attempt to do better padding on pieces being written into sectors

extern/sector-storage/ffiwrapper/sealer_cgo.go

@@ -195,19 +195,32 @@ func (sb *Sealer) AddPiece(ctx context.Context, sector storage.SectorRef, existi
 		return piecePromises[0]()
 	}
 
+	var payloadRoundedBytes abi.PaddedPieceSize
 	pieceCids := make([]abi.PieceInfo, len(piecePromises))
 	for i, promise := range piecePromises {
-		pieceCids[i], err = promise()
+		pinfo, err := promise()
 		if err != nil {
 			return abi.PieceInfo{}, err
 		}
+
+		pieceCids[i] = pinfo
+		payloadRoundedBytes += pinfo.Size
 	}
 
 	pieceCID, err := ffi.GenerateUnsealedCID(sector.ProofType, pieceCids)
 	if err != nil {
 		return abi.PieceInfo{}, xerrors.Errorf("generate unsealed CID: %w", err)
 	}
 
+	if payloadRoundedBytes < pieceSize.Padded() {
+		paddedCid, err := commpffi.ZeroPadPieceCommitment(pieceCID, payloadRoundedBytes.Unpadded(), pieceSize)
+		if err != nil {
+			return abi.PieceInfo{}, xerrors.Errorf("failed to pad data: %w", err)
+		}
+
+		pieceCID = paddedCid
+	}
+
 	// validate that the pieceCID was properly formed
 	if _, err := commcid.CIDToPieceCommitmentV1(pieceCID); err != nil {
 		return abi.PieceInfo{}, err