Addressing global removal race in Wayland

Summary

Global objects are one of the core abstractions in Wayland. They are used to announce supported protocols or announce semi-transient objects, for example available outputs. The compositor can add and remove global objects on the fly. For example, an output global object can be created when a new monitor becomes available, and be removed when the corresponding monitor gets disconnected. While there are no issues with announcing new global objects, the global removal is a racy process and if things go bad, your application will crash. Application crashes due to output removal were and still are fairly common, unfortunately. The race Let’s start from the start. If the compositor detects a new output, it may announce the corresponding wl_output global object. A client interested in outputs will then eventually bind it and receive the information about the output, e.g. the name, geometry, and so on: A new wl_output announcement. Things get interesting when it is time to remove the wl_output global. Ideally, after telling clients that the given global has been removed, nobody should use or bind it. If a client has bound the wl_output global object, it should only perform cleanup. The preferred way to handle wl_output global removal. However, it can happen that a wl_output global is removed shortly after it has been announced. If a client attempts to bind that wl_output at the same time, there is a big problem now. When the bind request finally arrives at the compositor side, the global won’t exist anymore and the only option left will be to post a protocol error and effectively crash the client. A wl_output global removal race. Attempt #1 Unfortunately, there is not a lot that can be done about the in-flight bind requests, they have to be handled. If we could only tell the clients that a given global has been removed but not destroy it yet, that would help with the race, we would still be able to process bind requests. Once the compositor knows that nobody will bind the g...