What ostree operations are safe to do concurrently

[Alexander Larsson <alexl redhat com>]

We're looking at starting to make gnome-software support xdg-app, and
as part of that it has to do things like installs and updates. At this
point we need to be more careful about things like concurrent
operations, as these affects the design of this stuff.

So, I'm pretty sure that most read-only operations are safe in
parallel. At least as long as nobody is removing anything, but i guess
if that happens we get a natural fail (i.e. partial listing, etc).

But, what about write changes like a pull? Is this safe for multiple
callers? If it is not, how should i protect things from problems. Do i
need a repo-global lock, or can i somehow do partial locking?