Cannot implement a custom user cancellation method

Hi, the patch applied successfully in Drupal 8.9.x version.
Great Work @claudiu.cristea

This is indeed a good notice. This whole thing starts from user_cancel_methods which throws a hook to allow generate a new way of cancelling the account.
Then, as noted in the issue description, the user_cancel method does fire the default method when a new method is defined.
All other default methods are handled explicitly.

A few nitpicks.

1. +++ b/core/modules/user/tests/src/Functional/UserCancelCustomMethodTest.php
   @@ -0,0 +1,60 @@
   +    $this->assertSession()->pageTextNotContains("{$account->getAccountName()} has been disabled.");
   

   Can we change the method used to ::getDisplayName? I don't think it will make the test fail though it is the one used by _user_cancel for the message display.

2. +++ b/core/modules/user/tests/src/Functional/UserCancelCustomMethodTest.php
   @@ -0,0 +1,60 @@
   +   * Clicks on the conformation link sent by email.
   

   Typo in "conformation".

3. +++ b/core/modules/user/tests/src/Functional/UserCancelCustomMethodTest.php
   @@ -0,0 +1,60 @@
   +    preg_match('#http.*#', $mail['body'], $found);
   

   If I am not mistaken, this will fetch a malformed URL unless the url is the last piece of string in the line.
   I tested this in https://www.functions-online.com/preg_match.html with the string "http://some.test.com. Hello world." and the whole sentense is returned.
   In the current case, this works because the default cancel email contains the \n\n[user:cancel-url]\n\n but I don't know if this is future proof. What do you think? We can keep it of course. The #http[^ \n]*# seems to cover all cases currently (it does not cover a change where the url ends with a fullstop as part of a sentence).

@claudiu.cristea to be honest, that was what I was thinking for the 3rd point but put it up there just to be sure. It is always better to have it written than just somebody having thought of it at some point.
This is fine as far as I am concerned. I don't know if more RTBCs have to be created. I am putting it in RTBC but I am not sure if any change record is needed since it is listed as a bug report, but stops sending notifications for custom cancellation methods.

1. +++ b/core/modules/user/tests/modules/user_cancel_test/user_cancel_test.info.yml
   @@ -0,0 +1,6 @@
   +core: 8.x
   

   This needs removing so we're compatible with 9.x

2. +++ b/core/modules/user/user.module
   @@ -781,7 +781,6 @@ function _user_cancel($edit, $account, $method) {
      switch ($method) {
        case 'user_cancel_block':
        case 'user_cancel_block_unpublish':
   -    default:
   

   This is quite a big change. Someone might have implemented and be relying on it. There's plenty of contrib that implements the hook - see http://codcontrib.hank.vps-private.net/search?text=user_cancel_methods_a... - so this code is currently being triggered for them and and now it won't be.

   I think we need to consider another way for code to opt out from this.

@claudiu.cristea we've just released an RC there's no way we can make this in the same way in 9.0.0. We need a BC layer. How about a new hook whose cancel methods don't result in this being called. Or moving to some event system where the default is always the last subscriber and you can stop propagation if you don't want its effects.

Fixed point 1 of #12. Point 2 is not yet addressed and requires a full rework of the patch.

Hiding patches because this is an MR based issue

There's a fair bit of overlap with #3043725: Provide a Entity Handler for user cancelation here, it would be good to compare notes and join forces with the folks over there. Having multiple people working towards fixing a legacy system in different ways isn't ideal, but I think working together will make things move faster as you have multiple hands for things like reviews etc

Left some comments on the MR

Updating title to describe all the things going on here for those seeing it in related links etc, please amend if I'm off piste

@claudiu.cristea any update here?

In addition, after creating a custom user cancellation handler, there's no way to select that method when using drush user:cancel. It'd be nice if there was a generic --method=[method] argument, rather than just --delete-content and --reassign-content.