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fix: Snap vault deltas to ValidVault's posterior assetsTotal scale

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Derive credits from the Downward-rounded posterior total so an off-grid
sfAssetsTotal cannot over-credit the depositor's payment, and round
debits down at that same posterior scale so decade-boundary amounts
that are representable after the tx succeed in full.
This commit is contained in:
Vito
2026-08-25 21:12:14 +02:00
parent 9b8e5d6672
commit 03236cc07e
7 changed files with 227 additions and 149 deletions
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38
include/xrpl/ledger/helpers/VaultHelpers.h
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@@ -10,6 +10,7 @@
#include <xrpl/protocol/TER.h>
#include <cstdint>
#include <expected>
#include <optional>
namespace xrpl {
@@ -45,30 +46,25 @@ assetsToSharesDeposit(SLE::const_ref vault, SLE::const_ref issuance, STAmount co
sharesToAssetsDeposit(SLE::const_ref vault, SLE::const_ref issuance, STAmount const& shares);
/**
* Returns the effective change to sfAssetsTotal after canonicalizing
* `sfAssetsTotal + delta` under `mode`, as a non-negative magnitude.
* `delta` is positive when crediting the vault and negative when debiting
* it; only its sign is used to select the rounding direction. This is not
* the same as simply rounding `delta`'s magnitude to sfAssetsTotal's
* scale: both the before and after totals are canonicalized under `mode`
* before subtracting, so the result also accounts for sfAssetsTotal
* itself sitting mid-grid. The caller adds the returned magnitude to
* sfAssetsTotal for credits, or subtracts it for debits, and applies it
* the same way to any related field (for example sfAssetsAvailable) so
* all rails stay in sync; the other fields' scale is at least as fine as
* sfAssetsTotal's.
* Returns a non-negative magnitude to apply to sfAssetsTotal and the related
* rails, snapped to the scale of the post-transaction sfAssetsTotal — the same
* scale ValidVault uses. Credits take the Downward-rounded posterior total
* minus the current total so the vault cannot be credited more than `|delta|`
* even when the anterior total sits off that grid. Debits round `|delta|`
* Downward on the posterior scale. A zero result is `tecPRECISION_LOSS`.
*
* The caller adds the returned magnitude for credits or subtracts it for
* debits, and applies it to every related field so all rails stay in sync.
* A zero result is returned as `tecPRECISION_LOSS` so callers reject rather
* than moving shares without assets.
*
* @param vault The vault SLE.
* @param delta The signed amount by which sfAssetsTotal will change; only
* its sign selects the rounding direction. The magnitude is
* what is quantized and returned.
* @param mode The rounding mode to apply when quantizing to the
* sfAssetsTotal scale.
*
* @return The rounded magnitude, always non-negative.
* @param delta The requested signed change to sfAssetsTotal.
* @return The rounded magnitude, always non-negative, or `tecPRECISION_LOSS`
* if the change is smaller than one ULP.
*/
[[nodiscard]] STAmount
clampToAssetsTotalScale(SLE::const_ref vault, STAmount const& delta, Number::RoundingMode mode);
[[nodiscard]] std::expected<STAmount, TER>
clampToAssetsTotalScale(SLE::const_ref vault, STAmount const& delta);
/**
* Controls whether to truncate shares instead of rounding.