From a410c5586319903b60438cf85549b3aa6753b009 Mon Sep 17 00:00:00 2001
From: Kevin Halliday <kevin@omni.network>
Date: Fri, 21 Feb 2025 09:55:30 -0500
Subject: [PATCH] fix(solver): handle tokens that decrement max allowance
 (#3112)

Do not error if below max allowance.

issue:  #3111
---
 solver/app/approvals.go | 16 +++++++++++++---
 solver/app/procdeps.go  |  9 +++++++--
 solver/app/reject.go    |  9 +++++++--
 3 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/solver/app/approvals.go b/solver/app/approvals.go
index bb698a9c6..326b26f78 100644
--- a/solver/app/approvals.go
+++ b/solver/app/approvals.go
@@ -22,6 +22,8 @@ import (
 )
 
 // approveOutboxes gives each outbox max allowance for all supported tokens.
+// Most tokens will not decrement allowance when set to max, though some do.
+// TODO: monitor allowances, alert or reset when "low" (half type(uint256).max).
 func approveOutboxes(ctx context.Context, network netconf.Network, backends ethbackend.Backends, solverAddr common.Address) error {
 	addrs, err := contracts.GetAddresses(ctx, network.ID)
 	if err != nil {
@@ -99,7 +101,9 @@ func approveToken(ctx context.Context, backend *ethbackend.Backend, token Token,
 		return errors.Wrap(err, "new token")
 	}
 
-	isApproved := func() (bool, error) { return isAppproved(ctx, token.Address, backend, solverAddr, outboxAddr) }
+	isApproved := func() (bool, error) {
+		return isAppproved(ctx, token.Address, backend, solverAddr, outboxAddr, umath.MaxUint256)
+	}
 
 	if approved, err := isApproved(); err != nil {
 		return err
@@ -128,7 +132,13 @@ func approveToken(ctx context.Context, backend *ethbackend.Backend, token Token,
 	return nil
 }
 
-func isAppproved(ctx context.Context, token common.Address, client ethclient.Client, solverAddr, outboxAddr common.Address) (bool, error) {
+func isAppproved(
+	ctx context.Context,
+	token common.Address,
+	client ethclient.Client,
+	solverAddr, outboxAddr common.Address,
+	spend *big.Int,
+) (bool, error) {
 	tkn, err := bindings.NewIERC20(token, client)
 	if err != nil {
 		return false, errors.Wrap(err, "new token")
@@ -139,5 +149,5 @@ func isAppproved(ctx context.Context, token common.Address, client ethclient.Cli
 		return false, errors.Wrap(err, "get allowance")
 	}
 
-	return new(big.Int).Sub(allowance, umath.MaxUint256).Sign() >= 0, nil
+	return spend.Cmp(allowance) <= 0, nil
 }
diff --git a/solver/app/procdeps.go b/solver/app/procdeps.go
index 99929e77e..43caa72c4 100644
--- a/solver/app/procdeps.go
+++ b/solver/app/procdeps.go
@@ -122,13 +122,18 @@ func newFiller(
 				return errors.New("unsupported token, should have been rejected [BUG]", "addr", tknAddr.Hex(), "chain_id", destChainID)
 			}
 
-			isAppproved, err := isAppproved(ctx, tknAddr, backend, solverAddr, outboxAddr)
+			isAppproved, err := isAppproved(ctx, tknAddr, backend, solverAddr, outboxAddr, output.Amount)
 			if err != nil {
 				return errors.Wrap(err, "is approved")
 			}
 
 			if !isAppproved {
-				return errors.New("outbox not approved to spend token [BUG] ", "token", tkn.Symbol, "chain_id", destChainID)
+				return errors.New("outbox not approved to spend token",
+					"token", tkn.Symbol,
+					"chain_id", destChainID,
+					"addr", tknAddr.Hex(),
+					"amount", output.Amount,
+				)
 			}
 		}
 
diff --git a/solver/app/reject.go b/solver/app/reject.go
index ee26ca90b..b57eda2ee 100644
--- a/solver/app/reject.go
+++ b/solver/app/reject.go
@@ -148,13 +148,18 @@ func checkApprovals(ctx context.Context, expenses []Payment, client ethclient.Cl
 			continue
 		}
 
-		isAppproved, err := isAppproved(ctx, tkn.Address, client, solverAddr, outboxAddr)
+		isAppproved, err := isAppproved(ctx, tkn.Address, client, solverAddr, outboxAddr, expense.Amount)
 		if err != nil {
 			return errors.Wrap(err, "is approved")
 		}
 
 		if !isAppproved {
-			return errors.New("outbox not approved to spend token [BUG]", "token", tkn.Symbol, "chain_id", tkn.ChainID)
+			return errors.New("outbox not approved to spend token",
+				"token", tkn.Symbol,
+				"chain_id", tkn.ChainID,
+				"addr", tkn.Address.Hex(),
+				"amount", expense.Amount,
+			)
 		}
 	}