If server is restarted in the middle of the execution of upgrade with DW, on next run DW is confused

To reproduce:

• upgrade a farm from 5.2.1 to 6.2.4, put the war and start the upgrade
• while the upgrade is running on a wiki, restart the server
• when the DW pops in again after the upgrade, depending on the moment when the restart was done, it can be confused, including (but not only):
  • not knowing anymore that a 5.2.1 version was installed before on that wiki (asking to repair something)
  • when going in the administration of that wiki (using a direct link) and looking at installed extensions it will show something about enterprise 6.2.4 being installed
  • I manually modified the extension repository on disk to mark the previous install 5.2.1 as installed on this wiki to re-do the upgrade, but upon restart, while DW recognized properly 5.2.1 as installed, on "Continue" on the first screen, it jumped directly to extensions upgrade instead of going through 6.2.4 install... I have no idea what the status was, if the upgrade to 6.2.4 was complete or not, etc.

Seems that the cause is that DW is writing its status on disk only at the end, and, as it didn't get the chance to end, it didn't write the status (tmortagne said).

While I understand that we can never make it perfect on restart, I think we can improve some of it by writing the status on the disk as it happens, especially as DW could take long (on wikis on which a lot of conflicts have to be resolved, for example, the user may take a long time to make the decisions).