_user_mail_notify() doesn't handle accounts without an email address

Problem/Motivation

User accounts that don't have an email address can be created by administrators. _user_mail_notify() assumes that when it's passed a user account that it has an address. When that happens errors occur. It happens for any user action that sends an email. It can happen via the admin UI or Drush. This issue has resulted in at least three different issues that I could find, but there may be more:

• #3312582: When a user account doesn't have an email, new user notification email attempt shouldn't be made
• #3355151: Exception thrown when unblocking a user via drush
• #3481894: Error when trying to reset password of the account without email address

These issues have proposed patches, but they all try to stop the email from being sent at the source, e.g. the User entity or the password reset form. None of them try to prevent _user_mail_notify() from sending an email to a NULL address, which means the inherent bug will persist. This issue will take that approach.

Afterward, some of the related issues may simply be closed. Some may be repurposed as follow-ups. For instance, the password reset form may want to print an extra message if an email can't be sent.

Steps to reproduce

1. Log into the site as an administrator.
2. Create a new user. The user should not have an email address.
3. Block the user.

Expected result

The user should be blocked with no attempt to send an email.

Actual result

[error]  TypeError: Drupal\Core\Mail\Plugin\Mail\PhpMail::doMail(): Argument #1 ($to) must be of type string, null given, called in /var/www/html/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php on line 123 in Drupal\Core\Mail\Plugin\Mail\PhpMail->doMail() (line 168 of /var/www/html/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php) #0 /var/www/html/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php(123): Drupal\Core\Mail\Plugin\Mail\PhpMail->doMail()
#1 /var/www/html/core/lib/Drupal/Core/Mail/MailManager.php(308): Drupal\Core\Mail\Plugin\Mail\PhpMail->mail()
#2 /var/www/html/core/lib/Drupal/Core/Mail/MailManager.php(181): Drupal\Core\Mail\MailManager->doMail()
#3 /var/www/html/core/lib/Drupal/Core/Render/Renderer.php(600): Drupal\Core\Mail\MailManager->Drupal\Core\Mail\{closure}()
#4 /var/www/html/core/lib/Drupal/Core/Mail/MailManager.php(180): Drupal\Core\Render\Renderer->executeInRenderContext()
#5 /var/www/html/core/modules/user/user.module(659): Drupal\Core\Mail\MailManager->mail()
#6 /var/www/html/core/modules/user/src/Entity/User.php(162): _user_mail_notify()
#7 /var/www/html/core/lib/Drupal/Core/Entity/EntityStorageBase.php(564): Drupal\user\Entity\User->postSave()
#8 /var/www/html/core/lib/Drupal/Core/Entity/ContentEntityStorageBase.php(796): Drupal\Core\Entity\EntityStorageBase->doPostSave()
#9 /var/www/html/core/lib/Drupal/Core/Entity/EntityStorageBase.php(490): Drupal\Core\Entity\ContentEntityStorageBase->doPostSave()
#10 /var/www/html/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php(806): Drupal\Core\Entity\EntityStorageBase->save()
#11 /var/www/html/core/lib/Drupal/Core/Entity/EntityBase.php(370): Drupal\Core\Entity\Sql\SqlContentEntityStorage->save()
#12 /var/www/html/vendor/drush/drush/src/Commands/core/UserCommands.php(154): Drupal\Core\Entity\EntityBase->save()
#13 [internal function]: Drush\Commands\core\UserCommands->unblock()
#14 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(276): call_user_func_array()
#15 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(212): Consolidation\AnnotatedCommand\CommandProcessor->runCommandCallback()
#16 /var/www/html/vendor/consolidation/annotated-command/src/CommandProcessor.php(175): Consolidation\AnnotatedCommand\CommandProcessor->validateRunAndAlter()
#17 /var/www/html/vendor/consolidation/annotated-command/src/AnnotatedCommand.php(387): Consolidation\AnnotatedCommand\CommandProcessor->process()
#18 /var/www/html/vendor/symfony/console/Command/Command.php(279): Consolidation\AnnotatedCommand\AnnotatedCommand->execute()
#19 /var/www/html/vendor/symfony/console/Application.php(1094): Symfony\Component\Console\Command\Command->run()
#20 /var/www/html/vendor/symfony/console/Application.php(342): Symfony\Component\Console\Application->doRunCommand()
#21 /var/www/html/vendor/symfony/console/Application.php(193): Symfony\Component\Console\Application->doRun()
#22 /var/www/html/vendor/drush/drush/src/Runtime/Runtime.php(110): Symfony\Component\Console\Application->run()
#23 /var/www/html/vendor/drush/drush/src/Runtime/Runtime.php(40): Drush\Runtime\Runtime->doRun()
#24 /var/www/html/vendor/drush/drush/drush.php(140): Drush\Runtime\Runtime->run()
#25 /var/www/html/vendor/bin/drush.php(119): include('...')
#26 {main}.

Proposed resolution

Add a check to _user_mail_notify() to verify that the account has an email address. Log info if it doesn't have one.

Remaining tasks

• Agree on the message to be displayed
• Review
• Commit
• Evaluate related issues to see if some can be closed

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet