switch /etc/systemd/system to "synced" mode

[ogra1]

http://manpages.ubuntu.com/manpages/xenial/man5/writable-paths.5.html

synced:
[ Any file appearing in the root filesystem will also be copied
over to writable storage. However file removals are still not
synced and files existing in both read-only and writeable
storage will not be updated. ]...

transition:
[ ...note that subsequent boots
will ignore any new files appearing or disappearing in the
original read-only rootfs location unless you perform a
factory reset. ]...

[zyga]

Why are we making this change?

[ogra1]

i would have expected the PR description to be explanatory enough :)

when a new systemd unit shows up in the squashfs it will not end up in /writable (and thus stay invisible to the system) when "transition" mode is used since transition only copies the dir contents once at first mount of a device. "synced" means all files from the squashfs that do not exist at the destination get copied at every boot, so new files that get added to the squashfs will show up.

[ogra1]

note that the same fix should be applied for snapcore/snapd#3807 ... the issue is identical ...

[zyga]

There's an extra space here before synced

[ogra1]

nope, the former "persistent" was wrongly indented (the file uses spaces for indentation) ...

[mvo5]

Why is this synced? Will we ship things in there (probably sensibel)?

[ogra1]

I dont think there will be anything in the squashfs initially ... @chipaca can probably elaborate why "synced" should be used...

[chipaca]

so that if we want to ship the caches pre-seeded, we can.

[ogra1]

you are aware that caches with the same filename in both places will not be synced i assume ?
(so if you seed them later, the generated ones will stay and override)

[chipaca]

that's exactly the behaviour I want, yes, thank you.

We're leaving ourselves the option of shipping a default cache that would be used if the snapd on the device doesn't get network for a while, say. E.g. if it's only rarely online and has been updated out-of-band.

If the file exists it'll probably be fresher / better than the seeded one.