XML encoder does not support UUIDs as keys: makes ImageStyle config entity XML serialization crash

Problem/Motivation

From See #2800873-124: Add XML GET REST test coverage, work around XML encoder quirks:

But for XML [serialization of the ImageStyle config entity], this is the response you get:

Fstring(2801) "The website encountered an unexpected error. Please try again later.</br></br><em class="placeholder">DOMException</em>: Invalid Character Error in <em class="placeholder">DOMDocument-&gt;createElement()</em> (line <em class="placeholder">452</em> of <em class="placeholder">vendor/symfony/serializer/Encoder/XmlEncoder.php</em>). <pre class="backtrace">Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;appendNode(Object, Array, &#039;574d9c11-33ec-4fa7-9a6b-6783600c9cfa&#039;) (Line: 408)
Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;buildXml(Object, Array) (Line: 490)
Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;selectNodeType(Object, Array) (Line: 456)
Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;appendNode(Object, Array, &#039;effects&#039;) (Line: 408)
Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;buildXml(Object, Array, &#039;response&#039;) (Line: 65)
Symfony\Component\Serializer\Encoder\XmlEncoder-&gt;encode(Array, &#039;xml&#039;, Array) (Line: 60)
Drupal\serialization\Encoder\XmlEncoder-&gt;encode(Array, &#039;xml&#039;, Array) (Line: 38)
Symfony\Component\Serializer\Encoder\ChainEncoder-&gt;encode(Array, &#039;xml&#039;, Array) (Line: 233)
Symfony\Component\Serializer\Serializer-&gt;encode(Array, &#039;xml&#039;, Array) (Line: 118)
Symfony\Component\Serializer\Serializer-&gt;serialize(Array, &#039;xml&#039;) (Line: 156)
Drupal\rest\EventSubscriber\ResourceResponseSubscriber-&gt;Drupal\rest\EventSubscriber\{closure}() (Line: 574)
Drupal\Core\Render\Renderer-&gt;executeInRenderContext(Object, Object) (Line: 157)
Drupal\rest\EventSubscriber\ResourceResponseSubscriber-&gt;renderResponseBody(Object, Object, Object, &#039;xml&#039;) (Line: 74)
Drupal\rest\EventSubscriber\ResourceResponseSubscriber-&gt;onResponse(Object, &#039;kernel.response&#039;, Object) (Line: 108)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher-&gt;dispatch(&#039;kernel.response&#039;, Object) (Line: 193)
Symfony\Component\HttpKernel\HttpKernel-&gt;filterResponse(Object, Object, 1) (Line: 175)
Symfony\Component\HttpKernel\HttpKernel-&gt;handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel-&gt;handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle-&gt;handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache-&gt;pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache-&gt;handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware-&gt;handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware-&gt;handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel-&gt;handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel-&gt;handle(Object) (Line: 19)
</pre>"

The root cause:

        $this->effectUuid => [
          'uuid' => $this->effectUuid,
          'id' => 'image_scale_and_crop',
          'weight' => 0,
          'data' => [
            'width' => 120,
            'height' => 121,
          ],
        ],

i.e. a UUID is being used as a key for an array, which means that the XML encoder will try to map this to an XML tag (element). And dashes aren't allowed in XML tag/element names, hence this exception.

I don't see a way to work around this. But it also shouldn't hold up everything else. So created a new issue for that: #2905655: XML encoder does not support UUIDs as keys: makes ImageStyle config entity XML serialization crash.

Therefore for now including stub test coverage.

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

None.

API changes

TBD

Data model changes

TBD