Add minimum deposit balance

[Kay-Zee]

Closes: #143

Description

Re-enforces a minimum balance on a position.

---

For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[zhangchiqing]

maybe continue to verify the same tx with minimum amount would pass

[Kay-Zee]

i do now continue with a successful create position

[turbolent]

There's already a function PositionView.trueBalance which pretty much does this. Maybe reuse it

[Kay-Zee]

the main argument against using positionView here was that it technically copies all balances, to create the new view. With that said, a position is unlikely to have too many different balances, so maybe that won't really be an "impactful" optimization. Will change implementation, but make a note

[jordanschalm]

Suggested change

	let minimumRequired = UFix128(tokenState.minimumTokenBalancePerPosition)
	assert(
	remainingBalance == 0.0 || remainingBalance >= minimumRequired,
	message: "Withdrawal would leave position below minimum balance requirement of \(minimumRequired). Remaining balance would be \(remainingBalance)."
	)
	assert(
	positionSatisfiesMinimumCreditBalance(type, remainingBalance)
	message: "Withdrawal would leave position below minimum balance requirement of \(minimumRequired). Remaining balance would be \(remainingBalance)."
	)

[Kay-Zee]

added, though i did have to do some type conversions. I think still worth though.

[jordanschalm]

Suggested change

	[positionId, FLOW_TOKEN_IDENTIFIER, amountAboveMin, true],
	[positionId, FLOW_TOKEN_IDENTIFIER, minimum-0.1, true],

To make sure we are withdrawing a amount smaller than current balance, independent of amountAboveMin

[Kay-Zee]

will adjust, but going to just div by 2

[jordanschalm]

Higher-value tokens may need lower minimums in token units

They just do need lower minimums, there is no may. :)

I think what we want to say is:

• Conceptually the minimum balance should reflect a minimum balance value, denominated in the default token ($ / MOET). We have implemented this as a per-token balance, denominated in the token itself, because it is easier to implement and reason about.
  • Possibly there are per-token reasons why the minim value should be higher or lower (like volatility), but the dominating factor is that we need the dollar value of the balance to exceed some threshold

And governance should:

• Pick a dollar-denominated minimum balance
• Set each token minimum so that it equates to the dollar-denominated minimum balance, maybe with an added buffer for expected volatility
• Periodically update the token minimums to match the dollar-denominated minimum (this could be automated)

[Kay-Zee]

did another pass with you suggestions

[nialexsan]

just a few nits

[jordanschalm]

I have a slight preference toward including the empty balance case here, so that this validator function captures the full range of valid balances.

return balance == 0.0 || balance >= UFix128(self.globalLedger[type]!.minimumTokenBalancePerPosition)

Then we can simplify the assertion on line 3014:

assert(self.positionSatisfiesMinimumBalance(type: type, balance: remainingBalance), ...)

[Kay-Zee]

my main problem with doing that was that technically, that would make this function "less" useful for the create position case. Since for that case, the balance == 0.0 is not a "true" case. Also, if i was to include the balance check, i'd probably also change the name of the function, as 0.0 obviously does NOT satisfy the minimum balance.

I think i'd still prefer to keep the balance == 0.0 check outside this function as it only applies in one of the locations where this function is used.