[systemd-devel] Masking mount units

[Phillip Susi]

Lennart Poettering <lennart at poettering.net> writes:

> Doing the locking on the fd you use for writing makes things a lot
> easier, because as mentioned udev will automatically retrigger block
> devices if an inotify event on it is seen that indicates
> "close-after-write". If you deal with multiple fds you need to make

That is exactly what I DON'T want to happen.  If udev retriggers when
the fd is closed, then that results in the partition being automatically
mounted just because libparted read the partition table.

> sure to close the fd used for writing *after* having released the
> lock on the other one, because if you do it the other way round, udev
> will retigger the device, but then not be able to lock device and not
> actually do anything, and then no further inotify is seen and things
> will remain out ofdate.

That is what I WANT to happen.  Well, at least I don't want a partition
to be automatically mounted.  It would be nice if udevdb could still be updated.

>> Then again, this will also prevent the udevdb from being updated.  Maybe
>> that is a bigger hammer than you really want when the goal is just to
>> stop auto mounting?
>
> If you change the superblock or part table of your disk, udev
> shouldn't read half-written data, so yes, it's *good* it doesn't
> update its udevdb. That's the whole point of the exercise!
>
> I am not sure I grok what the problem is supposed to be here...

Right, you don't want it to read half updated data, but if I
intentionally prevent udev from looking at the disk _at all_ ( so that
partitions don't get auto mounted ), then it also means that it never
looks at the finished updates either and updates udevdb.

> parted should just open the main block device for writing, take an
> exclusive BSD lock on it, then do its thing, and close the fd. It's
> very easy, and will bock generation of udev's uevents on the thing,
> will block udev from reading half-written data and everything else.

Yes, but then it reads the disk and auto mounts a partition just because
someone ran parted print.  Printing the partition table should not
trigger auto mount.

Hrm... now that I think about it, why does a CHANGE event trigger auto
mount?  Shouldn't that only be done on ADD?