[jsonrpc] fix estimated rewards during safe mode

[emmazzz]

Description

Currently if the exchange rate for an epoch is not found, we would assign it the default exchange rate while we should've used the exchange rate from the previous existing epoch instead. This PR fixes that.

Test plan

Added some unit tests.

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Release notes

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• Protocol:
• Nodes (Validators and Full nodes):
• Indexer:
• JSON-RPC: Fixed reward estimation in getStakes API to avoid overestimating stake rewards of stakes activated during safe mode epochs.
• GraphQL:
• CLI:
• Rust SDK:
• REST API:

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[lxfind]

It's probably easier to sort it first before processing.
But otherwise this code looks good to me.

[emmazzz]

I'm not sure if sorting it first will make things easier because then we'd have to keep track of the index we are at separately and insert the missing entry, etc. The logic may get messier. Unless I'm missing something.

[lxfind]

You just iterate the sorted vector. No need for min/max/find calls.

[emmazzz]

This is the alternative implementation I came up with:

fn backfill_rates(
    mut rates: Vec<(EpochId, PoolTokenExchangeRate)>,
) -> Vec<(EpochId, PoolTokenExchangeRate)> {
    if rates.is_empty() {
        return rates;
    }

    rates.sort_by(|(a, _), (b, _)| a.cmp(b));
    let mut filled_rates = vec![];


    let mut next_epoch_to_fill = rates[0].0;
    let mut prev_rate = rates[0].1.clone();
    for (epoch, rate) in rates {
        // Fill in all the gaps between the previous entry and the current entry.
        while epoch > next_epoch_to_fill {
            filled_rates.push((next_epoch_to_fill, prev_rate.clone()));
            next_epoch_to_fill += 1;
        }
        filled_rates.push((next_epoch_to_fill, rate.clone()));
        next_epoch_to_fill += 1;
        prev_rate = rate;
    }

    filled_rates.reverse();
    filled_rates
}

I think this is harder to understand and get right. It took me quite a few tries to get the code right, with off-by-one errors etc

[sblackshear]

Is line 466/467 attempting to do the equivalent of this?

[emmazzz]

The loop in backfill_rates does backfilling + sorting in ascending order by epoch number. And line 484 reverses the vector so it becomes sorted in descending order.

[longbowlu]

"we will use the value from the nearest prior epoch"