From 1ea5b888789c4c9cc8178e0f729125e6d2584a12 Mon Sep 17 00:00:00 2001
From: StrathCole <7449529+StrathCole@users.noreply.github.com>
Date: Wed, 25 Sep 2024 06:54:37 +0200
Subject: [PATCH] handle excessive fee deduction during simulation (#518)

---
 custom/auth/ante/expected_keeper.go |  2 ++
 custom/auth/ante/fee.go             | 23 +++++++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/custom/auth/ante/expected_keeper.go b/custom/auth/ante/expected_keeper.go
index 05957c63..2d00f13a 100644
--- a/custom/auth/ante/expected_keeper.go
+++ b/custom/auth/ante/expected_keeper.go
@@ -30,6 +30,8 @@ type BankKeeper interface {
 	SendCoins(ctx sdk.Context, from, to sdk.AccAddress, amt sdk.Coins) error
 	SendCoinsFromAccountToModule(ctx sdk.Context, senderAddr sdk.AccAddress, recipientModule string, amt sdk.Coins) error
 	SendCoinsFromModuleToModule(ctx sdk.Context, senderModule string, recipientModule string, amt sdk.Coins) error
+	SendCoinsFromModuleToAccount(ctx sdk.Context, senderModule string, recipientAddr sdk.AccAddress, amt sdk.Coins) error
+	MintCoins(ctx sdk.Context, moduleName string, amt sdk.Coins) error
 }
 
 type DistrKeeper interface {
diff --git a/custom/auth/ante/fee.go b/custom/auth/ante/fee.go
index a292be06..389ca909 100644
--- a/custom/auth/ante/fee.go
+++ b/custom/auth/ante/fee.go
@@ -10,6 +10,7 @@ import (
 	sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
 	"github.com/cosmos/cosmos-sdk/x/auth/ante"
 	"github.com/cosmos/cosmos-sdk/x/auth/types"
+	minttypes "github.com/cosmos/cosmos-sdk/x/mint/types"
 )
 
 // FeeDecorator deducts fees from the first signer of the tx
@@ -119,6 +120,28 @@ func (fd FeeDecorator) checkDeductFee(ctx sdk.Context, feeTx sdk.FeeTx, taxes sd
 				feesOrTax = feesOrTax.Add(sdk.NewCoin(tax.Denom, missingAmount))
 			}
 		}
+
+		// a further issue arises from the fact that simulations are sometimes run with
+		// the full bank balance of the account, which can lead to a situation where
+		// the fees are deducted from the account during simulation and so the account
+		// balance is not enough to complete the simulation.
+		// So ONLY during simulation, we MINT the fees to the account to avoid this issue.
+		// We only mint the fees we are adding on top of the original fee (sent by user).
+		if !feesOrTax.IsZero() {
+			needMint := feesOrTax.Sort().Sub(fee.Sort()...)
+			if !needMint.IsZero() {
+				err := fd.bankKeeper.MintCoins(ctx, minttypes.ModuleName, needMint)
+				if err != nil {
+					return err
+				}
+
+				// we need to add the fees to the account balance to avoid deduction errors
+				err = fd.bankKeeper.SendCoinsFromModuleToAccount(ctx, minttypes.ModuleName, deductFeesFromAcc.GetAddress(), needMint)
+				if err != nil {
+					return err
+				}
+			}
+		}
 	}
 
 	if !feesOrTax.IsZero() {