expression: fix tikv crash when bool like cast(bit as char)

pkg/expression/builtin_cast.go

@@ -300,10 +300,20 @@ func (c *castAsStringFunctionClass) getFunction(ctx BuildContext, args []Express
 	if err != nil {
 		return nil, err
 	}
-	if args[0].GetType(ctx.GetEvalCtx()).Hybrid() {
-		sig = &builtinCastStringAsStringSig{bf}
-		sig.setPbCode(tipb.ScalarFuncSig_CastStringAsString)
-		return sig, nil
+	if ft := args[0].GetType(ctx.GetEvalCtx()); ft.Hybrid() {
+		castBitAsUnBinary := ft.GetType() == mysql.TypeBit && c.tp.GetCharset() != charset.CharsetBin
+		if !castBitAsUnBinary {
+			sig = &builtinCastStringAsStringSig{bf}
+			sig.setPbCode(tipb.ScalarFuncSig_CastStringAsString)
+			return sig, nil
+		}
+		// for type BIT, it maybe an invalid value for the specified charset, we need to convert it to binary first,
+		// and then convert it to the specified charset with `HandleBinaryLiteral` in the following code.
+		tp := types.NewFieldType(mysql.TypeString)
+		tp.SetCharset(charset.CharsetBin)
+		tp.SetCollate(charset.CollationBin)
+		tp.AddFlag(mysql.BinaryFlag)
+		args[0] = BuildCastFunction(ctx, args[0], tp)
 	}
 	argTp := args[0].GetType(ctx.GetEvalCtx()).EvalType()
 	switch argTp {

pkg/expression/builtin_convert_charset.go

@@ -173,6 +173,7 @@ type builtinInternalFromBinarySig struct {
 func (b *builtinInternalFromBinarySig) Clone() builtinFunc {
 	newSig := &builtinInternalFromBinarySig{}
 	newSig.cloneFrom(&b.baseBuiltinFunc)
+	newSig.cannotConvertStringAsWarning = b.cannotConvertStringAsWarning
 	return newSig
 }
 

pkg/expression/expr_to_pb_test.go

@@ -1581,7 +1581,8 @@ func TestExprPushDownToTiKV(t *testing.T) {
 	require.Len(t, pushed, 0)
 	require.Len(t, remained, len(exprs))
 
-	// Test Conv function
+	// Test Conv function, `conv` function for a BIT column should not be pushed down for its special behavior which
+	// is only handled in TiDB currently.
 	exprs = exprs[:0]
 	function, err = NewFunction(mock.NewContext(), ast.Conv, types.NewFieldType(mysql.TypeString), stringColumn, intColumn, intColumn)
 	require.NoError(t, err)
@@ -1590,7 +1591,11 @@ func TestExprPushDownToTiKV(t *testing.T) {
 	require.Len(t, pushed, len(exprs))
 	require.Len(t, remained, 0)
 	exprs = exprs[:0]
-	castByteAsStringFunc, err := NewFunction(mock.NewContext(), ast.Cast, types.NewFieldType(mysql.TypeString), byteColumn)
+	// when conv a column with type BIT, a cast function will be used to cast bit to a binary string
+	castTp := types.NewFieldType(mysql.TypeString)
+	castTp.SetCharset(charset.CharsetBin)
+	castTp.SetCollate(charset.CollationBin)
+	castByteAsStringFunc, err := NewFunction(mock.NewContext(), ast.Cast, castTp, byteColumn)
 	require.NoError(t, err)
 	function, err = NewFunction(mock.NewContext(), ast.Conv, types.NewFieldType(mysql.TypeString), castByteAsStringFunc, intColumn, intColumn)
 	require.NoError(t, err)

tests/integrationtest/r/expression/cast.result

@@ -178,3 +178,18 @@ id	update_user
 2	李四
 4	李四
 1	张三
+drop table if exists test;
+create table test(a bit(24));
+insert into test values('中');
+select a from test where '中' like convert(a, char);
+a
+中
+select a from test where false not like convert(a, char);
+a
+中
+select a from test where false like convert(a, char);
+a
+truncate table test;
+insert into test values(0xffffff);
+select a from test where false not like convert(a, char);
+Error 1105 (HY000): Cannot convert string '\xFF\xFF\xFF' from binary to utf8mb4

tests/integrationtest/t/expression/cast.test

@@ -113,3 +113,14 @@ insert into test values(3,'张三');
 insert into test values(4,'李四');
 select * from test order by cast(update_user as char) desc , id limit 3;
 
+# issue #56494, cast bit as char
+drop table if exists test;
+create table test(a bit(24));
+insert into test values('中');
+select a from test where '中' like convert(a, char);
+select a from test where false not like convert(a, char);
+select a from test where false like convert(a, char);
+truncate table test;
+insert into test values(0xffffff);
+-- error 1105
+select a from test where false not like convert(a, char);