*: improve ErrKeyExists generation and MemoizeStr (#54412)

ref pingcap/tidb#53004

pkg/util/stringutil/string_util.go

@@ -360,10 +360,18 @@ func (l StringerFunc) String() string {
 	return l()
 }
 
-// MemoizeStr returns memoized version of stringFunc.
+// MemoizeStr returns memoized version of stringFunc. When the result of l is not
+// "", it will be cached and returned directly next time.
+//
+// MemoizeStr is not concurrency safe.
 func MemoizeStr(l func() string) fmt.Stringer {
+	var result string
 	return StringerFunc(func() string {
-		return l()
+		if result != "" {
+			return result
+		}
+		result = l()
+		return result
 	})
 }
 

pkg/util/stringutil/string_util_test.go

@@ -206,6 +206,18 @@ func TestEscapeGlobQuestionMark(t *testing.T) {
 	}
 }
 
+func TestMemoizeStr(t *testing.T) {
+	cnt := 0
+	slowStringFn := func() string {
+		cnt++
+		return "slow"
+	}
+	stringer := MemoizeStr(slowStringFn)
+	require.Equal(t, "slow", stringer.String())
+	require.Equal(t, "slow", stringer.String())
+	require.Equal(t, 1, cnt)
+}
+
 func BenchmarkDoMatch(b *testing.B) {
 	escape := byte('\\')
 	tbl := []struct {