Let plugin bags use the DependencySerializationTrait (was: EditorAdminTest fails on PHP 5.4 silently with unicorn editor)

The test has been broken since... #2205367: [HEAD BROKEN] PHP 5.4. ;)

By the way, my error message is funnier:

  call_user_func_array() expects parameter 1 to be a valid callback, class 'A
    list of HTML tags that can be used. JavaScript event attributes, JavaScript
    URLs, and CSS are always stripped.' not found

This only occurs on PHP 5.4 - I'm on PHP 5.4.16 - but this does not occur in PHP 5.5.

It appears to be an object serialisation issue since if I change the #element_validate assignment to use get_class() like so

  $settings_form['#element_validate'][] = array(get_class($plugin), 'settingsFormValidate');

I get a more reasonable error message of:

    call_user_func_array() expects parameter 1 to be a valid callback,
    non-static method
    Drupal\editor_test\Plugin\Editor\UnicornEditor::settingsFormValidate()
    should not be called statically

The patch attached fixes the issue for me. The issue seems to be with unserialising two instances of the same object.

Does this mean we want to clone every object we invoke a Form API callback on? (Because all of those may get serialized.)

No - the clone solution sucks - I was just posting the results so far of my investigation so maybe someone knows what's going on.

So we serialize the FilterPluginBag in the form which leads us to serializing the FilterPluginManager which means we serialize all sorts of other stuff. PHP 5.4 serialization does seem to be fragile but stopping this fixes this issue. Also offers a nice performance enhancement since the serialized form is much smaller.

Before

mysql> select length(value) from simpletest480873key_value_expire where name = 'form--BaLyxi46ixwojo7adHrLcThPK90G8r4sSNmfVl6D4s' and collection = 'form';
+---------------+
| length(value) |
+---------------+
|         48092 |
+---------------+

After

mysql> select length(value) from simpletest246321key_value_expire where name = 'form-ph-sBxFBf5zfy-PztVee8gC2pRdm8vUFyw_6i4c90C4' and collection = 'form';
+---------------+
| length(value) |
+---------------+
|         29331 |
+---------------+
1 row in set (0.00 sec)

#2199795: Make the Settings class prevent serialization of actual settings is proposing the plugin bags need to be serialized using dependency serialization too

#2208115: Add DependencySerializationTrait will make this simpler

What is this postponed on? Can we put the issue in the summary?

It was postponed on #2208115: Add DependencySerializationTrait and #2199795: Make the Settings class prevent serialization of actual settings. The first of those is now committed, the second one isn't yet, and that's what this is postponed on.

With #2208115: Add DependencySerializationTrait committed, #8 can be rerolled to be much simpler. So I did that here.

But, looking at #2199795-9: Make the Settings class prevent serialization of actual settings, it looks like a different solution was chosen (I think because of comment 5 in that issue). If that's the case, then this issue can implement that solution. I think alexpott should decide what to do here; I'm not knowledgeable enough in this area.

Patch #13 rerolled.

Patch no longer applies.

\Drupal\Core\Plugin\DefaultLazyPluginCollection uses DependencySerializationTrait already, I think this is not necessary, we don't want to add this trait to actual services.