raid assembly can break into divergent parts with conflicting changes

Consider a laptop with a raid setup that consists of one internal disk and a second external disk that resides in the laptops docking station.

Undocking the laptop during operation, causes the raid to switch into degraded operation and the laptop is turned off afterwards.

Back in the office, the laptop gets mounted in the dock and is switched on. The hardware setup is such that the disk in the docking station is found first.

Mdadm --incremental sets up the raid device with the external disk (the internal disk is refused by --incremental or may have been ejected). Because the raid does not come up completely, the raid device is then started in degraded mode with only the external disk.

We now have two divergent parts of the same raid device. The machine runns with the old disk state as of the undocking of the laptop. All changes made during the undocked state have been saved to the internal disks, but they are not used now. New changes will only be written to the external disk.

Suggested Fix:
Store the state of the event counter at the time of the degradation for each missing device in the superblocks on the remaining member devices.
--incremental should continue to (re)add a device automatically (only) if the event count shows the state of the new member device that is to be (re)added is equal or older than at the time the device failed.

When the incrementally assembled raid device is already running in the (auto) read only state from an device that had failed earlier, attempt to switch to the newer state of the added device, if that device correctly describes the failed state of the older device (older device event counter is equal or older than at the time it failed). Otherwise, abort and print an error message that the devices contain conflicting changes.