Go是如何实现protobuf的编解码的: 源码

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大家周末好,我是大彬。这是一篇姊妹篇文章,浅析一下Go是如何实现protobuf编解码的,本编是第二篇:

1.2.Go是如何实现protobuf的编解码的(2): 源码

前言

上一篇文章 中已经指出了Go语言数据和Protobuf数据的编解码是由包github.com/golang/protobuf/proto完成的,本编就来分析一下proto包是如何实现编解码的。

Go是如何实现protobuf的编解码的(2): 源码

编解码原理

编解码包都有支持的编解码类型,我们暂且把这些类型称为底层类型,编解码的本质是:

1.为每一个底层类型配备一个或多个编解码函数2.把一个结构体的字段,递归的拆解成底层类型,然后选择合适的函数进行编码或解码操作

接下来先看编码,再看解码。

编码

约定:以下所有的代码片,如果是request.pb.go或main.go中的代码,会在第一行标记文件名,否则都是proto包的源码。

// main.gopackage main
import ( "fmt"
"./types" "github.com/golang/protobuf/proto")
func main() { req := &types.Request{Data: "Hello Dabin"}
// Marshal encoded, err := proto.Marshal(req) if err != nil { fmt.Printf("Encode to protobuf data error: %v", err) } ...}

编码调用的是proto.Marshal函数,它可以完成的是Go语言数据序列化成protobuf数据,返回序列化结果或错误。

proto编译成的Go结构体都是符合Message接口的,从Marshal可知Go结构体有3种序列化方式:

1.pb Message满足newMarshaler接口,则调用XXX_Marshal()进行序列化。2.pb满足Marshaler接口,则调用Marshal()进行序列化,这种方式适合某类型自定义序列化规则的情况。3.否则,使用默认的序列化方式,创建一个Warpper,利用wrapper对pb进行序列化,后面会介绍方式1实际就是使用方式3。

// Marshal takes a protocol buffer message// and encodes it into the wire format, returning the data.// This is the main entry point.func Marshal(pb Message) ([]byte, error) { if m, ok := pb.(newMarshaler); ok { siz := m.XXX_Size() b := make([]byte, 0, siz) return m.XXX_Marshal(b, false) } if m, ok := pb.(Marshaler); ok { // If the message can marshal itself, let it do it, for compatibility. // NOTE: This is not efficient. return m.Marshal() } // in case somehow we didn't generate the wrapper if pb == nil { return nil, ErrNil } var info InternalMessageInfo siz := info.Size(pb) b := make([]byte, 0, siz) return info.Marshal(b, pb, false)}

newMarshalerMarshaler如下:

// newMarshaler is the interface representing objects that can marshal themselves.//// This exists to support protoc-gen-go generated messages.// The proto package will stop type-asserting to this interface in the future.//// DO NOT DEPEND ON THIS.type newMarshaler interface { XXX_Size() int XXX_Marshal(b []byte, deterministic bool) ([]byte, error)}
// Marshaler is the interface representing objects that can marshal themselves.type Marshaler interface { Marshal() ([]byte, error)}

Request实现了newMarshaler接口,XXX_Marshal实现如下,它实际是调用了xxx_messageInfo_Request.Marshalxxx_messageInfo_Request是定义在request.pb.go中的一个全局变量,类型就是InternalMessageInfo,实际就是前文提到的wrapper。

// request.pb.gofunc (m *Request) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) { print("Called xxx marshal
") panic("I want see stack trace") return xxx_messageInfo_Request.Marshal(b, m, deterministic)}
var xxx_messageInfo_Request proto.InternalMessageInfo

本质上,XXX_Marshal也是wrapper,后面才是真正序列化的主体函数在proto包中。

InternalMessageInfo主要是用来缓存序列化和反序列化需要用到的信息。

// InternalMessageInfo is a type used internally by generated .pb.go files.// This type is not intended to be used by non-generated code.// This type is not subject to any compatibility guarantee.type InternalMessageInfo struct { marshal *marshalInfo // marshal信息 unmarshal *unmarshalInfo // unmarshal信息 merge *mergeInfo discard *discardInfo}

InternalMessageInfo.Marshal首先是获取待序列化类型的序列化信息u marshalInfo,然后利用u.marshal进行序列化。

// Marshal is the entry point from generated code,// and should be ONLY called by generated code.// It marshals msg to the end of b.// a is a pointer to a place to store cached marshal info.func (a *InternalMessageInfo) Marshal(b []byte, msg Message, deterministic bool) ([]byte, error) { // 获取该message类型的MarshalInfo,这些信息都缓存起来了 // 大量并发时无需重复创建 u := getMessageMarshalInfo(msg, a) // 入参校验 ptr := toPointer(&msg) if ptr.isNil() { // We get here if msg is a typed nil ((*SomeMessage)(nil)), // so it satisfies the interface, and msg == nil wouldn't // catch it. We don't want crash in this case. return b, ErrNil } // 根据MarshalInfo对数据进行marshal return u.marshal(b, ptr, deterministic)}

由于每种类型的序列化信息是一致的,所以getMessageMarshalInfo对序列化信息进行了缓存,缓存在a.marshal中,如果a中不存在marshal信息,则去生成,但不进行初始化,然后保存到a中。

func getMessageMarshalInfo(msg interface{}, a *InternalMessageInfo) *marshalInfo { // u := a.marshal, but atomically. // We use an atomic here to ensure memory consistency. // 从InternalMessageInfo中读取 u := atomicLoadMarshalInfo(&a.marshal) // 读取不到代表未保存过 if u == nil { // Get marshal information from type of message. t := reflect.ValueOf(msg).Type() if t.Kind() != reflect.Ptr { panic(fmt.Sprintf("cannot handle non-pointer message type %v", t)) } u = getMarshalInfo(t.Elem()) // Store it in the cache for later users. // a.marshal = u, but atomically. atomicStoreMarshalInfo(&a.marshal, u) } return u}

getMarshalInfo只是创建了一个marshalInfo对象,填充了字段typ,剩余的字段未填充。

// getMarshalInfo returns the information to marshal a given type of message.// The info it returns may not necessarily initialized.// t is the type of the message (NOT the pointer to it).// 获取MarshalInfo结构体,如果不存在则使用message类型t创建1个func getMarshalInfo(t reflect.Type) *marshalInfo { marshalInfoLock.Lock() u, ok := marshalInfoMap[t] if !ok { u = &marshalInfo{typ: t} marshalInfoMap[t] = u } marshalInfoLock.Unlock() return u}
// marshalInfo is the information used for marshaling a message.type marshalInfo struct { typ reflect.Type fields []*marshalFieldInfo unrecognized field // offset of XXX_unrecognized extensions field // offset of XXX_InternalExtensions v1extensions field // offset of XXX_extensions sizecache field // offset of XXX_sizecache initialized int32 // 0 -- only typ is set, 1 -- fully initialized messageset bool // uses message set wire format hasmarshaler bool // has custom marshaler sync.RWMutex // protect extElems map, also for initialization extElems map[int32]*marshalElemInfo // info of extension elements}

marshalInfo.marshal是Marshal真实主体,会判断u是否已经初始化,如果未初始化调用computeMarshalInfo计算Marshal需要的信息,实际就是填充marshalInfo中的各种字段。

u.hasmarshaler代表当前类型是否实现了Marshaler接口,直接调用Marshal函数进行序列化。可以确定Marshal函数的序列化方式2,即实现Marshaler接口的方法,最后肯定也会调用marshalInfo.marshal

该函数的主体是一个for循环,依次遍历该类型的每一个字段,对required属性进行校验,然后按字段类型,调用f.marshaler对该字段类型进行序列化。这个f.marshaler哪来的呢?

// marshal is the main function to marshal a message. It takes a byte slice and appends// the encoded data to the end of the slice, returns the slice and error (if any).// ptr is the pointer to the message.// If deterministic is true, map is marshaled in deterministic order.// 该函数是Marshal的主体函数,把消息编码为数据后,追加到b之后,最后返回b。// deterministic为true代表map会以确定的顺序进行编码。func (u *marshalInfo) marshal(b []byte, ptr pointer, deterministic bool) ([]byte, error) { // 初始化marshalInfo的基础信息 // 主要是根据已有信息填充该结构体的一些字段 if atomic.LoadInt32(&u.initialized) == 0 { u.computeMarshalInfo() }
// If the message can marshal itself, let it do it, for compatibility. // NOTE: This is not efficient. // 如果该类型实现了Marshaler接口,即能够对自己Marshal,则自行Marshal // 结果追加到b if u.hasmarshaler { m := ptr.asPointerTo(u.typ).Interface().(Marshaler) b1, err := m.Marshal() b = append(b, b1...) return b, err }
var err, errLater error // The old marshaler encodes extensions at beginning. // 检查扩展字段,把message的扩展字段追加到b if u.extensions.IsValid() { // offset函数用来根据指针偏移量获取message的指定字段 e := ptr.offset(u.extensions).toExtensions() if u.messageset { b, err = u.appendMessageSet(b, e, deterministic) } else { b, err = u.appendExtensions(b, e, deterministic) } if err != nil { return b, err } } if u.v1extensions.IsValid() { m := *ptr.offset(u.v1extensions).toOldExtensions() b, err = u.appendV1Extensions(b, m, deterministic) if err != nil { return b, err } }
// 遍历message的每一个字段,检查并做编码,然后追加到b for _, f := range u.fields { if f.required { // 如果required的字段未设置,则记录错误,所有的marshal工作完成后再处理 if ptr.offset(f.field).getPointer().isNil() { // Required field is not set. // We record the error but keep going, to give a complete marshaling. if errLater == nil { errLater = &RequiredNotSetError{f.name} } continue } } // 字段为指针类型,并且为nil,代表未设置,该字段无需编码 if f.isPointer && ptr.offset(f.field).getPointer().isNil() { // nil pointer always marshals to nothing continue } // 利用这个字段的marshaler进行编码 b, err = f.marshaler(b, ptr.offset(f.field), f.wiretag, deterministic) if err != nil { if err1, ok := err.(*RequiredNotSetError); ok { // required字段但未设置错误 // Required field in submessage is not set. // We record the error but keep going, to give a complete marshaling. if errLater == nil { errLater = &RequiredNotSetError{f.name + "." + err1.field} } continue } // “动态数组”中包含nil元素 if err == errRepeatedHasNil { err = errors.New("proto: repeated field " + f.name + " has nil element") } if err == errInvalidUTF8 { if errLater == nil { fullName := revProtoTypes[reflect.PtrTo(u.typ)] + "." + f.name errLater = &invalidUTF8Error{fullName} } continue } return b, err } } // 为识别的类型字段,直接转为bytes,追加到b // computeMarshalInfo中已经收集这些字段 if u.unrecognized.IsValid() { s := *ptr.offset(u.unrecognized).toBytes() b = append(b, s...) } return b, errLater}

computeMarshalInfo实际上就是对要序列化的类型,进行一次全面检查,设置好序列化要使用的数据,这其中就包含了各字段的序列化函数f.marshaler。我们就重点关注下这部分,struct的每一个字段都会分配一个marshalFieldInfo,代表这个字段序列化需要的信息,会调用computeMarshalFieldInfo会填充这个对象。

// computeMarshalInfo initializes the marshal info.func (u *marshalInfo) computeMarshalInfo() { // 加锁,代表了不能同时计算marshal信息 u.Lock() defer u.Unlock() // 计算1次即可 if u.initialized != 0 { // non-atomic read is ok as it is protected by the lock return }
// 获取要marshal的message类型 t := u.typ u.unrecognized = invalidField u.extensions = invalidField u.v1extensions = invalidField u.sizecache = invalidField
// If the message can marshal itself, let it do it, for compatibility. // 判断当前类型是否实现了Marshal接口,如果实现标记为类型自有marshaler // 没用类型断言是因为t是Type类型,不是保存在某个接口的变量 // NOTE: This is not efficient. if reflect.PtrTo(t).Implements(marshalerType) { u.hasmarshaler = true atomic.StoreInt32(&u.initialized, 1) // 可以直接返回了,后面使用自有的marshaler编码 return }
// get oneof implementers // 看*t实现了以下哪个接口,oneof特性 var oneofImplementers []interface{} switch m := reflect.Zero(reflect.PtrTo(t)).Interface().(type) { case oneofFuncsIface: _, _, _, oneofImplementers = m.XXX_OneofFuncs() case oneofWrappersIface: oneofImplementers = m.XXX_OneofWrappers() }
n := t.NumField()
// deal with XXX fields first // 遍历t的每一个XXX字段 for i := 0; i < t.NumField(); i++ { f := t.Field(i) // 跳过非XXX开头的字段 if !strings.HasPrefix(f.Name, "XXX_") { continue } // 处理以下几个protobuf自带的字段 switch f.Name { case "XXX_sizecache": u.sizecache = toField(&f) case "XXX_unrecognized": u.unrecognized = toField(&f) case "XXX_InternalExtensions": u.extensions = toField(&f) u.messageset = f.Tag.Get("protobuf_messageset") == "1" case "XXX_extensions": u.v1extensions = toField(&f) case "XXX_NoUnkeyedLiteral": // nothing to do default: panic("unknown XXX field: " + f.Name) } n-- }
// normal fields // 处理message的普通字段 fields := make([]marshalFieldInfo, n) // batch allocation u.fields = make([]*marshalFieldInfo, 0, n) for i, j := 0, 0; i < t.NumField(); i++ { f := t.Field(i)
// 跳过XXX字段 if strings.HasPrefix(f.Name, "XXX_") { continue }
// 取fields的下一个有效字段,指针类型 // j代表了fields有效字段数量,n是包含了XXX字段的总字段数量 field := &fields[j] j++ field.name = f.Name // 填充到u.fields u.fields = append(u.fields, field) // 字段的tag里包含“protobuf_oneof”特殊处理 if f.Tag.Get("protobuf_oneof") != "" { field.computeOneofFieldInfo(&f, oneofImplementers) continue } // 字段里不包含“protobuf”,代表不是protoc自动生成的字段 if f.Tag.Get("protobuf") == "" { // field has no tag (not in generated message), ignore it // 删除刚刚保存的字段信息 u.fields = u.fields[:len(u.fields)-1] j-- continue } // 填充字段的marshal信息 field.computeMarshalFieldInfo(&f) }
// fields are marshaled in tag order on the wire. // 字段排序 sort.Sort(byTag(u.fields))
// 初始化完成 atomic.StoreInt32(&u.initialized, 1)}

回顾一下Request的定义,它包含1个字段Data,后面protobuf:...描述了protobuf要使用的信息,"bytes,..."这段被称为tags,用逗号进行分割后,其中:

tags[0]: bytes,代表Data类型的数据要被转换为bytestags[1]: 1,代表了字段的IDtags[2]: opt,代表可行,非必须tags[3]: name=data,proto文件中的名称tags[4]: proto3,代表使用的protobuf版本

// request.pb.gotype Request struct{ Data string `protobuf:"bytes,1,opt,name=data,proto3" json:"data,omitempty"` ...}

computeMarshalFieldInfo首先要获取字段ID和要转换的类型,填充到marshalFieldInfo,然后调用setMarshaler利用字段f和tags获取该字段类型的序列化函数。

// computeMarshalFieldInfo fills up the information to marshal a field.func (fi *marshalFieldInfo) computeMarshalFieldInfo(f *reflect.StructField) { // parse protobuf tag of the field. // tag has format of "bytes,49,opt,name=foo,def=hello!" // 获取"protobuf"的完整tag,然后使用,分割,得到上面的格式 tags := strings.Split(f.Tag.Get("protobuf"), ",") if tags[0] == "" { return } // tag的编号,即message中设置的string name = x,则x就是这个字段的tag id tag, err := strconv.Atoi(tags[1]) if err != nil { panic("tag is not an integer") } // 要转换成的类型,bytes,varint等等 wt := wiretype(tags[0]) // 设置字段是required还是opt if tags[2] == "req" { fi.required = true } // 设置field和tag信息到marshalFieldInfo fi.setTag(f, tag, wt) // 根据当前的tag信息(类型等),选择marshaler函数 fi.setMarshaler(f, tags)}

setMarshaler的重点是typeMarshalertypeMarshaler这个函数非常长,其实就是根据类型设置返回对于的序列化函数,比如Bool、Int32、Uint32...,如果是结构体、切片等复合类型,就可以形成递归了。

// setMarshaler fills up the sizer and marshaler in the info of a field.func (fi *marshalFieldInfo) setMarshaler(f *reflect.StructField, tags []string) { // map类型字段特殊处理 switch f.Type.Kind() { case reflect.Map: // map field fi.isPointer = true fi.sizer, fi.marshaler = makeMapMarshaler(f) return case reflect.Ptr, reflect.Slice: // 指针字段和切片字段标记指针类型 fi.isPointer = true }
// 根据字段类型和tag选择marshaler fi.sizer, fi.marshaler = typeMarshaler(f.Type, tags, true, false)}
// typeMarshaler returns the sizer and marshaler of a given field.// t is the type of the field.// tags is the generated "protobuf" tag of the field.// If nozero is true, zero value is not marshaled to the wire.// If oneof is true, it is a oneof field.// 函数非常长,省略内容func typeMarshaler(t reflect.Type, tags []string, nozero, oneof bool) (sizer, marshaler) { ... switch t.Kind() { case reflect.Bool: if pointer { return sizeBoolPtr, appendBoolPtr } if slice { if packed { return sizeBoolPackedSlice, appendBoolPackedSlice } return sizeBoolSlice, appendBoolSlice } if nozero { return sizeBoolValueNoZero, appendBoolValueNoZero } return sizeBoolValue, appendBoolValue case reflect.Uint32: ... case reflect.Int32: .... case reflect.Struct: ...}

以下是Bool和String类型的2个序列化函数示例:

func appendBoolValue(b []byte, ptr pointer, wiretag uint64, _ bool) ([]byte, error) { v := *ptr.toBool() b = appendVarint(b, wiretag) if v { b = append(b, 1) } else { b = append(b, 0) } return b, nil}
func appendStringValue(b []byte, ptr pointer, wiretag uint64, _ bool) ([]byte, error) { v := *ptr.toString() b = appendVarint(b, wiretag) b = appendVarint(b, uint64(len(v))) b = append(b, v...) return b, nil}

所以序列化后的[]byte,应当是符合这种模式:

| wiretag | data | wiretag | data | ... | data |

OK,以上就是编码的主要流程,简单回顾一下:

1.proto.Marshal会调用*.pb.go中自动生成的Wrapper函数,Wrapper函数会调用InternalMessageInfo进行序列化,然后才步入序列化的正题2.首先获取要序列化类型的marshal信息u,如果u没有初始化,则进行初始化,即设置好结构体每个字段的序列化函数,以及其他信息3.遍历结构体的每个字段,使用u中的信息为每个字段进行编码,并把加过追加到[]byte,所以字段编码完成,则返回序列化的结果[]byte或者错误。

解码

解码的流程其实与编码很类似,会是上面回顾的3大步骤,主要的区别在步骤2:它要获取的是序列化类型的unmarshal信息u,如果u没有初始化,会进行初始化,设置的是结构体每个字段的反序列化函数,以及其他信息。

所以解码的函数解析会简要的过一遍,不再有编码那么详细的解释。

下面是proto包中反序列化的接口和函数定义:

// Unmarshaler is the interface representing objects that can// unmarshal themselves. The argument points to data that may be// overwritten, so implementations should not keep references to the// buffer.// Unmarshal implementations should not clear the receiver.// Any unmarshaled data should be merged into the receiver.// Callers of Unmarshal that do not want to retain existing data// should Reset the receiver before calling Unmarshal.type Unmarshaler interface { Unmarshal([]byte) error}
// newUnmarshaler is the interface representing objects that can// unmarshal themselves. The semantics are identical to Unmarshaler.//// This exists to support protoc-gen-go generated messages.// The proto package will stop type-asserting to this interface in the future.//// DO NOT DEPEND ON THIS.type newUnmarshaler interface { // 实现了XXX_Unmarshal XXX_Unmarshal([]byte) error}
// Unmarshal parses the protocol buffer representation in buf and places the// decoded result in pb. If the struct underlying pb does not match// the data in buf, the results can be unpredictable.//// Unmarshal resets pb before starting to unmarshal, so any// existing data in pb is always removed. Use UnmarshalMerge// to preserve and append to existing data.func Unmarshal(buf []byte, pb Message) error { pb.Reset() // pb自己有unmarshal函数,实现了newUnmarshaler接口 if u, ok := pb.(newUnmarshaler); ok { return u.XXX_Unmarshal(buf) } // pb自己有unmarshal函数,实现了Unmarshaler接口 if u, ok := pb.(Unmarshaler); ok { return u.Unmarshal(buf) } // 使用默认的Unmarshal return NewBuffer(buf).Unmarshal(pb)}

Request实现了Unmarshaler接口:

// request.pb.gofunc (m *Request) XXX_Unmarshal(b []byte) error { return xxx_messageInfo_Request.Unmarshal(m, b)}

反序列化也是使用InternalMessageInfo进行。

// Unmarshal is the entry point from the generated .pb.go files.// This function is not intended to be used by non-generated code.// This function is not subject to any compatibility guarantee.// msg contains a pointer to a protocol buffer struct.// b is the data to be unmarshaled into the protocol buffer.// a is a pointer to a place to store cached unmarshal information.func (a *InternalMessageInfo) Unmarshal(msg Message, b []byte) error { // Load the unmarshal information for this message type. // The atomic load ensures memory consistency. // 获取保存在a中的unmarshal信息 u := atomicLoadUnmarshalInfo(&a.unmarshal) if u == nil { // Slow path: find unmarshal info for msg, update a with it. u = getUnmarshalInfo(reflect.TypeOf(msg).Elem()) atomicStoreUnmarshalInfo(&a.unmarshal, u) } // Then do the unmarshaling. // 执行unmarshal err := u.unmarshal(toPointer(&msg), b) return err}

以下是反序列化的主题函数,u未初始化时会调用computeUnmarshalInfo设置反序列化需要的信息。

// unmarshal does the main work of unmarshaling a message.// u provides type information used to unmarshal the message.// m is a pointer to a protocol buffer message.// b is a byte stream to unmarshal into m.// This is top routine used when recursively unmarshaling submessages.func (u *unmarshalInfo) unmarshal(m pointer, b []byte) error { if atomic.LoadInt32(&u.initialized) == 0 { // 为u填充unmarshal信息,以及设置每个字段类型的unmarshaler函数 u.computeUnmarshalInfo() } if u.isMessageSet { return unmarshalMessageSet(b, m.offset(u.extensions).toExtensions()) } var reqMask uint64 // bitmask of required fields we've seen. var errLater error for len(b) > 0 { // Read tag and wire type. // Special case 1 and 2 byte varints. var x uint64 if b[0] < 128 { x = uint64(b[0]) b = b[1:] } else if len(b) >= 2 && b[1] < 128 { x = uint64(b[0]&0x7f) + uint64(b[1])<<7 b = b[2:] } else { var n int x, n = decodeVarint(b) if n == 0 { return io.ErrUnexpectedEOF } b = b[n:] } // 获取tag和wire标记 tag := x >> 3 wire := int(x) & 7
// Dispatch on the tag to one of the unmarshal* functions below. // 根据tag选择该类型的unmarshalFieldInfo:f var f unmarshalFieldInfo if tag < uint64(len(u.dense)) { f = u.dense[tag] } else { f = u.sparse[tag] } // 如果该类型有unmarshaler函数,则执行解码和错误处理 if fn := f.unmarshal; fn != nil { var err error // 从b解析,然后填充到f的对应字段 b, err = fn(b, m.offset(f.field), wire) if err == nil { reqMask |= f.reqMask continue } if r, ok := err.(*RequiredNotSetError); ok { // Remember this error, but keep parsing. We need to produce // a full parse even if a required field is missing. if errLater == nil { errLater = r } reqMask |= f.reqMask continue } if err != errInternalBadWireType { if err == errInvalidUTF8 { if errLater == nil { fullName := revProtoTypes[reflect.PtrTo(u.typ)] + "." + f.name errLater = &invalidUTF8Error{fullName} } continue } return err } // Fragments with bad wire type are treated as unknown fields. }
// Unknown tag. // 跳过未知tag,可能是proto中的message定义升级了,增加了一些字段,使用老版本的,就不识别新的字段 if !u.unrecognized.IsValid() { // Don't keep unrecognized data; just skip it. var err error b, err = skipField(b, wire) if err != nil { return err } continue } // 检查未识别字段是不是extension // Keep unrecognized data around. // maybe in extensions, maybe in the unrecognized field. z := m.offset(u.unrecognized).toBytes() var emap map[int32]Extension var e Extension for _, r := range u.extensionRanges { if uint64(r.Start) <= tag && tag <= uint64(r.End) { if u.extensions.IsValid() { mp := m.offset(u.extensions).toExtensions() emap = mp.extensionsWrite() e = emap[int32(tag)] z = &e.enc break } if u.oldExtensions.IsValid() { p := m.offset(u.oldExtensions).toOldExtensions() emap = *p if emap == nil { emap = map[int32]Extension{} *p = emap } e = emap[int32(tag)] z = &e.enc break } panic("no extensions field available") } }
// Use wire type to skip data. var err error b0 := b b, err = skipField(b, wire) if err != nil { return err } *z = encodeVarint(*z, tag<<3|uint64(wire)) *z = append(*z, b0[:len(b0)-len(b)]...)
if emap != nil { emap[int32(tag)] = e } } // 校验解析到的required字段的数量,如果与u中记录的不匹配,则报错 if reqMask != u.reqMask && errLater == nil { // A required field of this message is missing. for _, n := range u.reqFields { if reqMask&1 == 0 { errLater = &RequiredNotSetError{n} } reqMask >>= 1 } } return errLater}

设置字段反序列化函数的过程不看了,看一下怎么选函数的,typeUnmarshaler是为字段类型,选择反序列化函数,这些函数选择与序列化函数是一一对应的。

// typeUnmarshaler returns an unmarshaler for the given field type / field tag pair.func typeUnmarshaler(t reflect.Type, tags string) unmarshaler { ... // Figure out packaging (pointer, slice, or both) slice := false pointer := false if t.Kind() == reflect.Slice && t.Elem().Kind() != reflect.Uint8 { slice = true t = t.Elem() } if t.Kind() == reflect.Ptr { pointer = true t = t.Elem() } ... switch t.Kind() { case reflect.Bool: if pointer { return unmarshalBoolPtr } if slice { return unmarshalBoolSlice } return unmarshalBoolValue }}

unmarshalBoolValue是默认的Bool类型反序列化函数,会把protobuf数据b解码,然后转换为bool类型v,最后赋值给字段f。

func unmarshalBoolValue(b []byte, f pointer, w int) ([]byte, error) { if w != WireVarint { return b, errInternalBadWireType } // Note: any length varint is allowed, even though any sane // encoder will use one byte. // See https://github.com/golang/protobuf/issues/76 x, n := decodeVarint(b) if n == 0 { return nil, io.ErrUnexpectedEOF } // TODO: check if x>1? Tests seem to indicate no. // toBool是返回bool类型的指针 // 完成对字段f的赋值 v := x != 0 *f.toBool() = v return b[n:], nil}

总结

本文分析了Go语言protobuf数据的序列化和反序列过程,可以简要概括为:

1.proto.Marshalproto.Unmarshal会调用*.pb.go中自动生成的Wrapper函数,Wrapper函数会调用InternalMessageInfo进行(反)序列化,然后才步入(反)序列化的正题2.首先获取要目标类型的(um)marshal信息u,如果u没有初始化,则进行初始化,即设置好结构体每个字段的(反)序列化函数,以及其他信息3.遍历结构体的每个字段,使用u中的信息为每个字段进行编码,生成序列化的结果,或进行解码,给结构体成员进行赋值

参考文章

以下参考文章都值得阅读:

https://tech.meituan.com/2015/02/26/serialization-vs-deserialization.html 《序列化和反序列化》出自美团技术团队,值得一读。https://github.com/golang/protobuf Go支持protocol buffer的仓库,Readme,值得详读。https://developers.google.com/protocol-buffers/docs/gotutorial Google Protocol Buffers的Go语言tutorial,值得详细阅读和实操。https://developers.google.com/protocol-buffers/docs/overview Google Protocol Buffers的Overview,介绍了什么是Protocol Buffers,它的原理、历史(起源),以及和XML的对比,必读。https://developers.google.com/protocol-buffers/docs/proto3 《Language Guide (proto3)》这篇文章介绍了proto3的定义,也可以理解为.proto文件的语法,就如同Go语言的语法,不懂语法怎么编写.proto文件?读这篇文章会了解很多原理,以及可以少踩坑,必读。https://developers.google.com/protocol-buffers/docs/reference/go-generated 《Go Generated Code》这篇文章详细介绍了protoc是怎么用.protoc生成.pb.go的,可选。https://developers.google.com/protocol-buffers/docs/encoding# 《Protocol Buffers Encoding》这篇介绍编码原理,可选。https://godoc.org/github.com/golang/protobuf/proto 《package proto文档》可以把proto包当做Go语言操作protobuf数据的SDK,它实现了结构体和protobuf数据的转换,它和.pb.go文件配合使用。
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