If resolvconf returns an error condition then NM overwrites /run/resolvconf/resolv.conf

(Arises from discussion here: https://bugs.launchpad.net/ubuntu/+source/resolvconf/+bug/1000244/comments/78)

The code in src/dns-manager/nm-dns-manager.c is essentially the following.

        success = dispatch_resolvconf (domain, searches, nameservers, iface, error);
        if (success == FALSE)
                success = update_resolv_conf (domain, searches, nameservers, iface, error);

When resolvconf runs if may or may not return an error. For example, if some /etc/resolvconf/update.d/ hook script returns a nonzero status then that status is returned by resolvconf to the caller, NetworkManager. But as NetworkManager is currently coded, if resolvconf returns an error for any reason then NM writes nameserver information directly to /etc/resolv.conf, i.e., generally, to /run/resolvconf/resolv.conf, clobbering the file that resolvconf has just written. This is not appropriate.

I mentioned this issue ten months ago in comment #30 on bug #324233.

    https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/324233/comments/30

Although the most serious of the several problems I discussed there have been fixed in Quantal, the problem remains that NetworkManager does not correctly handle a nonzero exit status returned by resolvconf.

The NM behavior in question dates from the time that resolvconf was an optional package in universe which returned nonzero status when /etc/resolv.conf failed to be a symbolic link. In Quantal resolvconf does not return nonzero status when /etc/resolv.conf fails to be a symbolic link. It returns nonzero only when there is some problem, e.g., non-writeable filesystem or whatever. NetworkManager should probably just report this failure and not immediately proceed to overwrite resolvconf's output.

Instead of the code quoted above, nm-dns-manager.c should implement the following logic.

    IF resolvconf is installed as evidenced by the presence of executable file /sbin/resolvconf
    THEN dispatch_resolvconf () # Send info to /sbin/resolvconf
    ELSE update_resolv_conf () # Write info directly to (the target of) /etc/resolv.conf