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Source file src/encoding/gob/encoder.go

Documentation: encoding/gob

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package gob
     6  
     7  import (
     8  	"errors"
     9  	"io"
    10  	"reflect"
    11  	"sync"
    12  )
    13  
    14  // An Encoder manages the transmission of type and data information to the
    15  // other side of a connection.
    16  type Encoder struct {
    17  	mutex      sync.Mutex              // each item must be sent atomically
    18  	w          []io.Writer             // where to send the data
    19  	sent       map[reflect.Type]typeId // which types we've already sent
    20  	countState *encoderState           // stage for writing counts
    21  	freeList   *encoderState           // list of free encoderStates; avoids reallocation
    22  	byteBuf    encBuffer               // buffer for top-level encoderState
    23  	err        error
    24  }
    25  
    26  // Before we encode a message, we reserve space at the head of the
    27  // buffer in which to encode its length. This means we can use the
    28  // buffer to assemble the message without another allocation.
    29  const maxLength = 9 // Maximum size of an encoded length.
    30  var spaceForLength = make([]byte, maxLength)
    31  
    32  // NewEncoder returns a new encoder that will transmit on the io.Writer.
    33  func NewEncoder(w io.Writer) *Encoder {
    34  	enc := new(Encoder)
    35  	enc.w = []io.Writer{w}
    36  	enc.sent = make(map[reflect.Type]typeId)
    37  	enc.countState = enc.newEncoderState(new(encBuffer))
    38  	return enc
    39  }
    40  
    41  // writer() returns the innermost writer the encoder is using
    42  func (enc *Encoder) writer() io.Writer {
    43  	return enc.w[len(enc.w)-1]
    44  }
    45  
    46  // pushWriter adds a writer to the encoder.
    47  func (enc *Encoder) pushWriter(w io.Writer) {
    48  	enc.w = append(enc.w, w)
    49  }
    50  
    51  // popWriter pops the innermost writer.
    52  func (enc *Encoder) popWriter() {
    53  	enc.w = enc.w[0 : len(enc.w)-1]
    54  }
    55  
    56  func (enc *Encoder) setError(err error) {
    57  	if enc.err == nil { // remember the first.
    58  		enc.err = err
    59  	}
    60  }
    61  
    62  // writeMessage sends the data item preceded by a unsigned count of its length.
    63  func (enc *Encoder) writeMessage(w io.Writer, b *encBuffer) {
    64  	// Space has been reserved for the length at the head of the message.
    65  	// This is a little dirty: we grab the slice from the bytes.Buffer and massage
    66  	// it by hand.
    67  	message := b.Bytes()
    68  	messageLen := len(message) - maxLength
    69  	// Length cannot be bigger than the decoder can handle.
    70  	if messageLen >= tooBig {
    71  		enc.setError(errors.New("gob: encoder: message too big"))
    72  		return
    73  	}
    74  	// Encode the length.
    75  	enc.countState.b.Reset()
    76  	enc.countState.encodeUint(uint64(messageLen))
    77  	// Copy the length to be a prefix of the message.
    78  	offset := maxLength - enc.countState.b.Len()
    79  	copy(message[offset:], enc.countState.b.Bytes())
    80  	// Write the data.
    81  	_, err := w.Write(message[offset:])
    82  	// Drain the buffer and restore the space at the front for the count of the next message.
    83  	b.Reset()
    84  	b.Write(spaceForLength)
    85  	if err != nil {
    86  		enc.setError(err)
    87  	}
    88  }
    89  
    90  // sendActualType sends the requested type, without further investigation, unless
    91  // it's been sent before.
    92  func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTypeInfo, actual reflect.Type) (sent bool) {
    93  	if _, alreadySent := enc.sent[actual]; alreadySent {
    94  		return false
    95  	}
    96  	info, err := getTypeInfo(ut)
    97  	if err != nil {
    98  		enc.setError(err)
    99  		return
   100  	}
   101  	// Send the pair (-id, type)
   102  	// Id:
   103  	state.encodeInt(-int64(info.id))
   104  	// Type:
   105  	enc.encode(state.b, reflect.ValueOf(info.wire), wireTypeUserInfo)
   106  	enc.writeMessage(w, state.b)
   107  	if enc.err != nil {
   108  		return
   109  	}
   110  
   111  	// Remember we've sent this type, both what the user gave us and the base type.
   112  	enc.sent[ut.base] = info.id
   113  	if ut.user != ut.base {
   114  		enc.sent[ut.user] = info.id
   115  	}
   116  	// Now send the inner types
   117  	switch st := actual; st.Kind() {
   118  	case reflect.Struct:
   119  		for i := 0; i < st.NumField(); i++ {
   120  			if isExported(st.Field(i).Name) {
   121  				enc.sendType(w, state, st.Field(i).Type)
   122  			}
   123  		}
   124  	case reflect.Array, reflect.Slice:
   125  		enc.sendType(w, state, st.Elem())
   126  	case reflect.Map:
   127  		enc.sendType(w, state, st.Key())
   128  		enc.sendType(w, state, st.Elem())
   129  	}
   130  	return true
   131  }
   132  
   133  // sendType sends the type info to the other side, if necessary.
   134  func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Type) (sent bool) {
   135  	ut := userType(origt)
   136  	if ut.externalEnc != 0 {
   137  		// The rules are different: regardless of the underlying type's representation,
   138  		// we need to tell the other side that the base type is a GobEncoder.
   139  		return enc.sendActualType(w, state, ut, ut.base)
   140  	}
   141  
   142  	// It's a concrete value, so drill down to the base type.
   143  	switch rt := ut.base; rt.Kind() {
   144  	default:
   145  		// Basic types and interfaces do not need to be described.
   146  		return
   147  	case reflect.Slice:
   148  		// If it's []uint8, don't send; it's considered basic.
   149  		if rt.Elem().Kind() == reflect.Uint8 {
   150  			return
   151  		}
   152  		// Otherwise we do send.
   153  		break
   154  	case reflect.Array:
   155  		// arrays must be sent so we know their lengths and element types.
   156  		break
   157  	case reflect.Map:
   158  		// maps must be sent so we know their lengths and key/value types.
   159  		break
   160  	case reflect.Struct:
   161  		// structs must be sent so we know their fields.
   162  		break
   163  	case reflect.Chan, reflect.Func:
   164  		// If we get here, it's a field of a struct; ignore it.
   165  		return
   166  	}
   167  
   168  	return enc.sendActualType(w, state, ut, ut.base)
   169  }
   170  
   171  // Encode transmits the data item represented by the empty interface value,
   172  // guaranteeing that all necessary type information has been transmitted first.
   173  // Passing a nil pointer to Encoder will panic, as they cannot be transmitted by gob.
   174  func (enc *Encoder) Encode(e interface{}) error {
   175  	return enc.EncodeValue(reflect.ValueOf(e))
   176  }
   177  
   178  // sendTypeDescriptor makes sure the remote side knows about this type.
   179  // It will send a descriptor if this is the first time the type has been
   180  // sent.
   181  func (enc *Encoder) sendTypeDescriptor(w io.Writer, state *encoderState, ut *userTypeInfo) {
   182  	// Make sure the type is known to the other side.
   183  	// First, have we already sent this type?
   184  	rt := ut.base
   185  	if ut.externalEnc != 0 {
   186  		rt = ut.user
   187  	}
   188  	if _, alreadySent := enc.sent[rt]; !alreadySent {
   189  		// No, so send it.
   190  		sent := enc.sendType(w, state, rt)
   191  		if enc.err != nil {
   192  			return
   193  		}
   194  		// If the type info has still not been transmitted, it means we have
   195  		// a singleton basic type (int, []byte etc.) at top level. We don't
   196  		// need to send the type info but we do need to update enc.sent.
   197  		if !sent {
   198  			info, err := getTypeInfo(ut)
   199  			if err != nil {
   200  				enc.setError(err)
   201  				return
   202  			}
   203  			enc.sent[rt] = info.id
   204  		}
   205  	}
   206  }
   207  
   208  // sendTypeId sends the id, which must have already been defined.
   209  func (enc *Encoder) sendTypeId(state *encoderState, ut *userTypeInfo) {
   210  	// Identify the type of this top-level value.
   211  	state.encodeInt(int64(enc.sent[ut.base]))
   212  }
   213  
   214  // EncodeValue transmits the data item represented by the reflection value,
   215  // guaranteeing that all necessary type information has been transmitted first.
   216  // Passing a nil pointer to EncodeValue will panic, as they cannot be transmitted by gob.
   217  func (enc *Encoder) EncodeValue(value reflect.Value) error {
   218  	if value.Kind() == reflect.Invalid {
   219  		return errors.New("gob: cannot encode nil value")
   220  	}
   221  	if value.Kind() == reflect.Ptr && value.IsNil() {
   222  		panic("gob: cannot encode nil pointer of type " + value.Type().String())
   223  	}
   224  
   225  	// Make sure we're single-threaded through here, so multiple
   226  	// goroutines can share an encoder.
   227  	enc.mutex.Lock()
   228  	defer enc.mutex.Unlock()
   229  
   230  	// Remove any nested writers remaining due to previous errors.
   231  	enc.w = enc.w[0:1]
   232  
   233  	ut, err := validUserType(value.Type())
   234  	if err != nil {
   235  		return err
   236  	}
   237  
   238  	enc.err = nil
   239  	enc.byteBuf.Reset()
   240  	enc.byteBuf.Write(spaceForLength)
   241  	state := enc.newEncoderState(&enc.byteBuf)
   242  
   243  	enc.sendTypeDescriptor(enc.writer(), state, ut)
   244  	enc.sendTypeId(state, ut)
   245  	if enc.err != nil {
   246  		return enc.err
   247  	}
   248  
   249  	// Encode the object.
   250  	enc.encode(state.b, value, ut)
   251  	if enc.err == nil {
   252  		enc.writeMessage(enc.writer(), state.b)
   253  	}
   254  
   255  	enc.freeEncoderState(state)
   256  	return enc.err
   257  }
   258  

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