Test compatibility of SplitMessages with/without share index

[rootulp]

Context

During non-interactive defaults, we added empty message shares that act as padding between messages. In order for transactions to be linked to the share(s) that they pay for, indexes were added to SplitMessages.

Problem Definition

Since indexes didn't exist in v0.6.0, we need to verify if v0.6.0 is compatible with main.

Ref:

• celestia-app/pkg/shares/split.go

  Line 69 in a3ef48c

  msgIndexes = append(msgIndexes, malleatedTx.ShareIndex)

• celestia-app/pkg/shares/split.go

  Line 96 in a3ef48c

  func SplitMessages(indexes []uint32, msgs []coretypes.Message) ([][]byte, error) {

Proposal

Write a unit test or integ test that verifies compatibility of SplitMessages with/without share index

[evan-forbes]

to expand on this slightly, the current tests do test comparability of the encoding and decoding, as we haven't changed them in #627 and won't be changing them when introducing non-interactive defaults either.

What we need to add is a mechanism to detect that we are decoding an old block, so that we don't add any padding or care if there is a share index in the wrapped transactions. This could be as simple (and implicit) as reading the first share index from a wrapped transaction. If that index is 0, then we automatically know that we are reading an old block, and can pass nil to SplitMessages(nil, msgs), which effectively uses identical share splitting code to v0.6.0

[evan-forbes]

ref #608