[Backport 2.1-develop] CMS Page - Force validate layout update xml in production mode when saving CMS Page - Handle layout update xml validation exceptions

[adrian-martinez-interactiv4]

When saving custom layout update xml in CMS page, in production mode, xml is not validated against schema file, due to \Magento\Framework\App\Arguments\ValidationState::isValidationRequiredmethod:

    /**
     * Retrieve current validation state
     *
     * @return boolean
     */
    public function isValidationRequired()
    {
        return $this->_appMode == \Magento\Framework\App\State::MODE_DEVELOPER;
    }

Actual result:

Desired behaviour would be check the layout update xml against the schema, and show error message to the user if it is invalid:

Also, when layout update is validated, validation exceptions are unhandled, showing a report error in production mode, and showing this error in developer mode:

For example, uncomment demo layout handle in home page:

Desired behaviour would be handle the exception properly instead of crash, and show error message to the user:

Description

• According to doc block, \Magento\Cms\Controller\Adminhtml\Page\PostDataProcessor::validate should always return a boolean value. It uses \Magento\Framework\View\Model\Layout\Update\Validator::isValid method, that may throw an exception if xml is not valid, so \Magento\Cms\Controller\Adminhtml\Page\PostDataProcessor::validate must handle possible exceptions thrown by xml validator, in order to always return a boolean value.
• \Magento\Cms\Model\Page\DomValidationState has been added, and it is injected from post data processor to force layout update xml validation, only upon cms page save.

This PR include changes of PR #11866

Manual testing scenarios

1. Enable developer mode

2. Edit or create a CMS page, enter some invalid xml in Layout Update XML textarea

3. Try to save the page, application crashes

4. Enable production mode

5. Edit or create a CMS page, enter some invalid xml in Layout Update XML textarea

6. Try to save the page, xml does not get validated

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[dmanners]

Will put this on hold until the PR to 2.2-develop is merged. Then we can process the backports.