我要评论使用 ctx.value 从 context 读取数据
// valuefromincomingcontext returns the metadata value corresponding to the metadata
// key from the incoming metadata if it exists. key must be lower-case.
//
// # experimental
//
// notice: this api is experimental and may be changed or removed in a
// later release.
func valuefromincomingcontext(ctx context.context, key string) []string {
md, ok := ctx.value(mdincomingkey{}).(md)
if !ok {
return nil
}
if v, ok := md[key]; ok {
return copyof(v)
}
for k, v := range md {
// we need to manually convert all keys to lower case, because md is a
// map, and there's no guarantee that the md attached to the context is
// created using our helper functions.
if strings.tolower(k) == key {
return copyof(v)
}
}
return nil
}使用 ctx.value 往 context 写入数据
// appendtooutgoingcontext returns a new context with the provided kv merged
// with any existing metadata in the context. please refer to the documentation
// of pairs for a description of kv.
func appendtooutgoingcontext(ctx context.context, kv ...string) context.context {
if len(kv)%2 == 1 {
panic(fmt.sprintf("metadata: appendtooutgoingcontext got an odd number of input pairs for metadata: %d", len(kv)))
}
md, _ := ctx.value(mdoutgoingkey{}).(rawmd)
added := make([][]string, len(md.added)+1)
copy(added, md.added)
kvcopy := make([]string, 0, len(kv))
for i := 0; i < len(kv); i += 2 {
kvcopy = append(kvcopy, strings.tolower(kv[i]), kv[i+1])
}
added[len(added)-1] = kvcopy
return context.withvalue(ctx, mdoutgoingkey{}, rawmd{md: md.md, added: added})
}metadata 是 grpc 内置的，用来往 rpc 服务传递 http 头数据，分 in 和 out 两种，对应的 key 都为一个空 struct，分别为：mdincomingkey 和 mdoutgoingkey 。
服务端的 ctx 和 md 直接打印出来，如下样子：
fmt.println(ctx) fmt.println(md) context.background.withvalue(type transport.connectionkey, val <not stringer>).withvalue(type peer.peerkey, val <not stringer>).withdeadline(2024-02-19 10:02:43.212614653 +0800 cst m=+41018.106555206 [1.999790196s]).withvalue(type metadata.mdincomingkey, val <not stringer>).withvalue(type grpc.streamkey, val <not stringer>).withvalue(type baggage.baggagecontextkeytype, val <not stringer>).withvalue(type trace.tracecontextkeytype, val <not stringer>).withvalue(type trace.tracecontextkeytype, val <not stringer>).withcancel map[:authority:[add.rpc] append:[append-value] content-type:[application/grpc] extra:[extra-value] grpc-accept-encoding:[gzip] noncestr:[abc] signature:[0123456789] timestamp:[2021-07-01 00:00:00] traceparent:[00-89415f99d44e6f8f6e14e3fe8f13ad20-bf33b29c4362ca6a-00] user-agent:[grpc-go/1.59.0]] signature: [0123456789]
注意 md 中的值会被加上中括号“[]”。
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