fungible/s decrease balance preserves account

[muharem]

Ensure preservation is respected when decreasing balance through fangible/s::Unbalanced::decrease_balance.

At present, an account might be removed even if preservation is set to Preserve or Protect. Given the default implementation of decrease_balance, we can't pinpoint the exact constraint that wasn't satisfied (frozen or ED). As a result, we return the FundsUnavailable error.

There's a more extensive PR that aims to address the same problem: #1296. Due to its broader scope, I'm maintaining this one separately, in the event it can be merged sooner. The new tests introduced here will eventually be superseded by the conformance tests from the referenced PR.

Closes: #1698

[liamaharon]

I have a fix and tests for what appear to be the same thing you're working on here: https://github.com/paritytech/polkadot-sdk/pull/1296/files#diff-e04f7799b91b4225b1923b49d8484495f7868868ab32ed78919f310ffe268fd9R190-R193

[muharem]

I have a fix and tests for what appear to be the same thing you're working on here: https://github.com/paritytech/polkadot-sdk/pull/1296/files#diff-e04f7799b91b4225b1923b49d8484495f7868868ab32ed78919f310ffe268fd9R190-R193

sure, lets work on your PR

[juangirini]

We shouldn't need this check anymore, as amount <= free <= old_balance (by definition reducible_balance must be always <= than balance)

polkadot-sdk/substrate/frame/support/src/traits/tokens/fungible/regular.rs

Lines 80 to 90 in 676bacd

	/// Get the maximum amount that `who` can withdraw/transfer successfully based on whether the
	/// account should be kept alive (`preservation`) or whether we are willing to force the
	/// reduction and potentially go below user-level restrictions on the minimum amount of the
	/// account.
	///
	/// Always less than or equal to `balance()`.
	fn reducible_balance(
	who: &AccountId,
	preservation: Preservation,
	force: Fortitude,
	) -> Self::Balance;

Same applies to fungibles

[muharem]

closing this PR, since #1296 is merged
thanks to @liamaharon