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Re: individual packages moving binaries from /bin to /usr/bin (was: Re: usrmerge -- plan B?)

On Thu, 22 Nov 2018 at 21:08:14 +0100, Ansgar Burchardt wrote:
> If we want to support packages such as iptables moving binaries from
> /{s,}bin to /usr/{s,}bin

To be honest, I'm not sure whether we do want this. We should be careful,
at least.

Now that we don't support booting without /usr[1], it is no longer
necessary to move an executable from /bin to /usr/bin if it gains
a dependency on a library that is in /usr/lib, and similarly, it is
unnecessary to move executables in the opposite direction to make them
available in early boot.

Unlike merged /usr, doing this move doesn't make things like containers
any simpler until literally every executable in /bin has undergone this
transition; the work happens now, but the benefit doesn't happen until
the transition is complete. If the compat symlink isn't created
carefully, it can easily break merged-/usr systems.

However, it can easily cause the same issue that started this thread -
when a dependent package detects "the" path to the iptables executable,
it will get different answers depending on $PATH order, just like it would
on a merged /usr system. That's fine if we can guarantee that iptables
exists in both places, but during a partial upgrade, we can't count on
that being the case without a versioned dependency on the moved iptables,
which I assume nobody is seriously intending to mass-bug-file?

For iptables, maybe that's no big deal, because not many packages depend
on it and hard-code its path, but we have evidence from our accidental
merged-/usr-buildds experiment (and its subsequent intentional equivalent
in reproducible-builds) that the paths to programs like grep, sed and
(of course) sh are more widespread.

(Everything I've said about /bin applies equally to /sbin of course,
I just didn't want to keep typing /{s,}bin.)

I'm not sure yet what the best plan for merged /usr is. I would definitely
like to make sure it's at least possible to continue to use merged
/usr for special-purpose systems (particularly containers and embedded
systems), even if it comes with major caveats like "can't reliably build
Debian packages suitable for other systems"; I personally think everyone
should be using sbuild or equivalent, either on a buildd or locally,
to build "release-quality" packages suitable for distribution to other
systems *anyway*, but I know that view isn't necessarily universal.

For at least special-purpose systems, merged /usr seems to work fine with
stretch, and I was able to get it working in an Ubuntu 12.04 derivative
by backporting a single-digit number of changes, so that particular genie
has been out of the bottle for quite some time anyway.


[1] in the sense that if /usr is separate, we require it to be mounted
    by the initramfs

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