 {
   326			error_(i.ovfl)
   327		} else {
   328			*(*float32)(p) = float32(v)
   329		}
   330	}
   331	
   332	// decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
   333	// number, and stores it through p.
   334	func decFloat32(i *decInstr, state *decoderState, p unsafe.Pointer) {
   335		if i.indir > 0 {
   336			if *(*unsafe.Pointer)(p) == nil {
   337				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(float32))
   338			}
   339			p = *(*unsafe.Pointer)(p)
   340		}
   341		storeFloat32(i, state, p)
   342	}
   343	
   344	// decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
   345	// number, and stores it through p.
   346	func decFloat64(i *decInstr, state *decoderState, p unsafe.Pointer) {
   347		if i.indir > 0 {
   348			if *(*unsafe.Pointer)(p) == nil {
   349				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(float64))
   350			}
   351			p = *(*unsafe.Pointer)(p)
   352		}
   353		*(*float64)(p) = floatFromBits(uint64(state.decodeUint()))
   354	}
   355	
   356	// decComplex64 decodes a pair of unsigned integers, treats them as a
   357	// pair of floating point numbers, and stores them as a complex64 through p.
   358	// The real part comes first.
   359	func decComplex64(i *decInstr, state *decoderState, p unsafe.Pointer) {
   360		if i.indir > 0 {
   361			if *(*unsafe.Pointer)(p) == nil {
   362				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(complex64))
   363			}
   364			p = *(*unsafe.Pointer)(p)
   365		}
   366		storeFloat32(i, state, p)
   367		storeFloat32(i, state, unsafe.Pointer(uintptr(p)+unsafe.Sizeof(float32(0))))
   368	}
   369	
   370	// decComplex128 decodes a pair of unsigned integers, treats them as a
   371	// pair of floating point numbers, and stores them as a complex128 through p.
   372	// The real part comes first.
   373	func decComplex128(i *decInstr, state *decoderState, p unsafe.Pointer) {
   374		if i.indir > 0 {
   375			if *(*unsafe.Pointer)(p) == nil {
   376				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(complex128))
   377			}
   378			p = *(*unsafe.Pointer)(p)
   379		}
   380		real := floatFromBits(uint64(state.decodeUint()))
   381		imag := floatFromBits(uint64(state.decodeUint()))
   382		*(*complex128)(p) = complex(real, imag)
   383	}
   384	
   385	// decUint8Slice decodes a byte slice and stores through p a slice header
   386	// describing the data.
   387	// uint8 slices are encoded as an unsigned count followed by the raw bytes.
   388	func decUint8Slice(i *decInstr, state *decoderState, p unsafe.Pointer) {
   389		if i.indir > 0 {
   390			if *(*unsafe.Pointer)(p) == nil {
   391				*(*unsafe.Pointer)(p) = unsafe.Pointer(new([]uint8))
   392			}
   393			p = *(*unsafe.Pointer)(p)
   394		}
   395		n := state.decodeUint()
   396		if n > uint64(state.b.Len()) {
   397			errorf("length of []byte exceeds input size (%d bytes)", n)
   398		}
   399		slice := (*[]uint8)(p)
   400		if uint64(cap(*slice)) < n {
   401			*slice = make([]uint8, n)
   402		} else {
   403			*slice = (*slice)[0:n]
   404		}
   405		if _, err := state.b.Read(*slice); err != nil {
   406			errorf("error decoding []byte: %s", err)
   407		}
   408	}
   409	
   410	// decString decodes byte array and stores through p 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, p unsafe.Pointer) {
   414		if i.indir > 0 {
   415			if *(*unsafe.Pointer)(p) == nil {
   416				*(*unsafe.Pointer)(p) = unsafe.Pointer(new(string))
   417			}
   418			p = *(*unsafe.Pointer)(p)
   419		}
   420		n := state.decodeUint()
   421		if n > uint64(state.b.Len()) {
   422			errorf("string length exceeds input size (%d bytes)", n)
   423		}
   424		b := make([]byte, n)
   425		state.b.Read(b)
   426		// It would be a shame to do the obvious thing here,
   427		//	*(*string)(p) = string(b)
   428		// because we've already allocated the storage and this would
   429		// allocate again and copy.  So we do this ugly hack, which is even
   430		// even more unsafe than it looks as it depends the memory
   431		// representation of a string matching the beginning of the memory
   432		// representation of a byte slice (a byte slice is longer).
   433		*(*string)(p) = *(*string)(unsafe.Pointer(&b))
   434	}
   435	
   436	// ignoreUint8Array skips over the data for a byte slice value with no destination.
   437	func ignoreUint8Array(i *decInstr, state *decoderState, p unsafe.Pointer) {
   438		b := make([]byte, state.decodeUint())
   439		state.b.Read(b)
   440	}
   441	
   442	// Execution engine
   443	
   444	// The encoder engine is an array of instructions indexed by field number of the incoming
   445	// decoder.  It is executed with random access according to field number.
   446	type decEngine struct {
   447		instr    []decInstr
   448		numInstr int // the number of active instructions
   449	}
   450	
   451	// allocate makes sure storage is available for an object of underlying type rtyp
   452	// that is indir levels of indirection through p.
   453	func allocate(rtyp reflect.Type, p uintptr, indir int) uintptr {
   454		if indir == 0 {
   455			return p
   456		}
   457		up := unsafe.Pointer(p)
   458		if indir > 1 {
   459			up = decIndirect(up, indir)
   460		}
   461		if *(*unsafe.Pointer)(up) == nil {
   462			// Allocate object.
   463			*(*unsafe.Pointer)(up) = unsafe.Pointer(reflect.New(rtyp).Pointer())
   464		}
   465		return *(*uintptr)(up)
   466	}
   467	
   468	// decodeSingle decodes a top-level value that is not a struct and stores it through p.
   469	// Such values are preceded by a zero, making them have the memory layout of a
   470	// struct field (although with an illegal field number).
   471	func (dec *Decoder) decodeSingle(engine *decEngine, ut *userTypeInfo, basep uintptr) {
   472		state := dec.newDecoderState(&dec.buf)
   473		state.fieldnum = singletonField
   474		delta := int(state.decodeUint())
   475		if delta != 0 {
   476			errorf("decode: corrupted data: non-zero delta for singleton")
   477		}
   478		instr := &engine.instr[singletonField]
   479		if instr.indir != ut.indir {
   480			errorf("internal error: inconsistent indirection instr %d ut %d", instr.indir, ut.indir)
   481		}
   482		ptr := unsafe.Pointer(basep) // offset will be zero
   483		if instr.indir > 1 {
   484			ptr = decIndirect(ptr, instr.indir)
   485		}
   486		instr.op(instr, state, ptr)
   487		dec.freeDecoderState(state)
   488	}
   489	
   490	// decodeStruct decodes a top-level struct and stores it through p.
   491	// Indir is for the value, not the type.  At the time of the call it may
   492	// differ from ut.indir, which was computed when the engine was built.
   493	// This state cannot arise for decodeSingle, which is called directly
   494	// from the user's value, not from the innards of an engine.
   495	func (dec *Decoder) decodeStruct(engine *decEngine, ut *userTypeInfo, p uintptr, indir int) {
   496		p = allocate(ut.base, p, indir)
   497		state := dec.newDecoderState(&dec.buf)
   498		state.fieldnum = -1
   499		basep := p
   500		for state.b.Len() > 0 {
   501			delta := int(state.decodeUint())
   502			if delta < 0 {
   503				errorf("decode: corrupted data: negative delta")
   504			}
   505			if delta == 0 { // struct terminator is zero delta fieldnum
   506				break
   507			}
   508			fieldnum := state.fieldnum + delta
   509			if fieldnum >= len(engine.instr) {
   510				error_(errRange)
   511				break
   512			}
   513			instr := &engine.instr[fieldnum]
   514			p := unsafe.Pointer(basep + instr.offset)
   515			if instr.indir > 1 {
   516				p = decIndirect(p, instr.indir)
   517			}
   518			instr.op(instr, state, p)
   519			state.fieldnum = fieldnum
   520		}
   521		dec.freeDecoderState(state)
   522	}
   523	
   524	// ignoreStruct discards the data for a struct with no destination.
   525	func (dec *Decoder) ignoreStruct(engine *decEngine) {
   526		state := dec.newDecoderState(&dec.buf)
   527		state.fieldnum = -1
   528		for state.b.Len() > 0 {
   529			delta := int(state.decodeUint())
   530			if delta < 0 {
   531				errorf("ignore decode: corrupted data: negative delta")
   532			}
   533			if delta == 0 { // struct terminator is zero delta fieldnum
   534				break
   535			}
   536			fieldnum := state.fieldnum + delta
   537			if fieldnum >= len(engine.instr) {
   538				error_(errRange)
   539			}
   540			instr := &engine.instr[fieldnum]
   541			instr.op(instr, state, unsafe.Pointer(nil))
   542			state.fieldnum = fieldnum
   543		}
   544		dec.freeDecoderState(state)
   545	}
   546	
   547	// ignoreSingle discards the data for a top-level non-struct value with no
   548	// destination. It's used when calling Decode with a nil value.
   549	func (dec *Decoder) ignoreSingle(engine *decEngine) {
   550		state := dec.newDecoderState(&dec.buf)
   551		state.fieldnum = singletonField
   552		delta := int(state.decodeUint())
   553		if delta != 0 {
   554			errorf("decode: corrupted data: non-zero delta for singleton")
   555		}
   556		instr := &engine.instr[singletonField]
   557		instr.op(instr, state, unsafe.Pointer(nil))
   558		dec.freeDecoderState(state)
   559	}
   560	
   561	// decodeArrayHelper does the work for decoding arrays and slices.
   562	func (dec *Decoder) decodeArrayHelper(state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, length, elemIndir int, ovfl error) {
   563		instr := &decInstr{elemOp, 0, elemIndir, 0, ovfl}
   564		for i := 0; i < length; i++ {
   565			up := unsafe.Pointer(p)
   566			if elemIndir > 1 {
   567				up = decIndirect(up, elemIndir)
   568			}
   569			elemOp(instr, state, up)
   570			p += uintptr(elemWid)
   571		}
   572	}
   573	
   574	// decodeArray decodes an array and stores it through p, that is, p points to the zeroth element.
   575	// The length is an unsigned integer preceding the elements.  Even though the length is redundant
   576	// (it's part of the type), it's a useful check and is included in the encoding.
   577	func (dec *Decoder) decodeArray(atyp reflect.Type, state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, length, indir, elemIndir int, ovfl error) {
   578		if indir > 0 {
   579			p = allocate(atyp, p, 1) // All but the last level has been allocated by dec.Indirect
   580		}
   581		if n := state.decodeUint(); n != uint64(length) {
   582			errorf("length mismatch in decodeArray")
   583		}
   584		dec.decodeArrayHelper(state, p, elemOp, elemWid, length, elemIndir, ovfl)
   585	}
   586	
   587	// decodeIntoValue is a helper for map decoding.  Since maps are decoded using reflection,
   588	// unlike the other items we can't use a pointer directly.
   589	func decodeIntoValue(state *decoderState, op decOp, indir int, v reflect.Value, ovfl error) reflect.Value {
   590		instr := &decInstr{op, 0, indir, 0, ovfl}
   591		up := unsafe.Pointer(unsafeAddr(v))
   592		if indir > 1 {
   593			up = decIndirect(up, indir)
   594		}
   595		op(instr, state, up)
   596		return v
   597	}
   598	
   599	// decodeMap decodes a map and stores its header through p.
   600	// Maps are encoded as a length followed by key:value pairs.
   601	// Because the internals of maps are not visible to us, we must
   602	// use reflection rather than pointer magic.
   603	func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, p uintptr, keyOp, elemOp decOp, indir, keyIndir, elemIndir int, ovfl error) {
   604		if indir > 0 {
   605			p = allocate(mtyp, p, 1) // All but the last level has been allocated by dec.Indirect
   606		}
   607		up := unsafe.Pointer(p)
   608		if *(*unsafe.Pointer)(up) == nil { // maps are represented as a pointer in the runtime
   609			// Allocate map.
   610			*(*unsafe.Pointer)(up) = unsafe.Pointer(reflect.MakeMap(mtyp).Pointer())
   611		}
   612		// Maps cannot be accessed by moving addresses around the way
   613		// that slices etc. can.  We must recover a full reflection value for
   614		// the iteration.
   615		v := reflect.NewAt(mtyp, unsafe.Pointer(p)).Elem()
   616		n := int(state.decodeUint())
   617		for i := 0; i < n; i++ {
   618			key := decodeIntoValue(state, keyOp, keyIndir, allocValue(mtyp.Key()), ovfl)
   619			elem := decodeIntoValue(state, elemOp, elemIndir, allocValue(mtyp.Elem()), ovfl)
   620			v.SetMapIndex(key, elem)
   621		}
   622	}
   623	
   624	// ignoreArrayHelper does the work for discarding arrays and slices.
   625	func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
   626		instr := &decInstr{elemOp, 0, 0, 0, errors.New("no error")}
   627		for i := 0; i < length; i++ {
   628			elemOp(instr, state, nil)
   629		}
   630	}
   631	
   632	// ignoreArray discards the data for an array value with no destination.
   633	func (dec *Decoder) ignoreArray(state *decoderState, elemOp decOp, length int) {
   634		if n := state.decodeUint(); n != uint64(length) {
   635			errorf("length mismatch in ignoreArray")
   636		}
   637		dec.ignoreArrayHelper(state, elemOp, length)
   638	}
   639	
   640	// ignoreMap discards the data for a map value with no destination.
   641	func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
   642		n := int(state.decodeUint())
   643		keyInstr := &decInstr{keyOp, 0, 0, 0, errors.New("no error")}
   644		elemInstr := &decInstr{elemOp, 0, 0, 0, errors.New("no error")}
   645		for i := 0; i < n; i++ {
   646			keyOp(keyInstr, state, nil)
   647			elemOp(elemInstr, state, nil)
   648		}
   649	}
   650	
   651	// decodeSlice decodes a slice and stores the slice header through p.
   652	// Slices are encoded as an unsigned length followed by the elements.
   653	func (dec *Decoder) decodeSlice(atyp reflect.Type, state *decoderState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl error) {
   654		nr := state.decodeUint()
   655		if nr > uint64(state.b.Len()) {
   656			errorf("length of slice exceeds input size (%d elements)", nr)
   657		}
   658		n := int(nr)
   659		if indir > 0 {
   660			up := unsafe.Pointer(p)
   661			if *(*unsafe.Pointer)(up) == nil {
   662				// Allocate the slice header.
   663				*(*unsafe.Pointer)(up) = unsafe.Pointer(new([]unsafe.Pointer))
   664			}
   665			p = *(*uintptr)(up)
   666		}
   667		// Allocate storage for the slice elements, that is, the underlying array,
   668		// if the existing slice does not have the capacity.
   669		// Always write a header at p.
   670		hdrp := (*reflect.SliceHeader)(unsafe.Pointer(p))
   671		if hdrp.Cap < n {
   672			hdrp.Data = reflect.MakeSlice(atyp, n, n).Pointer()
   673			hdrp.Cap = n
   674		}
   675		hdrp.Len = n
   676		dec.decodeArrayHelper(state, hdrp.Data, elemOp, elemWid, n, elemIndir, ovfl)
   677	}
   678	
   679	// ignoreSlice skips over the data for a slice value with no destination.
   680	func (dec *Decoder) ignoreSlice(state *decoderState, elemOp decOp) {
   681		dec.ignoreArrayHelper(state, elemOp, int(state.decodeUint()))
   682	}
   683	
   684	// setInterfaceValue sets an interface value to a concrete value,
   685	// but first it checks that the assignment will succeed.
   686	func setInterfaceValue(ivalue reflect.Value, value reflect.Value) {
   687		if !value.Type().AssignableTo(ivalue.Type()) {
   688			errorf("cannot assign value of type %s to %s", value.Type(), ivalue.Type())
   689		}
   690		ivalue.Set(value)
   691	}
   692	
   693	// decodeInterface decodes an interface value and stores it through p.
   694	// Interfaces are encoded as the name of a concrete type followed by a value.
   695	// If the name is empty, the value is nil and no value is sent.
   696	func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, p uintptr, indir int) {
   697		// Create a writable interface reflect.Value.  We need one even for the nil case.
   698		ivalue := allocValue(ityp)
   699		// Read the name of the concrete type.
   700		nr := state.decodeUint()
   701		if nr < 0 || nr > 1<<31 { // zero is permissible for anonymous types
   702			errorf("invalid type name length %d", nr)
   703		}
   704		b := make([]byte, nr)
   705		state.b.Read(b)
   706		name := string(b)
   707		if name == "" {
   708			// Copy the representation of the nil interface value to the target.
   709			// This is horribly unsafe and special.
   710			if indir > 0 {
   711				p = allocate(ityp, p, 1) // All but the last level has been allocated by dec.Indirect
   712			}
   713			*(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
   714			return
   715		}
   716		if len(name) > 1024 {
   717			errorf("name too long (%d bytes): %.20q...", len(name), name)
   718		}
   719		// The concrete type must be registered.
   720		typ, ok := nameToConcreteType[name]
   721		if !ok {
   722			errorf("name not registered for interface: %q", name)
   723		}
   724		// Read the type id of the concrete value.
   725		concreteId := dec.decodeTypeSequence(true)
   726		if concreteId < 0 {
   727			error_(dec.err)
   728		}
   729		// Byte count of value is next; we don't care what it is (it's there
   730		// in case we want to ignore the value by skipping it completely).
   731		state.decodeUint()
   732		// Read the concrete value.
   733		value := allocValue(typ)
   734		dec.decodeValue(concreteId, value)
   735		if dec.err != nil {
   736			error_(dec.err)
   737		}
   738		// Allocate the destination interface value.
   739		if indir > 0 {
   740			p = allocate(ityp, p, 1) // All but the last level has been allocated by dec.Indirect
   741		}
   742		// Assign the concrete value to the interface.
   743		// Tread carefully; it might not satisfy the interface.
   744		setInterfaceValue(ivalue, value)
   745		// Copy the representation of the interface value to the target.
   746		// This is horribly unsafe and special.
   747		*(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
   748	}
   749	
   750	// ignoreInterface discards the data for an interface value with no destination.
   751	func (dec *Decoder) ignoreInterface(state *decoderState) {
   752		// Read the name of the concrete type.
   753		b := make([]byte, state.decodeUint())
   754		_, err := state.b.Read(b)
   755		if err != nil {
   756			error_(err)
   757		}
   758		id := dec.decodeTypeSequence(true)
   759		if id < 0 {
   760			error_(dec.err)
   761		}
   762		// At this point, the decoder buffer contains a delimited value. Just toss it.
   763		state.b.Next(int(state.decodeUint()))
   764	}
   765	
   766	// decodeGobDecoder decodes something implementing the GobDecoder interface.
   767	// The data is encoded as a byte slice.
   768	func (dec *Decoder) decodeGobDecoder(state *decoderState, v reflect.Value) {
   769		// Read the bytes for the value.
   770		b := make([]byte, state.decodeUint())
   771		_, err := state.b.Read(b)
   772		if err != nil {
   773			error_(err)
   774		}
   775		// We know it's a GobDecoder, so just call the method directly.
   776		err = v.Interface().(GobDecoder).GobDecode(b)
   777		if err != nil {
   778			error_(err)
   779		}
   780	}
   781	
   782	// ignoreGobDecoder discards the data for a GobDecoder value with no destination.
   783	func (dec *Decoder) ignoreGobDecoder(state *decoderState) {
   784		// Read the bytes for the value.
   785		b := make([]byte, state.decodeUint())
   786		_, err := state.b.Read(b)
   787		if err != nil {
   788			error_(err)
   789		}
   790	}
   791	
   792	// Index by Go types.
   793	var decOpTable = [...]decOp{
   794		reflect.Bool:       decBool,
   795		reflect.Int8:       decInt8,
   796		reflect.Int16:      decInt16,
   797		reflect.Int32:      decInt32,
   798		reflect.Int64:      decInt64,
   799		reflect.Uint8:      decUint8,
   800		reflect.Uint16:     decUint16,
   801		reflect.Uint32:     decUint32,
   802		reflect.Uint64:     decUint64,
   803		reflect.Float32:    decFloat32,
   804		reflect.Float64:    decFloat64,
   805		reflect.Complex64:  decComplex64,
   806		reflect.Complex128: decComplex128,
   807		reflect.String:     decString,
   808	}
   809	
   810	// Indexed by gob types.  tComplex will be added during type.init().
   811	var decIgnoreOpMap = map[typeId]decOp{
   812		tBool:    ignoreUint,
   813		tInt:     ignoreUint,
   814		tUint:    ignoreUint,
   815		tFloat:   ignoreUint,
   816		tBytes:   ignoreUint8Array,
   817		tString:  ignoreUint8Array,
   818		tComplex: ignoreTwoUints,
   819	}
   820	
   821	// decOpFor returns the decoding op for the base type under rt and
   822	// the indirection count to reach it.
   823	func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProgress map[reflect.Type]*decOp) (*decOp, int) {
   824		ut := userType(rt)
   825		// If the type implements GobEncoder, we handle it without further processing.
   826		if ut.isGobDecoder {
   827			return dec.gobDecodeOpFor(ut)
   828		}
   829		// If this type is already in progress, it's a recursive type (e.g. map[string]*T).
   830		// Return the pointer to the op we're already building.
   831		if opPtr := inProgress[rt]; opPtr != nil {
   832			return opPtr, ut.indir
   833		}
   834		typ := ut.base
   835		indir := ut.indir
   836		var op decOp
   837		k := typ.Kind()
   838		if int(k) < len(decOpTable) {
   839			op = decOpTable[k]
   840		}
   841		if op == nil {
   842			inProgress[rt] = &op
   843			// Special cases
   844			switch t := typ; t.Kind() {
   845			case reflect.Array:
   846				name = "element of " + name
   847				elemId := dec.wireType[wireId].ArrayT.Elem
   848				elemOp, elemIndir := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   849				ovfl := overflow(name)
   850				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   851					state.dec.decodeArray(t, state, uintptr(p), *elemOp, t.Elem().Size(), t.Len(), i.indir, elemIndir, ovfl)
   852				}
   853	
   854			case reflect.Map:
   855				keyId := dec.wireType[wireId].MapT.Key
   856				elemId := dec.wireType[wireId].MapT.Elem
   857				keyOp, keyIndir := dec.decOpFor(keyId, t.Key(), "key of "+name, inProgress)
   858				elemOp, elemIndir := dec.decOpFor(elemId, t.Elem(), "element of "+name, inProgress)
   859				ovfl := overflow(name)
   860				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   861					up := unsafe.Pointer(p)
   862					state.dec.decodeMap(t, state, uintptr(up), *keyOp, *elemOp, i.indir, keyIndir, elemIndir, ovfl)
   863				}
   864	
   865			case reflect.Slice:
   866				name = "element of " + name
   867				if t.Elem().Kind() == reflect.Uint8 {
   868					op = decUint8Slice
   869					break
   870				}
   871				var elemId typeId
   872				if tt, ok := builtinIdToType[wireId]; ok {
   873					elemId = tt.(*sliceType).Elem
   874				} else {
   875					elemId = dec.wireType[wireId].SliceT.Elem
   876				}
   877				elemOp, elemIndir := dec.decOpFor(elemId, t.Elem(), name, inProgress)
   878				ovfl := overflow(name)
   879				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   880					state.dec.decodeSlice(t, state, uintptr(p), *elemOp, t.Elem().Size(), i.indir, elemIndir, ovfl)
   881				}
   882	
   883			case reflect.Struct:
   884				// Generate a closure that calls out to the engine for the nested type.
   885				enginePtr, err := dec.getDecEnginePtr(wireId, userType(typ))
   886				if err != nil {
   887					error_(err)
   888				}
   889				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   890					// indirect through enginePtr to delay evaluation for recursive structs.
   891					dec.decodeStruct(*enginePtr, userType(typ), uintptr(p), i.indir)
   892				}
   893			case reflect.Interface:
   894				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   895					state.dec.decodeInterface(t, state, uintptr(p), i.indir)
   896				}
   897			}
   898		}
   899		if op == nil {
   900			errorf("decode can't handle type %s", rt)
   901		}
   902		return &op, indir
   903	}
   904	
   905	// decIgnoreOpFor returns the decoding op for a field that has no destination.
   906	func (dec *Decoder) decIgnoreOpFor(wireId typeId) decOp {
   907		op, ok := decIgnoreOpMap[wireId]
   908		if !ok {
   909			if wireId == tInterface {
   910				// Special case because it's a method: the ignored item might
   911				// define types and we need to record their state in the decoder.
   912				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   913					state.dec.ignoreInterface(state)
   914				}
   915				return op
   916			}
   917			// Special cases
   918			wire := dec.wireType[wireId]
   919			switch {
   920			case wire == nil:
   921				errorf("bad data: undefined type %s", wireId.string())
   922			case wire.ArrayT != nil:
   923				elemId := wire.ArrayT.Elem
   924				elemOp := dec.decIgnoreOpFor(elemId)
   925				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   926					state.dec.ignoreArray(state, elemOp, wire.ArrayT.Len)
   927				}
   928	
   929			case wire.MapT != nil:
   930				keyId := dec.wireType[wireId].MapT.Key
   931				elemId := dec.wireType[wireId].MapT.Elem
   932				keyOp := dec.decIgnoreOpFor(keyId)
   933				elemOp := dec.decIgnoreOpFor(elemId)
   934				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   935					state.dec.ignoreMap(state, keyOp, elemOp)
   936				}
   937	
   938			case wire.SliceT != nil:
   939				elemId := wire.SliceT.Elem
   940				elemOp := dec.decIgnoreOpFor(elemId)
   941				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   942					state.dec.ignoreSlice(state, elemOp)
   943				}
   944	
   945			case wire.StructT != nil:
   946				// Generate a closure that calls out to the engine for the nested type.
   947				enginePtr, err := dec.getIgnoreEnginePtr(wireId)
   948				if err != nil {
   949					error_(err)
   950				}
   951				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   952					// indirect through enginePtr to delay evaluation for recursive structs
   953					state.dec.ignoreStruct(*enginePtr)
   954				}
   955	
   956			case wire.GobEncoderT != nil:
   957				op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   958					state.dec.ignoreGobDecoder(state)
   959				}
   960			}
   961		}
   962		if op == nil {
   963			errorf("bad data: ignore can't handle type %s", wireId.string())
   964		}
   965		return op
   966	}
   967	
   968	// gobDecodeOpFor returns the op for a type that is known to implement
   969	// GobDecoder.
   970	func (dec *Decoder) gobDecodeOpFor(ut *userTypeInfo) (*decOp, int) {
   971		rcvrType := ut.user
   972		if ut.decIndir == -1 {
   973			rcvrType = reflect.PtrTo(rcvrType)
   974		} else if ut.decIndir > 0 {
   975			for i := int8(0); i < ut.decIndir; i++ {
   976				rcvrType = rcvrType.Elem()
   977			}
   978		}
   979		var op decOp
   980		op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
   981			// Caller has gotten us to within one indirection of our value.
   982			if i.indir > 0 {
   983				if *(*unsafe.Pointer)(p) == nil {
   984					*(*unsafe.Pointer)(p) = unsafe.Pointer(reflect.New(ut.base).Pointer())
   985				}
   986			}
   987			// Now p is a pointer to the base type.  Do we need to climb out to
   988			// get to the receiver type?
   989			var v reflect.Value
   990			if ut.decIndir == -1 {
   991				v = reflect.NewAt(rcvrType, unsafe.Pointer(&p)).Elem()
   992			} else {
   993				v = reflect.NewAt(rcvrType, p).Elem()
   994			}
   995			state.dec.decodeGobDecoder(state, v)
   996		}
   997		return &op, int(ut.indir)
   998	
   999	}
  1000	
  1001	// compatibleType asks: Are these two gob Types compatible?
  1002	// Answers the question for basic types, arrays, maps and slices, plus
  1003	// GobEncoder/Decoder pairs.
  1004	// Structs are considered ok; fields will be checked later.
  1005	func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[reflect.Type]typeId) bool {
  1006		if rhs, ok := inProgress[fr]; ok {
  1007			return rhs == fw
  1008		}
  1009		inProgress[fr] = fw
  1010		ut := userType(fr)
  1011		wire, ok := dec.wireType[fw]
  1012		// If fr is a GobDecoder, the wire type must be GobEncoder.
  1013		// And if fr is not a GobDecoder, the wire type must not be either.
  1014		if ut.isGobDecoder != (ok && wire.GobEncoderT != nil) { // the parentheses look odd but are correct.
  1015			return false
  1016		}
  1017		if ut.isGobDecoder { // This test trumps all others.
  1018			return true
  1019		}
  1020		switch t := ut.base; t.Kind() {
  1021		default:
  1022			// chan, etc: cannot handle.
  1023			return false
  1024		case reflect.Bool:
  1025			return fw == tBool
  1026		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  1027			return fw == tInt
  1028		case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  1029			return fw == tUint
  1030		case reflect.Float32, reflect.Float64:
  1031			return fw == tFloat
  1032		case reflect.Complex64, reflect.Complex128:
  1033			return fw == tComplex
  1034		case reflect.String:
  1035			return fw == tString
  1036		case reflect.Interface:
  1037			return fw == tInterface
  1038		case reflect.Array:
  1039			if !ok || wire.ArrayT == nil {
  1040				return false
  1041			}
  1042			array := wire.ArrayT
  1043			return t.Len() == array.Len && dec.compatibleType(t.Elem(), array.Elem, inProgress)
  1044		case reflect.Map:
  1045			if !ok || wire.MapT == nil {
  1046				return false
  1047			}
  1048			MapType := wire.MapT
  1049			return dec.compatibleType(t.Key(), MapType.Key, inProgress) && dec.compatibleType(t.Elem(), MapType.Elem, inProgress)
  1050		case reflect.Slice:
  1051			// Is it an array of bytes?
  1052			if t.Elem().Kind() == reflect.Uint8 {
  1053				return fw == tBytes
  1054			}
  1055			// Extract and compare element types.
  1056			var sw *sliceType
  1057			if tt, ok := builtinIdToType[fw]; ok {
  1058				sw, _ = tt.(*sliceType)
  1059			} else if wire != nil {
  1060				sw = wire.SliceT
  1061			}
  1062			elem := userType(t.Elem()).base
  1063			return sw != nil && dec.compatibleType(elem, sw.Elem, inProgress)
  1064		case reflect.Struct:
  1065			return true
  1066		}
  1067		return true
  1068	}
  1069	
  1070	// typeString returns a human-readable description of the type identified by remoteId.
  1071	func (dec *Decoder) typeString(remoteId typeId) string {
  1072		if t := idToType[remoteId]; t != nil {
  1073			// globally known type.
  1074			return t.string()
  1075		}
  1076		return dec.wireType[remoteId].string()
  1077	}
  1078	
  1079	// compileSingle compiles the decoder engine for a non-struct top-level value, including
  1080	// GobDecoders.
  1081	func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1082		rt := ut.user
  1083		engine = new(decEngine)
  1084		engine.instr = make([]decInstr, 1) // one item
  1085		name := rt.String()                // best we can do
  1086		if !dec.compatibleType(rt, remoteId, make(map[reflect.Type]typeId)) {
  1087			remoteType := dec.typeString(remoteId)
  1088			// Common confusing case: local interface type, remote concrete type.
  1089			if ut.base.Kind() == reflect.Interface && remoteId != tInterface {
  1090				return nil, errors.New("gob: local interface type " + name + " can only be decoded from remote interface type; received concrete type " + remoteType)
  1091			}
  1092			return nil, errors.New("gob: decoding into local type " + name + ", received remote type " + remoteType)
  1093		}
  1094		op, indir := dec.decOpFor(remoteId, rt, name, make(map[reflect.Type]*decOp))
  1095		ovfl := errors.New(`value for "` + name + `" out of range`)
  1096		engine.instr[singletonField] = decInstr{*op, singletonField, indir, 0, ovfl}
  1097		engine.numInstr = 1
  1098		return
  1099	}
  1100	
  1101	// compileIgnoreSingle compiles the decoder engine for a non-struct top-level value that will be discarded.
  1102	func (dec *Decoder) compileIgnoreSingle(remoteId typeId) (engine *decEngine, err error) {
  1103		engine = new(decEngine)
  1104		engine.instr = make([]decInstr, 1) // one item
  1105		op := dec.decIgnoreOpFor(remoteId)
  1106		ovfl := overflow(dec.typeString(remoteId))
  1107		engine.instr[0] = decInstr{op, 0, 0, 0, ovfl}
  1108		engine.numInstr = 1
  1109		return
  1110	}
  1111	
  1112	// compileDec compiles the decoder engine for a value.  If the value is not a struct,
  1113	// it calls out to compileSingle.
  1114	func (dec *Decoder) compileDec(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  1115		rt := ut.base
  1116		srt := rt
  1117		if srt.Kind() != reflect.Struct ||
  1118			ut.isGobDecoder {
  1119			return dec.compileSingle(remoteId, ut)
  1120		}
  1121		var wireStruct *structType
  1122		// Builtin types can come from global pool; the rest must be defined by the decoder.
  1123		// Also we know we're decoding a struct now, so the client must have sent one.
  1124		if t, ok := builtinIdToType[remoteId]; ok {
  1125			wireStruct, _ = t.(*structType)
  1126		} else {
  1127			wire := dec.wireType[remoteId]
  1128			if wire == nil {
  1129				error_(errBadType)
  1130			}
  1131			wireStruct = wire.StructT
  1132		}
  1133		if wireStruct == nil {
  1134			errorf("type mismatch in decoder: want struct type %s; got non-struct", rt)
  1135		}
  1136		engine = new(decEngine)
  1137		engine.instr = make([]decInstr, len(wireStruct.Field))
  1138		seen := make(map[reflect.Type]*decOp)
  1139		// Loop over the fields of the wire type.
  1140		for fieldnum := 0; fieldnum < len(wireStruct.Field); fieldnum++ {
  1141			wireField := wireStruct.Field[fieldnum]
  1142			if wireField.Name == "" {
  1143				errorf("empty name for remote field of type %s", wireStruct.Name)
  1144			}
  1145			ovfl := overflow(wireField.Name)
  1146			// Find the field of the local type with the same name.
  1147			localField, present := srt.FieldByName(wireField.Name)
  1148			// TODO(r): anonymous names
  1149			if !present || !isExported(wireField.Name) {
  1150				op := dec.decIgnoreOpFor(wireField.Id)
  1151				engine.instr[fieldnum] = decInstr{op, fieldnum, 0, 0, ovfl}
  1152				continue
  1153			}
  1154			if !dec.compatibleType(localField.Type, wireField.Id, make(map[reflect.Type]typeId)) {
  1155				errorf("wrong type (%s) for received field %s.%s", localField.Type, wireStruct.Name, wireField.Name)
  1156			}
  1157			op, indir := dec.decOpFor(wireField.Id, localField.Type, localField.Name, seen)
  1158			engine.instr[fieldnum] = decInstr{*op, fieldnum, indir, uintptr(localField.Offset), ovfl}
  1159			engine.numInstr++
  1160		}
  1161		return
  1162	}
  1163	
  1164	// getDecEnginePtr returns the engine for the specified type.
  1165	func (dec *Decoder) getDecEnginePtr(remoteId typeId, ut *userTypeInfo) (enginePtr **decEngine, err error) {
  1166		rt := ut.user
  1167		decoderMap, ok := dec.decoderCache[rt]
  1168		if !ok {
  1169			decoderMap = make(map[typeId]**decEngine)
  1170			dec.decoderCache[rt] = decoderMap
  1171		}
  1172		if enginePtr, ok = decoderMap[remoteId]; !ok {
  1173			// To handle recursive types, mark this engine as underway before compiling.
  1174			enginePtr = new(*decEngine)
  1175			decoderMap[remoteId] = enginePtr
  1176			*enginePtr, err = dec.compileDec(remoteId, ut)
  1177			if err != nil {
  1178				delete(decoderMap, remoteId)
  1179			}
  1180		}
  1181		return
  1182	}
  1183	
  1184	// emptyStruct is the type we compile into when ignoring a struct value.
  1185	type emptyStruct struct{}
  1186	
  1187	var emptyStructType = reflect.TypeOf(emptyStruct{})
  1188	
  1189	// getDecEnginePtr returns the engine for the specified type when the value is to be discarded.
  1190	func (dec *Decoder) getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, err error) {
  1191		var ok bool
  1192		if enginePtr, ok = dec.ignorerCache[wireId]; !ok {
  1193			// To handle recursive types, mark this engine as underway before compiling.
  1194			enginePtr = new(*decEngine)
  1195			dec.ignorerCache[wireId] = enginePtr
  1196			wire := dec.wireType[wireId]
  1197			if wire != nil && wire.StructT != nil {
  1198				*enginePtr, err = dec.compileDec(wireId, userType(emptyStructType))
  1199			} else {
  1200				*enginePtr, err = dec.compileIgnoreSingle(wireId)
  1201			}
  1202			if err != nil {
  1203				delete(dec.ignorerCache, wireId)
  1204			}
  1205		}
  1206		return
  1207	}
  1208	
  1209	// decodeValue decodes the data stream representing a value and stores it in val.
  1210	func (dec *Decoder) decodeValue(wireId typeId, val reflect.Value) {
  1211		defer catchError(&dec.err)
  1212		// If the value is nil, it means we should just ignore this item.
  1213		if !val.IsValid() {
  1214			dec.decodeIgnoredValue(wireId)
  1215			return
  1216		}
  1217		// Dereference down to the underlying type.
  1218		ut := userType(val.Type())
  1219		base := ut.base
  1220		var enginePtr **decEngine
  1221		enginePtr, dec.err = dec.getDecEnginePtr(wireId, ut)
  1222		if dec.err != nil {
  1223			return
  1224		}
  1225		engine := *enginePtr
  1226		if st := base; st.Kind() == reflect.Struct && !ut.isGobDecoder {
  1227			if engine.numInstr == 0 && st.NumField() > 0 && len(dec.wireType[wireId].StructT.Field) > 0 {
  1228				name := base.Name()
  1229				errorf("type mismatch: no fields matched compiling decoder for %s", name)
  1230			}
  1231			dec.decodeStruct(engine, ut, uintptr(unsafeAddr(val)), ut.indir)
  1232		} else {
  1233			dec.decodeSingle(engine, ut, uintptr(unsafeAddr(val)))
  1234		}
  1235	}
  1236	
  1237	// decodeIgnoredValue decodes the data stream representing a value of the specified type and discards it.
  1238	func (dec *Decoder) decodeIgnoredValue(wireId typeId) {
  1239		var enginePtr **decEngine
  1240		enginePtr, dec.err = dec.getIgnoreEnginePtr(wireId)
  1241		if dec.err != nil {
  1242			return
  1243		}
  1244		wire := dec.wireType[wireId]
  1245		if wire != nil && wire.StructT != nil {
  1246			dec.ignoreStruct(*enginePtr)
  1247		} else {
  1248			dec.ignoreSingle(*enginePtr)
  1249		}
  1250	}
  1251	
  1252	func init() {
  1253		var iop, uop decOp
  1254		switch reflect.TypeOf(int(0)).Bits() {
  1255		case 32:
  1256			iop = decInt32
  1257			uop = decUint32
  1258		case 64:
  1259			iop = decInt64
  1260			uop = decUint64
  1261		default:
  1262			panic("gob: unknown size of int/uint")
  1263		}
  1264		decOpTable[reflect.Int] = iop
  1265		decOpTable[reflect.Uint] = uop
  1266	
  1267		// Finally uintptr
  1268		switch reflect.TypeOf(uintptr(0)).Bits() {
  1269		case 32:
  1270			uop = decUint32
  1271		case 64:
  1272			uop = decUint64
  1273		default:
  1274			panic("gob: unknown size of uintptr")
  1275		}
  1276		decOpTable[reflect.Uintptr] = uop
  1277	}
  1278	
  1279	// Gob assumes it can call UnsafeAddr on any Value
  1280	// in order to get a pointer it can copy data from.
  1281	// Values that have just been created and do not point
  1282	// into existing structs or slices cannot be addressed,
  1283	// so simulate it by returning a pointer to a copy.
  1284	// Each call allocates once.
  1285	func unsafeAddr(v reflect.Value) uintptr {
  1286		if v.CanAddr() {
  1287			return v.UnsafeAddr()
  1288		}
  1289		x := reflect.New(v.Type()).Elem()
  1290		x.Set(v)
  1291		return x.UnsafeAddr()
  1292	}
  1293	
  1294	// Gob depends on being able to take the address
  1295	// of zeroed Values it creates, so use this wrapper instead
  1296	// of the standard reflect.Zero.
  1297	// Each call allocates once.
  1298	func allocValue(t reflect.Type) reflect.Value {
  1299		return reflect.New(t).Elem()
  1300	}