shares: Implement the SharesService

[renaynay]

Implement SharesService in package shares.

Implement:

• type Share
• Constructor function for SharesService

The SharesService contains the following methods / functionality:

• Start
• Stop

For ShareExchange, a light node only requires the ability to request shares and perform sampling (random share requests across random peers), and store the shares they receive, while a full node should be able to serve those share requests as well as perform requests.

ShareStore only applies to light nodes as full nodes store full blocks, so if the node is a full node, ShareStore should be disabled.

---

type Service interface {
	GetShare(ctx context.Context, dah header.DataAvailabilityHeader, row, col int) (Share, error)
	GetShares(context.Context, DataAvailabilityHeader) ([]Shares, error)
	GetSharesByNamespace(context.Context, DataAvailabilityHeader, namespace.ID) ([]Shares, error)
	
	Start(context.Context) error
	Stop(context.Context) error
}

• GetSharesByNamespace depend on ipld: Implement API for application sovereignty: query data per namespace #5

[renaynay]

Should there be a pruning mechanism / threshold for light nodes ShareStore? cc @Wondertan @liamsi

[liamsi]

I think full storage nodes certainly need pruning (not necessarily for devnet/testnet but for mainnet).

Light nodes download even less shares. Hence, pruning for light nodes is even less of a priority. That said, if light nodes get subjectively re-initialized we might consider overwriting the existing header (and shares) store. So that way we would have implicit pruning: whenever the light node gets re-initialized it throws away old headers and corresponding shares.

[Wondertan]

Light nodes download even less shares.

Light nodes do DAS and still download a reasonable amount of shares, which should be pruned. I am not sure though what should be the default behavior between storing and pruning, or storing only for last n heights and prune.

[renaynay]

I believe @liamsi said something along the lines of we'll only prune when node is subjectively re-initialised? So we'll store all shares we get in the meantime. I don't know if pruning is within the scope of devnet tho but good to think about. We should make an issue for it.

[renaynay]

TODO:

• Integrate GetSharesByNamespace into shares.Service inside service/share, ipld: adds ability to retrieve shares by namespace ID #170
• Implement GetShares (ability to reconstruct whole square) -- this is already implemented somewhere, so mostly copy and paste, but better to refactor it a bit for networking, concurrency, and storage optimisations (but this can be left for post Devnet, just need to document it in an issue).

[Wondertan]

Reopen as there is still one simple todo left:

[renaynay]

TODO:

• GetShares

[Wondertan]

@renaynay, GetShares are required for Full Availability so start with this one