Merge pull request #5774 from filecoin-project/fix/post-fresh-rand

wdpost: Always use head tipset to get randomness

storage/wdpost_run.go

@@ -481,7 +481,12 @@ func (s *WindowPoStScheduler) runPost(ctx context.Context, di dline.Info, ts *ty
 		return nil, xerrors.Errorf("failed to marshal address to cbor: %w", err)
 	}
 
-	rand, err := s.api.ChainGetRandomnessFromBeacon(ctx, ts.Key(), crypto.DomainSeparationTag_WindowedPoStChallengeSeed, di.Challenge, buf.Bytes())
+	headTs, err := s.api.ChainHead(ctx)
+	if err != nil {
+		return nil, xerrors.Errorf("getting current head: %w", err)
+	}
+
+	rand, err := s.api.ChainGetRandomnessFromBeacon(ctx, headTs.Key(), crypto.DomainSeparationTag_WindowedPoStChallengeSeed, di.Challenge, buf.Bytes())
 	if err != nil {
 		return nil, xerrors.Errorf("failed to get chain randomness from beacon for window post (ts=%d; deadline=%d): %w", ts.Height(), di, err)
 	}