go-zero 的网关往 rpc 服务传递数据时，可以使用 headers，但需要注意前缀规则，否则会发现数据传递不过去，或者对方取不到数据。

go-zero 的网关对服务的调用使用了第三方库 grpcurl，入口函数为 InvokeRPC：

grpcurl.InvokeRPC(r.Context(), source, cli.Conn(), rpcPath, s.prepareMetadata(r.Header), handler, parser.Next)

调用在 https://github.com/zeromicro/go-zero/blob/master/gateway/server.go 中进行的，上述调用会处理 HTTP 的 headers 数据，对于不是以字符串“Grpc-Metadata-”打头的会过滤掉，对于以字符串“Grpc-Metadata-”打头的会将“Grpc-Metadata-”转为“gateway-”。

// go-zero/gateway/internal/headerprocessor.go
// ProcessHeaders builds the headers for the gateway from HTTP headers.
func ProcessHeaders(header http.Header) []string {var headers []stringfor k, v := range header {if !strings.HasPrefix(k, metadataHeaderPrefix) { // 判断是否包含了前缀“Grpc-Metadata-”continue // 如果没有前缀“Grpc-Metadata-”则直接过滤丢弃掉}// 将前缀“Grpc-Metadata-”替换为前缀“gateway-”key := fmt.Sprintf("%s%s", metadataPrefix, strings.TrimPrefix(k, metadataHeaderPrefix))for _, vv := range v {headers = append(headers, key+":"+vv)}}return headers
}

函数 MetadataFromHeaders 负责从 headers 解码数据：

// https://github.com/fullstorydev/grpcurl/blob/master/grpcurl.go
func MetadataFromHeaders(headers []string) metadata.MD {md := make(metadata.MD)for _, part := range headers {if part != "" {pieces := strings.SplitN(part, ":", 2)if len(pieces) == 1 {pieces = append(pieces, "") // if no value was specified, just make it "" (maybe the header value doesn't matter)}headerName := strings.ToLower(strings.TrimSpace(pieces[0]))val := strings.TrimSpace(pieces[1])if strings.HasSuffix(headerName, "-bin") {if v, err := decode(val); err == nil {val = v}}md[headerName] = append(md[headerName], val)}}return md
}

// https://github.com/fullstorydev/grpcurl/blob/master/invoke.go
func InvokeRPC(ctx context.Context, source DescriptorSource, ch grpcdynamic.Channel, methodName string,headers []string, handler InvocationEventHandler, requestData RequestSupplier) error {md := MetadataFromHeaders(headers)svc, mth := parseSymbol(methodName)if svc == "" || mth == "" {return fmt.Errorf("given method name %q is not in expected format: 'service/method' or 'service.method'", methodName)}dsc, err := source.FindSymbol(svc)if err != nil {// return a gRPC status error if hasStatus is trueerrStatus, hasStatus := status.FromError(err)switch {case hasStatus && isNotFoundError(err):return status.Errorf(errStatus.Code(), "target server does not expose service %q: %s", svc, errStatus.Message())case hasStatus:return status.Errorf(errStatus.Code(), "failed to query for service descriptor %q: %s", svc, errStatus.Message())case isNotFoundError(err):return fmt.Errorf("target server does not expose service %q", svc)}return fmt.Errorf("failed to query for service descriptor %q: %v", svc, err)}sd, ok := dsc.(*desc.ServiceDescriptor)if !ok {return fmt.Errorf("target server does not expose service %q", svc)}mtd := sd.FindMethodByName(mth)if mtd == nil {return fmt.Errorf("service %q does not include a method named %q", svc, mth)}handler.OnResolveMethod(mtd)// we also download any applicable extensions so we can provide full support for parsing user-provided datavar ext dynamic.ExtensionRegistryalreadyFetched := map[string]bool{}if err = fetchAllExtensions(source, &ext, mtd.GetInputType(), alreadyFetched); err != nil {return fmt.Errorf("error resolving server extensions for message %s: %v", mtd.GetInputType().GetFullyQualifiedName(), err)}if err = fetchAllExtensions(source, &ext, mtd.GetOutputType(), alreadyFetched); err != nil {return fmt.Errorf("error resolving server extensions for message %s: %v", mtd.GetOutputType().GetFullyQualifiedName(), err)}msgFactory := dynamic.NewMessageFactoryWithExtensionRegistry(&ext)req := msgFactory.NewMessage(mtd.GetInputType())handler.OnSendHeaders(md)ctx = metadata.NewOutgoingContext(ctx, md)stub := grpcdynamic.NewStubWithMessageFactory(ch, msgFactory)ctx, cancel := context.WithCancel(ctx)defer cancel()if mtd.IsClientStreaming() && mtd.IsServerStreaming() {return invokeBidi(ctx, stub, mtd, handler, requestData, req)} else if mtd.IsClientStreaming() {return invokeClientStream(ctx, stub, mtd, handler, requestData, req)} else if mtd.IsServerStreaming() {return invokeServerStream(ctx, stub, mtd, handler, requestData, req)} else {return invokeUnary(ctx, stub, mtd, handler, requestData, req)}
}

网关可如下实现：

newReq := r.WithContext(r.Context())
newReq.Header.Set("Grpc-Metadata-myuid", userId)
next.ServeHTTP(w, newReq)

服务端的实现：

vals := metadata.ValueFromIncomingContext(l.ctx, "gateway-myuid")
userId := vals[0]