events.xml cant have no childrens, others can [Magento 2.2.4]

[bytes-commerce]

Preconditions

1. Using Magento 2.2.4
2. PHP 7.0.30

Steps to reproduce

1. Create new module
2. Create di.xml, events.xml, crontab.xml additionally to needed module.xml
3. Fill all files correctly with its nessecary headers but dont put any children in it
   Example for di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
</config>

4. run php bin/magento setup:upgrade && php bin/magento cache:flush && php bin/magento cache:clean

Expected result

1. Upgrade + Flush + Clean should run fine trough, page is loading afterwards in developer mode

Actual result

1. Upgrade runs fine, Cache Flush triggers the error (cache:clean does not)
2. Error Message:

  [Magento\Framework\Exception\LocalizedException]                               
  Invalid XML in file /var/www/html/app/code/Vendor/Modulename/etc/events.xml:  
  Element 'config': Missing child element(s). Expected is ( event ).             
  Line: 2        

This is not logically as it should render fine, process the file even with no config in it, as it process other files.

[engcom-backlog-nickolas]

Hello @NopeNopeNope, thank you for your report.
We've acknowledged the issue and added to our backlog.

Looks like the only thing which should be changed is set minOccurs from "1" to "0":

magento2/lib/internal/Magento/Framework/Event/etc/events.xsd

Line 12 in 075de96

<xs:element name="event" type="eventDeclaration" minOccurs="1" maxOccurs="unbounded">

If you'd like, you can create pull request to fix this issue.

[arnoudhgz]

@engcom-backlog-nickolas are you sure you need to fix this by setting minOccurs from "1" to "0"? Should the developer just not create the events.xml file since no events are added?

The crontab.xsd also has the minOccurs set to "1" for the group element in the config part. Maybe the di.xml should have a more strict xsd to require at least one preference, type or virtualType.

I think your suggestion will make the code less strict and therefore not a step in the good direction.

Please advice to not add an empty events.xml when you do not need one.

[bytes-commerce]

@arnoudhgz

Whatever way they choose, it should be unified. Right now there can be empty XMLs, but no empty events.xml.

[bytes-commerce]

@arnoudhgz if you downvote something, one should have the courage to explain why its a bad idea. Please elaborate if possible.

[arnoudhgz]

@NopeNopeNope I don't think everything should work the same, a di.xml and a events.xml have a different purpose. It is nice that events.xml can't be added empty, that the xsd blocks this for you.

Don't add empty files if you need them. My opinion is that it should not be unified with 'whatever way they choose'. The can improve other xsds, but they should never downgrade the xsd for the events by making it less strict just to have in a unified way.