plugin: Write out state file via create-new-then-rename

[cgwalters]

This makes updates atomic, so it's useful generally. However
I'm making this change specifically to support rpm-ostree.

For more information, see:
https://bugzilla.redhat.com/show_bug.cgi?id=1352152

[sethvargo]

Yea, so there's a reason we don't do this...

1. File.rename has historically been a problem on Windows
2. File.rename does not work across multiple volumes, particularly if the volumes are different formats
3. The tmpfile could be removed by GC before the mv takes place (aka there's a slight race between lines 97 and 98 here)

I think it's probably a better idea to do something like:

Tempfile.open("vagrant-plugins") do |f|
  f.binmode
  f.write(JSON.dump(@data))
  f.close
  FileUtils.mv(f.path, @path)
end

What do you think?

[cgwalters]

Re 2: The tempfile is created in the same directory as the target
Re 3: I'll be honest and say this is my first Ruby patch, but I have a strong suspicion it can't be removed by GC because the local variable holds a strong reference across the function scope

But yes, this is just an implementation of a general pattern for "replace file atomically" which is harder on Windows indeed. I'm an upstream developer for GLib where for g_file_set_contents() we do https://git.gnome.org/browse/glib/tree/glib/gfileutils.c#n1217

It's the sort of thing I'd expect Ruby to have in the standard library, but if it exists I couldn't find it.

[sethvargo]

I think FileUtils.mv handles this better than File.rename

[cgwalters]

Okay, updated ⬆️ to use FileUtils.mv

[cgwalters]

FileUtils.mv will use rename() on Unix which does unlink the
tempfile, and on Windows it will to after deleting the original.

The tempfile does still call unlink in its cleanup on the old name,
but that's relatively harmless...not sure offhand if there's a way to
avoid that.

(There has to be some Ruby codebase that does things like this right and
has a "write file atomically" utility function but I don't know Ruby
well enough to know which codebases are good to look at)

On Thu, Jul 7, 2016, at 03:19 PM, Seth Vargo wrote:

This is still going to leave the tempfile lying around.
The Ruby docs are wrong... you have to call #unlink on the file to
remove it, but the block form that I posted in my earlier comment
automatically does that.
 

[cgwalters]

Sorry, when reworking this I introduced a regression - we need to chmod back to 0644. Will fix.