filer.remote.sync: an unanswered lookup after a failed attempt ends the retry loop; an unread offset never resolves to now minus timeAgo

After a failed upload attempt the supersession lookup's error ended in
isSuperseded's boolean and the loop could upload the snapshot again
unverified. The lookup error now ends the loop (errLookupUnanswered, not
retried) and the processor retries the event. With -timeAgo set, a failed
read of the saved offset resolved to now minus timeAgo, skipping everything
older; the daemon exits instead so a restart retries.

(cherry picked from commit 55ca2a88e01d7b556d494a404b9cc263c27b2566)

weed/command/filer_remote_sync_dir_test.go

@@ -591,6 +591,52 @@ func TestFilerLookupsOnTheSyncPathCarryADeadline(t *testing.T) {
 	}
 }
 
+// answerThenBlockFilerClient answers the first lookup and blocks every later
+// one until its context ends.
+type answerThenBlockFilerClient struct {
+	stubFilerClient
+	entry *filer_pb.Entry
+}
+
+func (c *answerThenBlockFilerClient) LookupDirectoryEntry(ctx context.Context, _ *filer_pb.LookupDirectoryEntryRequest, _ ...grpc.CallOption) (*filer_pb.LookupDirectoryEntryResponse, error) {
+	c.lookups++
+	if c.lookups == 1 {
+		return &filer_pb.LookupDirectoryEntryResponse{Entry: c.entry}, nil
+	}
+	<-ctx.Done()
+	return nil, ctx.Err()
+}
+
+func (c *answerThenBlockFilerClient) WithFilerClient(_ bool, fn func(filer_pb.SeaweedFilerClient) error) error {
+	return fn(c)
+}
+
+func TestRetriedWriteFileStopsWhenTheLookupAfterAFailureIsUnanswered(t *testing.T) {
+	remoteSyncLookupTimeout = 50 * time.Millisecond
+	t.Cleanup(func() { remoteSyncLookupTimeout = 30 * time.Second })
+	const dir = "/buckets/ingest"
+	event := entryWith("lease.json", nil, chunk("3,01", "e1"))
+	dest := &remote_pb.RemoteStorageLocation{Name: "b2", Bucket: "tier", Path: "/" + event.Name}
+	transient := errors.New("RequestError: send request failed\ncaused by: Put \"https://s3.example.com/tier/x\": 503 Service Unavailable")
+	remote := &failingRemote{err: transient}
+	filerClient := &answerThenBlockFilerClient{entry: entryWith(event.Name, nil, chunk("3,01", "e1"))}
+
+	started := time.Now()
+	_, err := retriedWriteFile(remote, filerClient, dir, event, dest)
+	if !errors.Is(err, errLookupUnanswered) || !errors.Is(err, context.DeadlineExceeded) {
+		t.Fatalf("err = %v, want the unanswered-lookup error wrapping the deadline", err)
+	}
+	if remote.writes != 1 {
+		t.Errorf("wrote %d times, want 1: no further attempt until the filer answers", remote.writes)
+	}
+	if filerClient.lookups != 2 {
+		t.Errorf("looked up the filer %d times, want one before the attempt and one after it failed", filerClient.lookups)
+	}
+	if elapsed := time.Since(started); elapsed > 5*time.Second {
+		t.Fatalf("took %v; the retry loop did not stop on the unanswered lookup", elapsed)
+	}
+}
+
 func TestResolveLastSyncOffsetKeepsTheSavedOffsetWhenTheLookupFails(t *testing.T) {
 	saved := time.Date(2026, 11, 4, 17, 57, 10, 0, time.UTC)
 	lookupFails := func() (*filer_pb.Entry, error) { return nil, context.DeadlineExceeded }