filer.remote.sync: no upload on an unanswered supersession lookup; bounded offset read

retriedWriteFile returns the supersession lookup's error instead of
treating an unanswered lookup as "not superseded": in a replay an older
snapshot could otherwise overwrite a newer remote object. The processor
retries the event. isSuperseded keeps its boolean form for the other call
sites. GetSyncOffset's KvGet carries a 30 s deadline so a startup that falls
back to the saved offset cannot hang on it.

TestGatewayUncachedRewriteIsStillUploaded asserted the upload on a failed
lookup; it now asserts the withheld upload and the lookup error.

(cherry picked from commit 8fc2e718f7cd8322615b4e40007be903ade2dbfa)

weed/remote_storage/track_sync_offset.go

@@ -3,6 +3,7 @@ package remote_storage
 import (
 	"context"
 	"errors"
+	"time"
 
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
@@ -26,7 +27,10 @@ func SyncOffsetKey(dir string) []byte {
 func GetSyncOffset(grpcDialOption grpc.DialOption, filer pb.ServerAddress, dir string) (lastOffsetTsNs int64, readErr error) {
 
 	readErr = pb.WithFilerClient(false, 0, filer, grpcDialOption, func(client filer_pb.SeaweedFilerClient) error {
-		resp, err := client.KvGet(context.Background(), &filer_pb.KvGetRequest{Key: SyncOffsetKey(dir)})
+		// bounded: a read that never answers would hold the daemon's start
+		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+		defer cancel()
+		resp, err := client.KvGet(ctx, &filer_pb.KvGetRequest{Key: SyncOffsetKey(dir)})
 		if err != nil {
 			return err
 		}