adds feature-gate code to enforce retransmitter signature verification

[behzadnouri]

Problem

Need to verify if the shred is retransmitted by the expected parent node in Turbine tree.

Summary of Changes

Added feature-gate code to enforce retransmitter signature verification

[AshwinSekar]

LGTM.

The last step is then for replay to check if the last FEC set is of the resigned variant when this feature flag is turned on, similar to

agave/ledger/src/blockstore.rs

Line 3691 in 9ee36fa

pub fn is_last_fec_set_full(&self, slot: Slot) -> Result<bool> {

?

Or would we prefer to wait until the resigned variant is turned on, and discard any shred that is not resigned, similar to how we discard legacy shreds, which skips us having to check during replay.

[behzadnouri]

wait until the resigned variant is turned on, and discard any shred that is not resigned, similar to how we discard legacy shreds, which skips us having to check during replay.

I don't think that would work.
We only require the very last erasure batch to be of resigned variant. But when you receive a shred you cannot tell if it belongs to the last erasure batch until you have received the data shred with LAST_SHRED_IN_SLOT flag.
So at the moment that you receive a shred, you cannot tell if it was supposed to be of resigned variant, and determine to drop or not if it is not resigned.

So I think we actually need to do what you described earlier:

for replay to check if the last FEC set is of the resigned variant when this feature flag is turned on

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.