share: WriteEDS

[Wondertan]

Context

As per #1099, we want to store EDSes as a whole. For this, the CAR format is chosen.

Implementation Details

To write EDS into a stream/file, WriteEDS is introduced. Internally it:

• Re-imports EDS similarly to
  ipld.ImportShares
  • Using Blockservice with offline
    exchange and in-memory Blockstore
  • With NodeVisitor, which saves to the
    Blockstore only NMT Merkle proofs(no shares) NOTE: len(node.Links()) == 2
    • Actual shares are written further in a particular way explained further
• Creates and writes header CARv1Header
  • Fills up Roots field with EDS.RowRoots/EDS.ColRoots roots converted into CIDs
• Iterates over shares in quadrant-by-quadrant order via EDS.GetCell
  • Writes the shares in row-by-row order
• Iterates over in-memory Blockstore and writes NMT Merkle
  proofs stored in it

NOTES:

• CAR provides a utility to serialize any DAG into the file
  and there is a way to serialize EDS into DAG(share/ipld.ImportShares). This approach is the simplest and traverses
  shares and Merkle Proofs in a depth-first manner packing them in a CAR file. However, this is incompatible with the
  requirement of being able to truncate the CAR file reading out only the first quadrant out of it without NMT proofs,
  so serialization must be different from the utility to support that.
• Alternatively to WriteEDS, an EDSReader could be introduced to make EDS-to-stream handling more idiomatic
  and efficient in some cases, with the cost of more complex implementation.

// WriteEDS writes the whole EDS into the given io.Writer as CARv1 file.
// All its shares and recomputed NMT proofs.
func WriteEDS(context.Context, *rsmt2d.ExtendedDataSquare, io.Writer) error