UPDATE: the bug is already fixed after a few hours, and only affected a minority of users (of a now deprecated, experimental option in the ‘unstable’ distribution, and only users that rebooted with the affected version).

The sysvinit version that hit unstable today has a grave bug if you have been running “startpar” or maybe “shell” style parallel booting. Read this bug report, if you have been using these (they were not enabled by default, so unless you’ve been giving parallel boot a try before, you should be ok.)

How to check if you are affected:

grep CONCURRENCY /etc/default/rcS

If this command says “startpar”, then you ARE affected. If it says “shell” you MIGHT be affected. If you have not set CONCURRENCY or if it’s “none” or “makefile”, then you should be ok (according to the bug).

The workaround is as simple: just put either “none” or “makefile” in there, these are the only two values that are still distinct.

How to recover a broken system:

  1. Boot recovery mode aka “single-user”. At some point you should be asked for the root password. Login.
  2. Run mount -o remount,rw / to enable write mode on your disk.
  3. $EDITOR /etc/default/rcS and change the value of “CUNCURRENCY”
  4. reboot

You should have a working system again.

I can only confirm that changing “startpar” to “none” helped me. I havn’t tried “makefile” yet, and “none” seemed more likely to fix things.