v2.0: runtime: use leader schedule epoch to serve sol_get_epoch_stake (backport of #3279)

[mergify]

Problem

The bank epoch stakes cache stores stakes according to the leader schedule epoch, which means the epoch stake for a given epoch N actually corresponds to the N + 1 epoch in the epoch stakes cache.

The sol_get_epoch_stake syscall is designed to vend to on-chain programs the epoch stakes for the current epoch, but the existing implementation is returning the epoch stakes for leader schedule epoch N, when it actually should return the epoch stakes for leader schedule epoch N + 1.

For example, if you activate stake in epoch N, it becomes active in epoch N + 1, but the existing bank implementation for serving the sol_get_epoch_stakes syscall actually wouldn't return this new stake until epoch N + 2.

Summary of Changes

Use Bank's current_epoch_stakes method - which fetches the leader schedule epoch bank.epoch() + 1 - to serve the epoch stake information for sol_get_epoch_stakes, properly returning the current epoch stake.

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This is an automatic backport of pull request #3279 done by Mergify.

[buffalojoec]

As specified in the backport instructions, I've only included the change, and not the additional tests or the BPF program tests. Lmk if anyone wants those added as well.

Case for backport: fixes a bug in the SVM API for accessing the epoch stakes cache, done through the sol_get_epoch_stake syscall. Small change, significant bugfix.

[bw-solana]

I'm probably missing something, but couldn't this break consensus? Does it need a feature gate?

[jstarry]

I'm probably missing something, but couldn't this break consensus? Does it need a feature gate?

This change is already behind the enable_get_epoch_stake_syscall feature gate which hasn't been activated yet. So this bug fix is fixing unactivated code in case that wasn't clear.

[bw-solana]

This change is already behind the enable_get_epoch_stake_syscall feature gate which hasn't been activated yet. So this bug fix is fixing unactivated code in case that wasn't clear.

Ah, I knew I was missing something. Thanks Justin!

[bw-solana]

LGTM