Add MINT_MAX_BALANCE_ISSUED setting

[minibits-cash]

MINT_MAX_BALANCE_ISSUED would serve the purposes, when the mint operator wants to make sure the mint can not issue more units of ecash than the configured value.

This can serve multiple purposes - as a security limit as well as an imposed limit in case the unit issued is backed by a derivative position at the exchange that is pre-funded - i.e. fully hedged against exchange rate changes to bitcoin upfront it is available to be issued as ecash.

[lollerfirst]

I wonder if this is information we should display in a GET /v1/info request.
If so, we should probably do a NUT about it.

[minibits-cash]

Agree, but I think it's not a prerequisite for implementation. From user pov max one time mint / melt limits that are already there are more important then a total.

[lollerfirst]

@minibits-cash There's two ways I would do this:

1. When generating a mint quote, sneak in a balance request to the LN backend right before requesting the invoice.
   If balance >= MINT_MAX_BALANCE_ISSUED then abort
2. Add a table to the DB that keeps track of the amounts worth of issued ecash for each unit, then for every mint quote
   request check that the balance for the particular unit is not exceeded.

What do you think?

[minibits-cash]

I think it's fully upon mint to enforce it and just fail the first mint request hitting the limit.

[lollerfirst]

Yes. I was referring to how I would implement this here in Nutshell.

bff30d4#diff-563ab1fc2a5b2676a9794be3b08bb863afc36b495cbe4653ced3fdeb430bf530R252-R256

Looks like there already is a check of the first type I mentioned. Although it doesn't seem to account for different units.

[minibits-cash]

Yes. I was referring to how I would implement this here in Nutshell.

bff30d4#diff-563ab1fc2a5b2676a9794be3b08bb863afc36b495cbe4653ced3fdeb430bf530R252-R256

Looks like there already is a check of the first type I mentioned. Although it doesn't seem to account for different units.

Ah sorry, I misunderstood that.

Ad balance check: I believe nutshell already uses database views to calculate overall minted and melted amounts as well as issued balance.

Not sure how it handles the units tho.
I remind calle mentioning that max mint and melt configs do not handle units currently.

[minibits-cash]

I now found that there already is MINT_MAX_BALANCE existing setting doing the same, so I close this one.

[minibits-cash]

Related PR #652 could then focus to extend existing limits so they support multiple units (as they would likely be incorrectly applied as of now):

Limits
Max mint balance in satoshis
MINT_MAX_BALANCE=1000000
Max peg-in amount in satoshis
MINT_MAX_PEG_IN=100000
Max peg-out amount in satoshis
MINT_MAX_PEG_OUT=100000

[minibits-cash]

There is now separate issue for this extended scope: #650