feat: Add zero IR strategy

[miguelmtzinf]

Closes #817

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Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (94e571f) 99.65% compared to head (87d3cf7) 99.65%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #818   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
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  Files          45       46    +1     
  Lines        2300     2320   +20     
  Branches      421      421           
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+ Hits         2292     2312   +20     
  Misses          8        8           

Impacted Files	Coverage Δ
contracts/misc/ZeroReserveInterestRateStrategy.sol	100.00% <100.00%> (ø)

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[eboadom]

Why immutable and just not constant?

[miguelmtzinf]

good catch, fixed now

[eboadom]

Generally, I don't see any major problem with the implementation from the perspective of the protocol having a ZERO rate strategy configured, just an small consideration.
More specifically:

• There is no problem with returning 0 on all functions non-calculateInterestRates(), as there is no dependency on them in the protocol.
• It is correct to keep the addresses provider initialization, as there could be some external dependency checking it for some reason.
• Regarding where calculateInterestRates() is used:
  • On updateInterestRates() there should not cause any problem: rates on the storage of the reserve data will be update to 0, and this will just cause that on the next pool action, indexes will not increase, and there is no "inflow" coming from rates. Additionally, this is the same case as having RF configured to 100%, which we know is perfectly fine since 3.0.1
  • On validateRebalanceStableBorrowRate() is it slightly more interesting, but I would say no big problem and definitely not related to this implementation. If there is no stable debt supply of an asset, no matter if the stable rate mode is active or not, the rebalancing will just revert().
    If there would be stable rate active and the rate strategy ZERO would be set, it would actually allow to continuously rebalance of any borrower with a positive balance of stable debt. This should not cause any problem but seems antinatural to allow continuous minting/burn of stable debt for the user, so maybe it could be an option to circuit-break somehow the rebalance.

[miguelmtzinf]

Thanks for the analysis and review @eboadom .

Regarding the side-effect on validateRebalanceStableBorrowRate(), these hypothetical rebalance actions would not make sense at all, because the rate is 0 no matter what.

[eboadom]

Thanks for the analysis and review @eboadom .

Regarding the side-effect on validateRebalanceStableBorrowRate(), these hypothetical rebalance actions would not make sense at all, because the rate is 0 no matter what.

Yes, agree. Just something to take into account, because it feels weird that we allow repeated actions that make no change