Feat: decrease debt

[mustermeiszer]

Description

Allows issuers to decrease the debt of any loan without actually repaying to the pool. Mostly useful for ensuring offchain cash is reflected accurately. This is basically an expense in most cases, should be covered by fees, but is a good increase of flexibility.

NOTE:

• The deployed capital ratio of the tranches will decrease during the next epoch close but until then the tranches expect to generate the same revenue as of before - i.e. with a higher deployed capital ratio

Changes and Descriptions

• adds extrinsic fn decrease_debt(..) - same as fn repay(..) but without moving tokens to the pool

Checklist:

• I have added Rust doc comments to structs, enums, traits and functions
• I have made corresponding changes to the documentation
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 46.65%. Comparing base (4b9e47e) to head (4395ff8).

Files	Patch %	Lines
pallets/loans/src/lib.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1857      +/-   ##
==========================================
+ Coverage   46.52%   46.65%   +0.12%     
==========================================
  Files         167      167              
  Lines       13105    13112       +7     
==========================================
+ Hits         6097     6117      +20     
+ Misses       7008     6995      -13     

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[mustermeiszer]

Subquery ticket: centrifuge/api#187

[wischli]

CR: Add unit test(s) for this. We must not merge PRs which add/alter code without adding/updating tests.

[mustermeiszer]

Not adding new logic IMO, just sparing the T::Pool::repay(..) which will be mocked. So pub fn repay(..) should already cover everything.

If you disagree, can still add a test.

[lemunozm]

There is a super simple fn increase_debt_does_not_withdraw() test for the increase_debt() case, maybe we can create a similar one in tests/repay_loan.rs checking that nothing goes to the pool. Of course, paranoid mode-on just ensuring future refactors does not break the invariant here

[lemunozm]

Apart from my suggestion above, LGTM!

[lemunozm]

Yes, amazing test haha. I had this modified in the cashflows. This is an example of how to not test correctly 😆

[lemunozm]

Thanks for tackling the suggestions!