fix: Fix VaultInvariant and VaultDeposit precision bugs at IOU scale boundaries (#7272)

Co-authored-by: Bart <bthomee@users.noreply.github.com>

include/xrpl/protocol/STAmount.h

@@ -189,7 +189,6 @@ public:
     /**
      * Checks if this amount evaluates to zero when constrained to a specific
      * accounting scale.
-     *
      * For XRP and MPT `roundToScale` is a no-op, returns true only when the amount itself is zero.
      * The `scale` argument is ignored in that case.
      * For IOU, the amount is rounded to the given scale using Number::RoundingMode::ToNearest mode

include/xrpl/tx/invariants/VaultInvariant.h

@@ -4,9 +4,11 @@
 #include <xrpl/basics/base_uint.h>
 #include <xrpl/beast/utility/Journal.h>
 #include <xrpl/ledger/ReadView.h>
+#include <xrpl/protocol/AccountID.h>
 #include <xrpl/protocol/MPTIssue.h>
 #include <xrpl/protocol/STTx.h>
 #include <xrpl/protocol/TER.h>
+#include <xrpl/protocol/XRPAmount.h>
 
 #include <optional>
 #include <unordered_map>
@@ -79,16 +81,83 @@ private:
     std::vector<Shares> beforeMPTs_;
     std::unordered_map<uint256, DeltaInfo> deltas_;
 
+    /**
+     * @brief Compute the minimum STAmount scale for rounding invariant
+     *        calculations.
+     *
+     * Post-amendment (@c fixCleanup3_2_0) this is simply the posterior
+     * @c assetsTotal scale.  Pre-amendment it is the coarsest scale across
+     * @p vaultDelta and both asset-field deltas.
+     *
+     * @param vaultDelta Delta of the vault's asset balance for this transaction.
+     * @param rules      Active ledger rules (used to check the amendment).
+     * @returns The minimum scale to apply when rounding vault-related amounts.
+     */
+    [[nodiscard]] std::int32_t
+    computeVaultMinScale(DeltaInfo const& vaultDelta, Rules const& rules) const;
+
+    /**
+     * @brief Return the vault-asset balance-change delta for an account.
+     *
+     * Looks up the ledger-entry delta recorded during @c visitEntry for the
+     * account entry (XRP), trust line (IOU), or MPToken (MPT) that corresponds
+     * to the vault asset held by @p id.
+     *
+     * @param id Account whose asset delta is requested.
+     * @returns The delta, or @c std::nullopt if the entry was not touched.
+     */
+    [[nodiscard]] std::optional<DeltaInfo>
+    deltaAssets(AccountID const& id) const;
+
+    /**
+     * @brief Return the vault-asset delta for the transaction's sending
+     *        account, adjusted for the fee.
+     *
+     * Calls @c deltaAssets for @c tx[sfAccount] and, for non-delegated XRP
+     * transactions, adds the consumed fee back so the invariant sees the net
+     * asset movement rather than the fee-reduced balance change.
+     *
+     * @param tx  The transaction being applied.
+     * @param fee Fee charged by this transaction.
+     * @returns The fee-adjusted delta, or @c std::nullopt if the net delta is
+     *          zero or the account entry was not touched.
+     */
+    [[nodiscard]] std::optional<DeltaInfo>
+    deltaAssetsTxAccount(STTx const& tx, XRPAmount fee) const;
+
+    /**
+     * @brief Return the vault-share balance-change delta for an account.
+     *
+     * For the vault's pseudo-account the @c MPTokenIssuance outstanding-amount
+     * delta is returned; for all other accounts the @c MPToken delta is
+     * returned.
+     *
+     * @param id Account whose share delta is requested.
+     * @returns The delta, or @c std::nullopt if the entry was not touched.
+     */
+    [[nodiscard]] std::optional<DeltaInfo>
+    deltaShares(AccountID const& id) const;
+
+    /**
+     * @brief Check whether a vault holds no assets.
+     *
+     * @param vault Snapshot of the vault to test.
+     * @returns @c true when both @c assetsAvailable and @c assetsTotal are
+     *          zero.
+     */
+    [[nodiscard]] static bool
+    isVaultEmpty(Vault const& vault);
+
 public:
+    // Compute the coarsest scale required to represent all numbers
+    [[nodiscard]] static std::int32_t
+    computeCoarsestScale(std::vector<DeltaInfo> const& numbers);
+
     void
     visitEntry(bool, std::shared_ptr<SLE const> const&, std::shared_ptr<SLE const> const&);
 
     bool
     finalize(STTx const&, TER const, XRPAmount const, ReadView const&, beast::Journal const&);
-
-    // Compute the coarsest scale required to represent all numbers
-    [[nodiscard]] static std::int32_t
-    computeCoarsestScale(std::vector<DeltaInfo> const& numbers);
 };
 
 }  // namespace xrpl