Fixes #8884: Introduce proper VirtualBox shared folders for BSD

This commit adds proper VirtualBox shared folder support for BSD guests.
It is essentially a copy of the linux capability.

plugins/guests/bsd/cap/virtualbox.rb

@@ -1,14 +1,78 @@
+require_relative "../../../synced_folders/unix_mount_helpers"
+
 module VagrantPlugins
   module GuestBSD
     module Cap
       class VirtualBox
+        extend SyncedFolder::UnixMountHelpers
         # BSD-based guests do not currently support VirtualBox synced folders.
         # Instead of raising an error about a missing capability, this defines
         # the capability and then provides a more detailed error message,
         # linking to sources on the Internet where the problem is
         # better-described.
         def self.mount_virtualbox_shared_folder(machine, name, guestpath, options)
-          raise Vagrant::Errors::VirtualBoxMountNotSupportedBSD
+          guest_path = Shellwords.escape(guestpath)
+
+          @@logger.debug("Mounting #{name} (#{options[:hostpath]} to #{guestpath})")
+
+          builtin_mount_type = "-cit vboxsf"
+          addon_mount_type = "-t vboxsf"
+
+          mount_options = options.fetch(:mount_options, [])
+          detected_ids = detect_owner_group_ids(machine, guest_path, mount_options, options)
+          mount_uid = detected_ids[:uid]
+          mount_gid = detected_ids[:gid]
+
+          mount_options << "uid=#{mount_uid}"
+          mount_options << "gid=#{mount_gid}"
+          mount_options = mount_options.join(',')
+          mount_command = "mount #{addon_mount_type} -o #{mount_options} #{name} #{guest_path}"
+
+          # Create the guest path if it doesn't exist
+          machine.communicate.sudo("mkdir -p #{guest_path}")
+
+          stderr = ""
+          result = machine.communicate.sudo(mount_command, error_check: false) do |type, data|
+            stderr << data if type == :stderr
+          end
+
+          if result != 0
+            if stderr.include?("-cit")
+              @@logger.info("Detected builtin vboxsf module, modifying mount command")
+              mount_command.sub!(addon_mount_type, builtin_mount_type)
+            end
+
+            # Attempt to mount the folder. We retry here a few times because
+            # it can fail early on.
+            stderr = ""
+            retryable(on: Vagrant::Errors::VirtualBoxMountFailed, tries: 3, sleep: 5) do
+              machine.communicate.sudo(mount_command,
+                error_class: Vagrant::Errors::VirtualBoxMountFailed,
+                error_key: :virtualbox_mount_failed,
+                command: mount_command,
+                output: stderr,
+              ) { |type, data| stderr = data if type == :stderr }
+            end
+          end
+
+          # Chown the directory to the proper user. We skip this if the
+          # mount options contained a readonly flag, because it won't work.
+          if !options[:mount_options] || !options[:mount_options].include?("ro")
+            chown_command = "chown #{mount_uid}:#{mount_gid} #{guest_path}"
+            machine.communicate.sudo(chown_command)
+          end
+
+          emit_upstart_notification(machine, guest_path)
+        end
+
+
+        def self.unmount_virtualbox_shared_folder(machine, guestpath, options)
+          guest_path = Shellwords.escape(guestpath)
+
+          result = machine.communicate.sudo("umount #{guest_path}", error_check: false)
+          if result == 0
+            machine.communicate.sudo("rmdir #{guest_path}", error_check: false)
+          end
         end
       end
     end