Change Fedora 16+ NFS support.

[smerrill]

Here's a quick-and-dirty pull request to add NFS support to Fedora 17 and up, as reported in #1125.

This simply parses the redhat-release file, and if the version of Fedora is 17 or greater, it changes @nfs_server_binary to /usr/sbin/service nfs-server instead of /etc/init.d/nfs, which does not exist any more in Fedora 17+ due to the switch to systemd.

This works because the service command in Fedora 17 will rewrite service nfs-server restart to systemctl restart nfs-server.service behind the scenes. This is the simplest way to solve this issue for now without changing too much code since lib/vagrant/hosts/linux.rb calls

system("sudo #{@nfs_server_binary} restart")

at the end of nfs_export, whereas calling systemctl would require copying that method and just changing the last line.

[smerrill]

I have tested this code change on a Fedora 17 system and it all works. The only noticeable difference is that the service command will output a notice that it's redirecting to systemctl, like so:

[default] VM already created. Booting if it's not already running...
[default] Clearing any previously set forwarded ports...
[default] Forwarding ports...
[default] -- 22 => 22230 (adapter 1)
[default] Exporting NFS shared folders...
[vagrant] Preparing to edit /etc/exports. Administrator privileges will be required...
Redirecting to /bin/systemctl restart  nfs-server.service
[default] Creating shared folders metadata...
[default] Clearing any previously set network interfaces...
[default] Preparing network interfaces based on configuration...
[default] Running any VM customizations...
[default] Booting VM...
[default] Waiting for VM to boot. This can take a few minutes.

[skottler]

This should be if version_number >= 16 since systemd was introduced in Fedora 16.

[skottler]

This looks good, just added one quick comment.

[smerrill]

Duly added. Thanks, @skottler. :)

[mitchellh]

This looks good. Thanks a ton. And thanks @skottler for doing open source right.

Merging. I'll handle porting this to the master branch.

[mitchellh]

One nitpick @smerrill:

Instead of checking if a file exists, I usually try to open it and just rescue the Errno::ENOENT error. This is safer because there is actually a TINY race condition between an exist? check and an open call. For example, the file may exist when exist? is called, then it can be deleted by some outside source, then open is called and then BOOM! exception!.

So instead, I recommend just always opening and catching the exception. Less error cases this way.

I made this fix here: 2d710f5