Handle existential deposit BelowMinimum error

[chungquantin]

Expand the error thrown on specific cases of decrease_balance method

• TokeError::BelowMinimum if existential deposit case happens. This happens when Precision::Exact is provided and Preservation is Protect or Provided. If the check fails, FundsUnavailable should not be a right message as there might be a fund but it is below the existential deposit balance.

Issue: #2240

polkadot address: 19nSqFQorfF2HxD3oBzWM3oCh4SaCRKWt1yvmgaPYGCo71J

[liamaharon]

Could you please write in the pr comment each way the method could fail, and the corresponding err you're returning in each case?

Would be nice to also have a conformance test covering each of these cases with a comment explaining the rationale.

[liamaharon]

Can we also add a conformance test for FundsUnavaliable?

[chungquantin]

Yes I will add one

[liamaharon]

I don't understand why to always return BelowMinimum here.

[chungquantin]

From what I see, preservation is provided to ensure the balance does not fall below the existential deposit. Hence, if the reducible < amount we want to throw Token::BelowMinimum. There is a test case for this saying Decreasing the balance below the minimum when Precision::Exact should fail..

// Decreasing the balance below the minimum when Precision::Exact should fail.
	let amount = 11.into();
	assert_eq!(
		T::decrease_balance(
			&account_0,
			amount,
			Precision::Exact,
			Preservation::Preserve,
			Fortitude::Polite,
		),
		Err(TokenError::BelowMinimum.into()),
	);

[chungquantin]

	/// Hold some funds in an account. If a hold for `reason` is already in place, then this
	/// will increase it.
	fn hold(reason: &Self::Reason, who: &AccountId, amount: Self::Balance) -> DispatchResult {
		// NOTE: This doesn't change the total balance of the account so there's no need to
		// check liquidity.

		Self::ensure_can_hold(reason, who, amount)?;
		// Should be infallible now, but we proceed softly anyway.
>>>>		Self::decrease_balance(who, amount, Exact, Protect, Force)?;
		Self::increase_balance_on_hold(reason, who, amount, BestEffort)?;
		Self::done_hold(reason, who, amount);
		Ok(())
	}

Because method hold() has the method decrease_balance called with Preservation::Protect, if it fails, it must throw TokenError::BelowMinimum instead