// google.golang.org/protobuf@v1.25.0/encoding/protowire/wire.go
// AppendVarint appends v to b as a varint-encoded uint64.
func AppendVarint(b []byte, v uint64) []byte {
switch {
case v < 1<<7:
b = append(b, byte(v))
case v < 1<<14:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte(v>>7))
case v < 1<<21:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte(v>>14))
case v < 1<<28:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte(v>>21))
case v < 1<<35:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte(v>>28))
case v < 1<<42:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte((v>>28)&0x7f|0x80),
byte(v>>35))
case v < 1<<49:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte((v>>28)&0x7f|0x80),
byte((v>>35)&0x7f|0x80),
byte(v>>42))
case v < 1<<56:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte((v>>28)&0x7f|0x80),
byte((v>>35)&0x7f|0x80),
byte((v>>42)&0x7f|0x80),
byte(v>>49))
case v < 1<<63:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte((v>>28)&0x7f|0x80),
byte((v>>35)&0x7f|0x80),
byte((v>>42)&0x7f|0x80),
byte((v>>49)&0x7f|0x80),
byte(v>>56))
default:
b = append(b,
byte((v>>0)&0x7f|0x80),
byte((v>>7)&0x7f|0x80),
byte((v>>14)&0x7f|0x80),
byte((v>>21)&0x7f|0x80),
byte((v>>28)&0x7f|0x80),
byte((v>>35)&0x7f|0x80),
byte((v>>42)&0x7f|0x80),
byte((v>>49)&0x7f|0x80),
byte((v>>56)&0x7f|0x80),
1)
}
return b
}
// google.golang.org/protobuf@v1.25.0/encoding/protowire/wire.go
// EncodeTag encodes the field Number and wire Type into its unified form.
func EncodeTag(num Number, typ Type) uint64 {
return uint64(num)<<3 | uint64(typ&7)
}
源码中生成的 tag 是 uint64，代表着 field number 可以使用 61 个 bit 吗？
并非如此！
事实上：tag 的长度不能超过 32 bits，意味着 field number 的最大取值为 2^29-1 (536870911)。
而且在这个范围内，有一些数是不能被使用的：
1）0 ：protobuf 规定 field number 必须为正整数；
2）19000 到 19999： protobuf 仅供内部使用的保留位。
理解了生成 tag 的规则之后，不难得出以下结论：
1）field number 不必从 1 开始，可以从合法范围内的任意数字开始；
2）不同字段间的 field number 不必连续，只要合法且不同即可。
但是实际上：大多数人分配 field number 还是会从 1 开始，因为 tag 最终要经过 Base 128 Varints 编码，较小的 field number 有助于压缩空间，field number 为 1 到 15 的 tag 最终仅需占用一个字节。
假设接收方的 proto3 中定义了某个字段（假设 field number=1），当接收方从字节流中读取到多个 field number=1 的字段时，会执行 merge 操作。
merge 的规则如下：
1）如果字段为不可分割的类型，则直接覆盖；
2）如果字段为 repeated，则 append 到已有字段；
3）如果字段为嵌套消息，则递归执行 merge；
如果字段的 field number 相同但是结构不同，则出现 error。
以下为 Go 版本 Protobuf 中 merge 的部分：
// google.golang.org/protobuf@v1.25.0/proto/merge.go
// Merge merges src into dst, which must be a message with the same descriptor.
//
// Populated scalar fields in src are copied to dst, while populated
// singular messages in src are merged into dst by recursively calling Merge.
// The elements of every list field in src is appended to the corresponded
// list fields in dst. The entries of every map field in src is copied into
// the corresponding map field in dst, possibly replacing existing entries.
// The unknown fields of src are appended to the unknown fields of dst.
//
// It is semantically equivalent to unmarshaling the encoded form of src
// into dst with the UnmarshalOptions.Merge option specified.
func Merge(dst, src Message) {
// TODO: Should nil src be treated as semantically equivalent to a
// untyped, read-only, empty message? What about a nil dst?
dstMsg, srcMsg := dst.ProtoReflect(), src.ProtoReflect()
if dstMsg.Descriptor() != srcMsg.Descriptor() {
if got, want := dstMsg.Descriptor().FullName(), srcMsg.Descriptor().FullName(); got != want {
panic(fmt.Sprintf("descriptor mismatch: %v != %v", got, want))
}
panic("descriptor mismatch")
}
mergeOptions{}.mergeMessage(dstMsg, srcMsg)
}
func (o mergeOptions) mergeMessage(dst, src protoreflect.Message) {
methods := protoMethods(dst)
if methods != nil && methods.Merge != nil {
in := protoiface.MergeInput{
Destination: dst,
Source: src,
}
out := methods.Merge(in)
if out.Flags&protoiface.MergeComplete != 0 {
return
}
}
if !dst.IsValid() {
panic(fmt.Sprintf("cannot merge into invalid %v message", dst.Descriptor().FullName()))
}
src.Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) bool {
switch {
case fd.IsList():
o.mergeList(dst.Mutable(fd).List(), v.List(), fd)
case fd.IsMap():
o.mergeMap(dst.Mutable(fd).Map(), v.Map(), fd.MapValue())
case fd.Message() != nil:
o.mergeMessage(dst.Mutable(fd).Message(), v.Message())
case fd.Kind() == protoreflect.BytesKind:
dst.Set(fd, o.cloneBytes(v))
default:
dst.Set(fd, v)
}
return true
})
if len(src.GetUnknown()) > 0 {
dst.SetUnknown(append(dst.GetUnknown(), src.GetUnknown()...))
}
}
默认
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