[Rpm-maint] [rpm-software-management/rpm] WIP/RFE: Hint to users to use ostree/rpm-ostree if we get EROFS (#320)

[Colin Walters]

> @cgwalters: This patch looks very specific to RPM-OSTree. Is there not a better, more general way to do this?

More general to...other image systems that happen to use rpm?  Possibly.  As far as I'm aware though rpm-ostree is fairly unique in the way it's a hybrid image/package system.   I guess the old oVirt Node "classic" model might apply, AFAIK they shipped rpm but had no unlock functionality even.  (slapping an overlayfs on /usr is really handy!).  But they switched to https://github.com/fabiand/imgbased with everything writable so `yum` works.  (But `yum` is totally unaware of the underlying image system)

> if ostree permits live update even with -EROFS, then your patch should teach rpm to do similar live-update, not spew nagware adverts.

Well, it's not an advertisement - we don't and will never support librpm doing (persistent¹) writes.  This isn't like how `dnf` (used to thankfully) print a message and continue for people who type `yum`.  This case is a hard error. 

So...a path we could pursue instead of this would be having `/usr/bin/rpm` be a symlink → `/usr/bin/rpm-ostree`.  There's a lot of advantages to that, but it'd also be obviously a large maintenance overhead of detecting operations we want to intercept (basically writes like `-i`, `-U`, but not `-q`).  If consensus favors that I'd be OK with doing that instead.

¹ `rpm -Uvh` etc work fine on top of `ostree admin unlock`, which is intentional and very handy (and in fact how I tested this patch, by building a new rpm of rpm and installing it that way).  But the way persistent changes work is totally different.



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