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Source file src/pkg/encoding/gob/encoder.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	import (
     8		"bytes"
     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    bytes.Buffer            // 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(bytes.Buffer))
    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 *bytes.Buffer) {
    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		// Encode the length.
    70		enc.countState.b.Reset()
    71		enc.countState.encodeUint(uint64(messageLen))
    72		// Copy the length to be a prefix of the message.
    73		offset := maxLength - enc.countState.b.Len()
    74		copy(message[offset:], enc.countState.b.Bytes())
    75		// Write the data.
    76		_, err := w.Write(message[offset:])
    77		// Drain the buffer and restore the space at the front for the count of the next message.
    78		b.Reset()
    79		b.Write(spaceForLength)
    80		if err != nil {
    81			enc.setError(err)
    82		}
    83	}
    84	
    85	// sendActualType sends the requested type, without further investigation, unless
    86	// it's been sent before.
    87	func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTypeInfo, actual reflect.Type) (sent bool) {
    88		if _, alreadySent := enc.sent[actual]; alreadySent {
    89			return false
    90		}
    91		typeLock.Lock()
    92		info, err := getTypeInfo(ut)
    93		typeLock.Unlock()
    94		if err != nil {
    95			enc.setError(err)
    96			return
    97		}
    98		// Send the pair (-id, type)
    99		// Id:
   100		state.encodeInt(-int64(info.id))
   101		// Type:
   102		enc.encode(state.b, reflect.ValueOf(info.wire), wireTypeUserInfo)
   103		enc.writeMessage(w, state.b)
   104		if enc.err != nil {
   105			return
   106		}
   107	
   108		// Remember we've sent this type, both what the user gave us and the base type.
   109		enc.sent[ut.base] = info.id
   110		if ut.user != ut.base {
   111			enc.sent[ut.user] = info.id
   112		}
   113		// Now send the inner types
   114		switch st := actual; st.Kind() {
   115		case reflect.Struct:
   116			for i := 0; i < st.NumField(); i++ {
   117				if isExported(st.Field(i).Name) {
   118					enc.sendType(w, state, st.Field(i).Type)
   119				}
   120			}
   121		case reflect.Array, reflect.Slice:
   122			enc.sendType(w, state, st.Elem())
   123		case reflect.Map:
   124			enc.sendType(w, state, st.Key())
   125			enc.sendType(w, state, st.Elem())
   126		}
   127		return true
   128	}
   129	
   130	// sendType sends the type info to the other side, if necessary.
   131	func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Type) (sent bool) {
   132		ut := userType(origt)
   133		if ut.externalEnc != 0 {
   134			// The rules are different: regardless of the underlying type's representation,
   135			// we need to tell the other side that the base type is a GobEncoder.
   136			return enc.sendActualType(w, state, ut, ut.base)
   137		}
   138	
   139		// It's a concrete value, so drill down to the base type.
   140		switch rt := ut.base; rt.Kind() {
   141		default:
   142			// Basic types and interfaces do not need to be described.
   143			return
   144		case reflect.Slice:
   145			// If it's []uint8, don't send; it's considered basic.
   146			if rt.Elem().Kind() == reflect.Uint8 {
   147				return
   148			}
   149			// Otherwise we do send.
   150			break
   151		case reflect.Array:
   152			// arrays must be sent so we know their lengths and element types.
   153			break
   154		case reflect.Map:
   155			// maps must be sent so we know their lengths and key/value types.
   156			break
   157		case reflect.Struct:
   158			// structs must be sent so we know their fields.
   159			break
   160		case reflect.Chan, reflect.Func:
   161			// If we get here, it's a field of a struct; ignore it.
   162			return
   163		}
   164	
   165		return enc.sendActualType(w, state, ut, ut.base)
   166	}
   167	
   168	// Encode transmits the data item represented by the empty interface value,
   169	// guaranteeing that all necessary type information has been transmitted first.
   170	func (enc *Encoder) Encode(e interface{}) error {
   171		return enc.EncodeValue(reflect.ValueOf(e))
   172	}
   173	
   174	// sendTypeDescriptor makes sure the remote side knows about this type.
   175	// It will send a descriptor if this is the first time the type has been
   176	// sent.
   177	func (enc *Encoder) sendTypeDescriptor(w io.Writer, state *encoderState, ut *userTypeInfo) {
   178		// Make sure the type is known to the other side.
   179		// First, have we already sent this type?
   180		rt := ut.base
   181		if ut.externalEnc != 0 {
   182			rt = ut.user
   183		}
   184		if _, alreadySent := enc.sent[rt]; !alreadySent {
   185			// No, so send it.
   186			sent := enc.sendType(w, state, rt)
   187			if enc.err != nil {
   188				return
   189			}
   190			// If the type info has still not been transmitted, it means we have
   191			// a singleton basic type (int, []byte etc.) at top level.  We don't
   192			// need to send the type info but we do need to update enc.sent.
   193			if !sent {
   194				typeLock.Lock()
   195				info, err := getTypeInfo(ut)
   196				typeLock.Unlock()
   197				if err != nil {
   198					enc.setError(err)
   199					return
   200				}
   201				enc.sent[rt] = info.id
   202			}
   203		}
   204	}
   205	
   206	// sendTypeId sends the id, which must have already been defined.
   207	func (enc *Encoder) sendTypeId(state *encoderState, ut *userTypeInfo) {
   208		// Identify the type of this top-level value.
   209		state.encodeInt(int64(enc.sent[ut.base]))
   210	}
   211	
   212	// EncodeValue transmits the data item represented by the reflection value,
   213	// guaranteeing that all necessary type information has been transmitted first.
   214	func (enc *Encoder) EncodeValue(value reflect.Value) error {
   215		// Gobs contain values. They cannot represent nil pointers, which
   216		// have no value to encode.
   217		if value.Kind() == reflect.Ptr && value.IsNil() {
   218			panic("gob: cannot encode nil pointer of type " + value.Type().String())
   219		}
   220	
   221		// Make sure we're single-threaded through here, so multiple
   222		// goroutines can share an encoder.
   223		enc.mutex.Lock()
   224		defer enc.mutex.Unlock()
   225	
   226		// Remove any nested writers remaining due to previous errors.
   227		enc.w = enc.w[0:1]
   228	
   229		ut, err := validUserType(value.Type())
   230		if err != nil {
   231			return err
   232		}
   233	
   234		enc.err = nil
   235		enc.byteBuf.Reset()
   236		enc.byteBuf.Write(spaceForLength)
   237		state := enc.newEncoderState(&enc.byteBuf)
   238	
   239		enc.sendTypeDescriptor(enc.writer(), state, ut)
   240		enc.sendTypeId(state, ut)
   241		if enc.err != nil {
   242			return enc.err
   243		}
   244	
   245		// Encode the object.
   246		enc.encode(state.b, value, ut)
   247		if enc.err == nil {
   248			enc.writeMessage(enc.writer(), state.b)
   249		}
   250	
   251		enc.freeEncoderState(state)
   252		return enc.err
   253	}
   254	

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