Add a new force_empty_mountpoints configuration option

vagrant-bindfs was not used to do anything to mount points before mount,
except ensuring they exist. Hence, if you tried to bind a folder to a
non-empty directory without explicitly allowing Fuse to do so (with the
`nonempty` Fuse option), mount will fail.

This add a new configuration option to the vagrant-bindfs plugin to
make sure mount points are empty. Unless you allow Fuse to not mind with
the `nonempty` Fuse option, mount point will first be destroyed (with
`rm -rf`) then recreated (with `mkdir -p`) before mount.

As this can be dangerous and covers only really specific use cases, it
is disabled by default.

README.md

@@ -171,6 +171,34 @@ end
 **This feature only works with exact version match and does not try to resolve dependencies.**
 In particular, Fuse will always be installed from the latest version available in repositories (_via_ Homebrew Cask for OS X guests).
 
+### `force_empty_mountpoints`
+
+By default, `vagrant-bindfs` won't try to empty an existing mount point before mount. Hence, if you try to bind a folder to a non-empty directory without explicitly allowing Fuse to do so (with the `nonempty` Fuse option), mount will fail.
+
+You can ask `vagrant-bindfs` to make sure the mount points are empty with the `config.bindfs.force_empty_mountpoints` option.
+Mount poitns will then be destroyed (with `rm -rf`) prior to mount, unless you allow Fuse to not mind (with the `nonempty` Fuse option).
+
+
+```ruby
+Vagrant.configure("2") do |config|
+
+  config.bindfs.force_empty_mountpoint = true
+
+  # This exemple assume two directories exist in your VM:
+  # - `a/non/empty/mount/point/and/its/content`
+  # - `another/non/empty/mount/point/and/its/content` 
+
+  # `a/non/empty/mount/point` will be destroyed before mount and
+  #  all its content will be lost.
+  config.bindfs.bind_folder "/vagrant", "a/non/empty/mount/point"
+
+  # `a/non/empty/mount/point` will not be destroyed before mount
+  # but its content will not be accessible while in use by bindfs.
+  config.bindfs.bind_folder "/vagrant", "a/non/empty/mount/point", o: :nonempty
+
+end
+```
+
 ## Contributing
 
 If you find this plugin useful, we could use a few enhancements!

Vagrantfile

@@ -35,6 +35,7 @@ Vagrant.configure('2') do |config|
 
   # For a more detailled output when you test, set this to true.
   config.bindfs.debug = false
+  config.bindfs.force_empty_mountpoints = true
 
   test_machines.each do |distro, options|
     config.vm.define "vagrant-bindfs-test-#{distro}" do |machine|

lib/vagrant-bindfs/vagrant/actions/mounter.rb

@@ -56,12 +56,28 @@ def bind_folders!
             )
 
             machine.communicate.tap do |comm|
+
+              if empty_mountpoint?(folder)
+                comm.sudo("rm -rf #{folder.destination}")
+                detail I18n.t(
+                  'vagrant-bindfs.actions.mounter.force_empty_mountpoints',
+                  dest: folder.destination
+                )
+              end
+
               comm.sudo("mkdir -p #{folder.destination}")
               comm.sudo(command.to_s(bindfs_full_path), error_class: VagrantBindfs::Vagrant::Error, error_key: 'bindfs.mount_failed')
               debug(command.to_s(bindfs_full_path))
             end
           end
         end
+
+        def empty_mountpoint?(folder)
+          return false unless config.force_empty_mountpoints
+          return false if folder.options.key?('o') and !folder.options['o'].match(%r{nonempty}).nil?
+          true
+        end
+
       end
     end
   end

lib/vagrant-bindfs/vagrant/config.rb

@@ -12,6 +12,7 @@ class Config < ::Vagrant.plugin('2', :config)
       attr_accessor :binded_folders
 
       attr_accessor :skip_validations
+      attr_reader   :force_empty_mountpoints
 
       def initialize
         @debug                      = false
@@ -26,6 +27,7 @@ def initialize
                                                             'perms'       => 'u=rwX:g=rD:o=rD')
 
         @skip_validations           = []
+        @force_empty_mountpoints    = false
       end
 
       def debug=(value)
@@ -44,6 +46,10 @@ def default_options=(options = {})
         @default_options = Bindfs::OptionSet.new(nil, options)
       end
 
+      def force_empty_mountpoints=(value)
+        @force_empty_mountpoints = (value == true)
+      end
+
       def binded_folder=(*_any_variant)
         raise VagrantBindfs::Vagrant::ConfigError, :binded_folders
       end
@@ -66,6 +72,7 @@ def merge(other)
           result.binded_folders             = binded_folders.merge(other.binded_folders)
 
           result.skip_validations           = (skip_validations + other.skip_validations).uniq
+          result.force_empty_mountpoints    = (force_empty_mountpoints || other.force_empty_mountpoints)
         end
       end
 

locales/en.yml

@@ -20,6 +20,7 @@ en:
       mounter:
         start: "Creating bind mounts after %{hook}..."
         entry: "%{source} => %{dest}"
+        force_empty_mountpoints: "All content in '%{dest}' as been removed before mount"
 
 
     validations:

spec/vagrantfile_helper.rb

@@ -8,4 +8,21 @@ def tests_setup(machine, options = {})
   machine.bindfs.bind_folder '/etc',
                              '/etc-binded-after-provisioning',
                              options.merge(owner: 'root', after: :provision)
+
+
+  machine.vm.provision :shell do |p|
+    p.name   = "create non emtpy mountpoints"
+    p.inline = <<-COMMANDS
+      mkdir -p /home/vagrant/mountpoint/non/empty
+      mkdir -p /home/vagrant/mountpoint-nonempty/non/empty
+    COMMANDS
+  end
+
+  machine.bindfs.bind_folder '/etc',
+                             '/home/vagrant/mountpoint',
+                             options.merge(owner: 'root', after: :provision)
+
+  machine.bindfs.bind_folder '/etc',
+                             '/home/vagrant/mountpoint-nonempty',
+                             options.merge(owner: 'root', o: 'nonempty', after: :provision)
 end