From: Antonio Terceiro <terceiro@debian.org>
Date: Wed, 27 May 2015 09:36:17 -0300
Subject: Support system-installed plugins

Plugins must be installed as regular Ruby libraries, and they must
contain /usr/share/vagrant-plugins/plugins.d/$PLUGINNAME.json with the
following content:

{
  "${PLUGINNAME}": {
    "ruby_version":"$(ruby -e 'puts RUBY_VERSION')",
    "vagrant_version":"$(cat /usr/share/vagrant/version.txt)",
    "gem_version":"",
    "require":"",
    "sources":[]
  }
}
---
 lib/vagrant/plugin/manager.rb    |  4 ++--
 lib/vagrant/plugin/state_file.rb | 28 +++++++++++++++++++++++++---
 2 files changed, 27 insertions(+), 5 deletions(-)

diff --git a/lib/vagrant/plugin/manager.rb b/lib/vagrant/plugin/manager.rb
index dfaab2e..35122e0 100644
--- a/lib/vagrant/plugin/manager.rb
+++ b/lib/vagrant/plugin/manager.rb
@@ -18,7 +18,7 @@ module Vagrant
 
       # Returns the path to the [StateFile] for system plugins.
       def self.system_plugins_file
-        dir = Vagrant.installer_embedded_dir
+        dir = '/usr/share/vagrant-plugins'
         return nil if !dir
         Pathname.new(dir).join("plugins.json")
       end
@@ -33,7 +33,7 @@ module Vagrant
 
         system_path  = self.class.system_plugins_file
         @system_file = nil
-        @system_file = StateFile.new(system_path) if system_path && system_path.file?
+        @system_file = StateFile.new(system_path, true) if system_path && system_path.file?
       end
 
       # Installs another plugin into our gem directory.
diff --git a/lib/vagrant/plugin/state_file.rb b/lib/vagrant/plugin/state_file.rb
index f41da1b..717b353 100644
--- a/lib/vagrant/plugin/state_file.rb
+++ b/lib/vagrant/plugin/state_file.rb
@@ -5,8 +5,9 @@ module Vagrant
     # This is a helper to deal with the plugin state file that Vagrant
     # uses to track what plugins are installed and activated and such.
     class StateFile
-      def initialize(path)
+      def initialize(path, system = false)
         @path = path
+        @system = system
 
         @data = {}
         if @path.exist?
@@ -22,6 +23,21 @@ module Vagrant
 
         @data["version"] ||= "1"
         @data["installed"] ||= {}
+        load_extra_plugins
+      end
+
+      def load_extra_plugins
+        extra_plugins = Dir.glob(@path.dirname.join('plugins.d', '*.json'))
+        extra_plugins.each do |filename|
+          json = File.read(filename)
+          begin
+            plugin_data = JSON.parse(json)
+            @data["installed"].merge!(plugin_data)
+          rescue JSON::ParserError => e
+            raise Vagrant::Errors::PluginStateFileParseError,
+              path: filename, message: e.message
+          end
+        end
       end
 
       # Add a plugin that is installed to the state file.
@@ -91,8 +107,14 @@ module Vagrant
 
       # This saves the state back into the state file.
       def save!
-        @path.open("w+") do |f|
-          f.write(JSON.dump(@data))
+        begin
+          @path.open("w+") do |f|
+            f.write(JSON.dump(@data))
+          end
+        rescue Errno::EACCES
+          # Ignore permission denied against system-installed plugins; regular
+          # users are not supposed to write there.
+          raise unless @system
         end
       end