Why rebuild extended shares?

[rootulp]

Context

rsmt2d/extendeddatacrossword.go

Lines 244 to 275 in 328fe4d

	func (eds *ExtendedDataSquare) rebuildShares(
	isExtendedPartIncomplete bool,
	shares [][]byte,
	) ([][]byte, bool, error) {
	rebuiltShares, err := eds.codec.Decode(shares)
	if err != nil {
	// repair unsuccessful
	return nil, false, nil
	}
	if isExtendedPartIncomplete {
	// If needed, rebuild the parity shares too.
	rebuiltExtendedShares, err := eds.codec.Encode(rebuiltShares[0:eds.originalDataWidth])
	if err != nil {
	return nil, true, err
	}
	startIndex := len(rebuiltExtendedShares) - int(eds.originalDataWidth)
	rebuiltShares = append(
	rebuiltShares[0:eds.originalDataWidth],
	rebuiltExtendedShares[startIndex:]...,
	)
	} else {
	// Otherwise copy them from the EDS.
	startIndex := len(shares) - int(eds.originalDataWidth)
	rebuiltShares = append(
	rebuiltShares[0:eds.originalDataWidth],
	shares[startIndex:]...,
	)
	}
	return rebuiltShares, true, nil
	}

Question

Given that Decode returns the original shares + parity shares, why does rebuildShares have logic to separately rebuild extended shares? It seems possible to entirely remove the isExtendedPartIncomplete conditional.

[adlerjohn]

IIRC, Infectious behaves differently and Decode doesn't return all shares. Could you confirm?

The other reason for this is probably refactoring that left redundant code in.

[rootulp]

Ahh I see, thanks for pointing this out. It appears Infectious Decode did only return the rebuilt shares here.

[adlerjohn]

This should probably have been originally fixed within the Infectious codec IMO, rather that doubling work for other codecs.