fix: auto gas don't work (#12590)

## Description

Closes: #12588
Solution:
- allow zero gas in simulation mode
- don't check min-gas-price in simulation mode

Please backport to 0.46.





---

### Author Checklist

*All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.*

I have...

- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [ ] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/main/docs/building-modules)
- [ ] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/main/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

### Reviewers Checklist

*All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.*

I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

CHANGELOG.md

@@ -45,6 +45,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 ### Improvements
 
+* [#12589](https://github.com/cosmos/cosmos-sdk/pull/12589) Allow zero gas in simulation mode.
 * [#12576](https://github.com/cosmos/cosmos-sdk/pull/12576) Remove dependency on cosmos/keyring and upgrade to 99designs/keyring v1.2.1
 * [#12089](https://github.com/cosmos/cosmos-sdk/pull/12089) Mark the `TipDecorator` as beta, don't include it in simapp by default.
 * [#12153](https://github.com/cosmos/cosmos-sdk/pull/12153) Add a new `NewSimulationManagerFromAppModules` constructor, to simplify simulation wiring.

x/auth/ante/fee.go

@@ -42,13 +42,21 @@ func (dfd DeductFeeDecorator) AnteHandle(ctx sdk.Context, tx sdk.Tx, simulate bo
 		return ctx, sdkerrors.Wrap(sdkerrors.ErrTxDecode, "Tx must be a FeeTx")
 	}
 
-	if ctx.BlockHeight() > 0 && feeTx.GetGas() == 0 {
+	if !simulate && ctx.BlockHeight() > 0 && feeTx.GetGas() == 0 {
 		return ctx, sdkerrors.Wrap(sdkerrors.ErrInvalidGasLimit, "must provide positive gas")
 	}
 
-	fee, priority, err := dfd.txFeeChecker(ctx, tx)
-	if err != nil {
-		return ctx, err
+	var (
+	  priority int64
+	  err error
+	)
+
+	fee := feeTx.GetFee()
+	if !simulate {
+		fee, priority, err = dfd.txFeeChecker(ctx, tx)
+		if err != nil {
+			return ctx, err
+		}
 	}
 	if err := dfd.checkDeductFee(ctx, tx, fee); err != nil {
 		return ctx, err

x/auth/ante/fee_test.go

@@ -36,6 +36,10 @@ func (s *AnteTestSuite) TestDeductFeeDecorator_ZeroGas() {
 
 	_, err = antehandler(s.ctx, tx, false)
 	s.Require().Error(err)
+
+	// zero gas is accepted in simulation mode
+	_, err = antehandler(s.ctx, tx, true)
+	s.Require().NoError(err)
 }
 
 func (s *AnteTestSuite) TestEnsureMempoolFees() {
@@ -74,6 +78,11 @@ func (s *AnteTestSuite) TestEnsureMempoolFees() {
 	_, err = antehandler(s.ctx, tx, false)
 	s.Require().NotNil(err, "Decorator should have errored on too low fee for local gasPrice")
 
+	// antehandler should not error since we do not check minGasPrice in simulation mode
+	cacheCtx, _ := s.ctx.CacheContext()
+	_, err = antehandler(cacheCtx, tx, true)
+	s.Require().Nil(err, "Decorator should not have errored in simulation mode")
+
 	// Set IsCheckTx to false
 	s.ctx = s.ctx.WithIsCheckTx(false)