saveConfig fails if config file location and /tmp/ are on different filesystems

[LongHairedHacker]

This is another problem I've noticed, while debugging #4.

On many OpenWRT device /etc/config is a read-only filesystem overlaid with a writable filesystem, while/tmp/ is usually an actual tmpfs/ramdisk.

Consider saveConfig() being called for network.
The call to ioutil.TempFile in saveConfig creates a temporary file in\tmp\.
A subsequent call to os.Rename() tries to hardlink the file to /etc/network.
While this a considered best practice to ensure atomic file changes it will fail for this case as the hardlink crosses filesystem boundaries.
A typical error looks like this:
rename /tmp/network012772557 /etc/config/network: invalid cross link

As the user can't choose a different location for the temporary files, this bug renders this library useless on many OpenWRT installations.

[dmke]

Huh. I didn't think of that (or misremembered how os.Rename operates).

[dmke]

saveConfig now places a temporary file in /etc/config (or whatever the config root is), named /etc/config/.012772557.network, before writing the config to it and renaming it to /etc/config/network. OverlayFS does support hardlinks, doesn't it?

While this may leave dot files behind if (for whatever reason) the cleanup mechanism fails, I'd prefer those files being present over half-written config files if we directly write to /etc/config/network.

As far as I can tell, UCI itself ignores dot files in /etc/config. I didn't find the C source responsible for this behaviour, but I've empirically tested this on one of our OpenWRT devices.

[LongHairedHacker]

Works on our system and looks okay to me.