fix: auto gas don't work (backport cosmos#12590) (cosmos#12602)

CHANGELOG.md

@@ -46,6 +46,7 @@ It is strongly recommended to upgrade to these releases as well.
 
 ### Improvements
 
+* [#12589](https://github.com/cosmos/cosmos-sdk/pull/12589) Allow zero gas in simulation mode.
 * [#12453](https://github.com/cosmos/cosmos-sdk/pull/12453) Add `NewInMemoryWithKeyring` function which allows the creation of in memory `keystore` instances with a specified set of existing items.
 * [#11390](https://github.com/cosmos/cosmos-sdk/pull/11390) `LatestBlockResponse` & `BlockByHeightResponse` types' `Block` filed has been deprecated and they now contains new field `sdk_block` with `proposer_address` as `string`
 

x/auth/ante/fee.go

@@ -52,13 +52,21 @@ 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
+		}
 	}
 
 	newCtx := ctx.WithPriority(txPriority)

x/auth/ante/fee_test.go

@@ -45,6 +45,10 @@ 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() {
@@ -83,6 +87,11 @@ 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)