control: send current timestamp header with event streams

Status and ListenHistory API return events with one-sided timestamp
for events that have not been completed yet. Because client and
server may not have exactly the same time the client can't really
be sure what these values mean without knowing the current server
time as well.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

control/control.go

@@ -44,6 +44,7 @@ import (
 	"golang.org/x/sync/errgroup"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/status"
 )
 
@@ -248,6 +249,9 @@ func (c *Controller) Export(ctx context.Context, req *tracev1.ExportTraceService
 }
 
 func (c *Controller) ListenBuildHistory(req *controlapi.BuildHistoryRequest, srv controlapi.Control_ListenBuildHistoryServer) error {
+	if err := sendTimestampHeader(srv); err != nil {
+		return err
+	}
 	return c.history.Listen(srv.Context(), req, func(h *controlapi.BuildHistoryEvent) error {
 		if err := srv.Send(h); err != nil {
 			return err
@@ -438,6 +442,9 @@ func (c *Controller) Solve(ctx context.Context, req *controlapi.SolveRequest) (*
 }
 
 func (c *Controller) Status(req *controlapi.StatusRequest, stream controlapi.Control_StatusServer) error {
+	if err := sendTimestampHeader(stream); err != nil {
+		return err
+	}
 	ch := make(chan *client.SolveStatus, 8)
 
 	eg, ctx := errgroup.WithContext(stream.Context())
@@ -647,3 +654,9 @@ func (cs *roContentStore) Update(ctx context.Context, info content.Info, fieldpa
 func (cs *roContentStore) Abort(ctx context.Context, ref string) error {
 	return errors.Errorf("read-only content store")
 }
+
+const timestampKey = "buildkit-current-timestamp"
+
+func sendTimestampHeader(srv grpc.ServerStream) error {
+	return srv.SendHeader(metadata.Pairs(timestampKey, time.Now().Format(time.RFC3339Nano)))
+}