dirs: use the right snap mount dir for the distribution

[zyga]

Stacked on #2844

Some distributions use /var/lib/snapd/snap as the place where snaps are mounted
so make this a dynamic runtime choice made on startup of snapd and other tools.

Signed-off-by: Zygmunt Krynicki me@zygoon.pl

[Conan-Kudo]

For now, I suppose this is fine, since we don't have support for different bases and core snaps yet, but just a fair warning that this assumption is not likely to stick around either...

[Conan-Kudo]

It occurs to me that you could test for the existence of /usr/libexec/snapd, and if it exists, use it, otherwise just fall back to /usr/lib/snapd. That's much simpler than trying to use crazy case statements to switch things around.

[zyga]

For core that's a good assumption. For base that won't be a factor (I think).

As for the second advice I agree, I will look at making all the runtime checks comprehensive and not based on the os-release but rather on the important property at hand, I just wanted to land this quickly as it bears a lot of fruit.

[Conan-Kudo]

@zyga Like I said, for now the core snap libexecdir thing is fine, but once it becomes possible for me to make a Fedora Snappy Core using a core snap based from the build artifacts from Fedora infrastructure, that will not be valid anymore.

If the change to support this difference is trivial, I'd prefer to see it handled now rather than later, since you're already messing with this code.

[zyga]

We will not do fedora snappy core, we will do a fedora base, the core will stay unique but it will not be used as the root for snap execution anymore. I think we are in agreement, it was just the naming that got confusing.

[Conan-Kudo]

Anyway, aside from my comments above, it looks fine to me.

[Conan-Kudo]

@zyga Can you rebase this PR against master?

[vosst]

LGTM