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

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