reset_machine_id: ask for forgiveness, not permission

[fsateler]

Avoid TOCTOU by always deleting and handling the FileNotFoundError

[keszybz]

OK. But why not use try..else, and get rid of the boolean?

[fsateler]

I'm not sure what the else would be for. I could lump the symlink into the same try block though..

[keszybz]

try:
  os.unlink(dbus_machine_id)
except FileNotFoundError:
  pass
else:
  os.symlink('../../../etc/machine-id', dbus_machine_id)

[poettering]

i figure that was my bad suggestion. I forgot that you can do the "else" syntax when catching exceptions...

[fsateler]

Actually, what I want is to create the symlink only if the file already existed. I do not want to create it if we didn't remove it. That would imply creating the /var/lib/dbus dir if it doesn't exist, which I don't think we should be doing. That leaves us with either this PR or:

try:
  os.unlink(dbus_machine_id)
  os.symlink('../../../etc/machine-id', dbus_machine_id)
except FileNotFoundError:
  pass

[keszybz]

My code does what you described.

[poettering]

@fsateler no. the "else" branch of a "try" statement is only executed when no exception is thrown. The difference between the version you are now proposing and @keszybz's is that in your version "os.symlink()" throwing FileNotFoundError is ignored, while in @keszybz's version it is not. And I think @keszybz' version is right here: we should only ignore the errors we really want to ignore, and that's the ones from os.unlink().

[fsateler]

Oh, its clear I did not know how the else clause works. Force pushed a new version using that.

Thanks for the patience!