common/collections: Always make a copy of the input slice in Append

Fixes #10458.

common/collections/append.go

@@ -31,6 +31,13 @@ func Append(to any, from ...any) (any, error) {
 	var tot reflect.Type
 
 	if !toIsNil {
+		if tov.Kind() == reflect.Slice {
+			// Create a copy of tov, so we don't modify the original.
+			c := reflect.MakeSlice(tov.Type(), tov.Len(), tov.Len()+len(from))
+			reflect.Copy(c, tov)
+			tov = c
+		}
+
 		if tov.Kind() != reflect.Slice {
 			return nil, fmt.Errorf("expected a slice, got %T", to)
 		}

common/collections/append_test.go

@@ -129,3 +129,15 @@ func TestAppendToMultiDimensionalSlice(t *testing.T) {
 	}
 
 }
+
+func TestAppendShouldMakeACopyOfTheInputSlice(t *testing.T) {
+	t.Parallel()
+	c := qt.New(t)
+	slice := make([]string, 0, 100)
+	slice = append(slice, "a", "b")
+	result, err := Append(slice, "c")
+	c.Assert(err, qt.IsNil)
+	slice[0] = "d"
+	c.Assert(result, qt.DeepEquals, []string{"a", "b", "c"})
+	c.Assert(slice, qt.DeepEquals, []string{"d", "b"})
+}