fix types.ChainAnteDecorators() panic (cosmos#5742)

* fix types.ChainAnteDecorators() panic

ChainAnteDecorators() panics when no arguments are supplied.
This change its behaviour and the function now returns a nil
AnteHandler in case no AnteDecorator instances are supplied.

Closes: cosmos#5741

* Append Terminator to non-terminated chains.

* Fix test case

CHANGELOG.md

@@ -74,6 +74,7 @@ resulted in a panic when the tx execution mode was `CheckTx`.
 * (client) [\#5618](https://github.com/cosmos/cosmos-sdk/pull/5618) Fix crash on the client when the verifier is not set.
 * (x/distribution) [\#5620](https://github.com/cosmos/cosmos-sdk/pull/5620) Fix nil pointer deref in distribution tax/rewward validation helpers.
 * (genesis) [\#5086](https://github.com/cosmos/cosmos-sdk/issues/5086) Ensure `gentxs` are always an empty array instead of `nil`
+* (types) [\#5741](https://github.com/cosmos/cosmos-sdk/issues/5741) Prevent ChainAnteDecorators() from panicking when empty AnteDecorator slice is supplied.
 
 ### State Machine Breaking
 

types/handler.go

@@ -25,15 +25,15 @@ type AnteDecorator interface {
 // MUST set GasMeter with the FIRST AnteDecorator. Failing to do so will cause
 // transactions to be processed with an infinite gasmeter and open a DOS attack vector.
 // Use `ante.SetUpContextDecorator` or a custom Decorator with similar functionality.
+// Returns nil when no AnteDecorator are supplied.
 func ChainAnteDecorators(chain ...AnteDecorator) AnteHandler {
-	if (chain[len(chain)-1] != Terminator{}) {
-		chain = append(chain, Terminator{})
+	if len(chain) == 0 {
+		return nil
 	}
 
-	if len(chain) == 1 {
-		return func(ctx Context, tx Tx, simulate bool) (Context, error) {
-			return chain[0].AnteHandle(ctx, tx, simulate, nil)
-		}
+	// handle non-terminated decorators chain
+	if (chain[len(chain)-1] != Terminator{}) {
+		chain = append(chain, Terminator{})
 	}
 
 	return func(ctx Context, tx Tx, simulate bool) (Context, error) {

types/handler_test.go

@@ -0,0 +1,28 @@
+package types_test
+
+import (
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/require"
+
+	"github.com/cosmos/cosmos-sdk/tests/mocks"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+)
+
+func TestChainAnteDecorators(t *testing.T) {
+	t.Parallel()
+	// test panic
+	require.Nil(t, sdk.ChainAnteDecorators([]sdk.AnteDecorator{}...))
+
+	ctx, tx := sdk.Context{}, sdk.Tx(nil)
+	mockCtrl := gomock.NewController(t)
+	mockAnteDecorator1 := mocks.NewMockAnteDecorator(mockCtrl)
+	mockAnteDecorator1.EXPECT().AnteHandle(gomock.Eq(ctx), gomock.Eq(tx), true, gomock.Any()).Times(1)
+	sdk.ChainAnteDecorators(mockAnteDecorator1)(ctx, tx, true)
+
+	mockAnteDecorator2 := mocks.NewMockAnteDecorator(mockCtrl)
+	mockAnteDecorator1.EXPECT().AnteHandle(gomock.Eq(ctx), gomock.Eq(tx), true, mockAnteDecorator2).Times(1)
+	mockAnteDecorator2.EXPECT().AnteHandle(gomock.Eq(ctx), gomock.Eq(tx), true, nil).Times(1)
+	sdk.ChainAnteDecorators(mockAnteDecorator1, mockAnteDecorator2)
+}