Problem/Motivation
Let's assume two users enable two different modules on the admin/modules page at the same time.
Then the slightly slower attempt will win, as its comes after the first one
Proposed resolution
Add a lock around module install.
Inject the lock service into the module installer service (\Drupal\Core\Extension\ModuleInstaller)
Wrap the ::install method in a lock - see the Lock API for details.
If the lock can't be acquired after waiting, throw an exception
Add a test to \Drupal\KernelTests\Core\Extension\ModuleInstallerTest to ensure the lock prevents two people from installing at the same time
Add code to \Drupal\system\Form\ModulesListForm::submitForm and \Drupal\system\Form\ModulesListConfirmForm::submitForm to catch the thrown exception and display a message to the user
Add tests to \Drupal\Tests\system\Functional\Form\ModulesListFormWebTest for these scenarios
Remaining tasks
Review + Merge
User interface changes
N/A
API changes
ModuleInstaller now has a 6th (currently optional) parameter, Lock. This is optional for BC purposes only, this is actually required and defaulted if not present.
Data model changes
N/A