Make server decide the value of request-id.

Also, use header rather than trailer.

internal/controlplane/server.go

@@ -250,8 +250,10 @@ func (s *Server) StartGRPCServer(ctx context.Context) error {
 	interceptors := []grpc.UnaryServerInterceptor{
 		// TODO: this has no test coverage!
 		util.SanitizingInterceptor(),
+		// This adds `Grpc-Metadata-Request-Id` to the
+		// response.
+		logger.RequestIDInterceptor("request-id"),
 		logger.Interceptor(s.cfg.LoggingConfig),
-		logger.RequestIDInterceptor(),
 		TokenValidationInterceptor,
 		EntityContextProjectInterceptor,
 		ProjectAuthorizationInterceptor,

internal/logger/logging_interceptor.go

@@ -105,42 +105,20 @@ func Interceptor(cfg config.LoggingConfig) grpc.UnaryServerInterceptor {
 
 // RequestIDInterceptor traces request ids.
 //
-// It tries to use the request id from the request context, creating a
-// new one if that is missing. It also sends back in the trailer the
-// request id, ensuring that the client receives it.
-func RequestIDInterceptor() grpc.UnaryServerInterceptor {
+// It's job is to add a request id (UUID) to the context so that all
+// subsequent logs inherit it, making it easier to track down problems
+// on a per-request basis. It also sends back it back in a header.
+func RequestIDInterceptor(headerSuffix string) grpc.UnaryServerInterceptor {
 	return func(ctx context.Context, req interface{}, _ *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
-		rID := maybeGetRequestID(ctx)
+		rID := uuid.New().String()
 		ctx = zerolog.Ctx(ctx).With().Str("request_id", rID).Logger().WithContext(ctx)
 
 		resp, err := handler(ctx, req)
 
-		if err := grpc.SetTrailer(ctx, metadata.Pairs("request-id", rID)); err != nil {
+		if err := grpc.SendHeader(ctx, metadata.Pairs(headerSuffix, rID)); err != nil {
 			zerolog.Ctx(ctx).Trace().Err(err).Msg("unable to attach request id to trailer")
 		}
 
 		return resp, err
 	}
 }
-
-func maybeGetRequestID(ctx context.Context) string {
-	var rID string
-	if md, ok := metadata.FromIncomingContext(ctx); ok {
-		if rIDs, ok := md["request-id"]; ok {
-			if len(rIDs) != 0 {
-				rID = rIDs[0]
-			}
-		}
-	}
-
-	if rID == "" {
-		return uuid.New().String()
-	}
-
-	if _, err := uuid.Parse(rID); err != nil {
-		zerolog.Ctx(ctx).Trace().Err(err).Msg("request id is not valid uuid")
-		return uuid.New().String()
-	}
-
-	return rID
-}