[Libguestfs] supermin root: race condition with multiple drives

[Brian Candler]

Hi,

I discovered an issue when using libguestfs with large numbers of 
attached disks.  I submitted the details to github:

https://github.com/libguestfs/libguestfs/issues/69

... and then discovered that the mailing list is the right place, not 
github.  Sorry about that!

The problem is: I have batches of 40 or 50 qcow2 images to write files 
to.  It is very slow to start a separate libguestfs appliance for each 
one, so what I do is to start a single one with 40 or 50 disks attached, 
and then mount, upload and unmount each one in turn.

What I find is that sometimes the disks are attached in the wrong order, 
such that the kernel tries to use one of these qcow2 files as its root 
disk, instead of the supermin appliance image.  This seems to happen 
more often when the system is under load, such as when running multiple 
libguestfs instances concurrently (I have 15 or 20 different versions of 
these batches of 40-50 disks to create, so to speed things up, I run 
them concurrently).

This is all "userland" stuff so ought to work fine under load, but the 
supermin kernel booting issue messes it up intermittently.

Anyway, the github ticket has full details, including standalone scripts 
which can reproduce the problem on my system.  I'd be grateful if 
someone could take a look.

Many thanks,

Brian Candler.