pallas-validate: Conway phase-1 validation fails on value check when comparing Coin(x) vs Multiasset(x, {})

[a-osiecki]

Summary

A transaction that burns all of its native assets should leave both sides of the preservation of value check as pure ADA. However the arithmetic can produce Multiasset(ada, {}) on the lhs while the rhs resolves to Coin(ada).

The equality check fails here:

pallas/pallas-validate/src/phase1/conway.rs

Lines 329 to 331 in 996893a

	if !conway_values_are_equal(&input, &output) {
	return Err(PostAlonzo(PreservationOfValue));
	}

pallas/pallas-validate/src/utils.rs

Lines 610 to 623 in 996893a

	pub fn conway_values_are_equal(first: &ConwayValue, second: &ConwayValue) -> bool {
	match (first, second) {
	(ConwayValue::Coin(f), ConwayValue::Coin(s)) => f == s,
	(ConwayValue::Multiasset(..), ConwayValue::Coin(..)) => false,
	(ConwayValue::Coin(..), ConwayValue::Multiasset(..)) => false,
	(ConwayValue::Multiasset(f, fma), ConwayValue::Multiasset(s, sma)) => {
	if f != s {
	false
	} else {
	conway_multi_assets_are_equal(fma, sma)
	}
	}
	}
	}

Observed behaviour

Validation fails with transaction values are not preserved correctly, despite the values being equal. This was captured after adding tracing to the Conway phase-1 validation:

conway preservation of value mismatch
  consumed         = Ok(Multiasset(821828716, {policy: {Bounty_23: 1}}))
  produced         = Coin(820149135)
  fee              = 1679581
  mint             = Some({policy: {Bounty_23: -1}})
  lhs_input_plus_mint = Multiasset(821828716, {})
  rhs_output_plus_fee = Coin(821828716)