Provider-specific configuration overrides via 2nd block param

Example:

Vagrant.configure("2") do |config|
  # ...

  config.vm.provider :virtualbox do |p, vb_config|
    vb_config.vm.box = "virtualbox_box"
  end
end

CHANGELOG.md

@@ -9,6 +9,12 @@ FEATURES:
   - Box downloads are now done via `curl` rather than Ruby's built-in
     HTTP library. This results in massive speedups, support for SSL
     verification, FTP downloads, and more.
+  - `config.vm.provider` now takes an optional second parameter to the block,
+    allowing you to override any configuration value. These overrides are
+    applied last, and therefore override any other configuration value.
+    Note that while this feature is available, the "Vagrant way" is instead
+    to use box manifests to ensure that the "box" for every provider matches,
+    so these sorts of overrides are unnecessary.
 
 IMPROVEMENTS:
 

lib/vagrant/environment.rb

@@ -355,6 +355,17 @@ module Vagrant
         end
       end
 
+      # If there are provider overrides for the machine, then we run
+      # those as well.
+      provider_overrides = config.vm.get_provider_overrides(provider)
+      if provider_overrides.length > 0
+        @logger.info("Applying #{provider_overrides.length} provider overrides. Reloading config.")
+        provider_override_key = "vm_#{name}_#{config.vm.box}_#{provider}".to_sym
+        @config_loader.set(provider_override_key, provider_overrides)
+        config, config_warnings, config_errors = \
+          @config_loader.load([:default, box_config_key, :home, :root, vm_config_key, provider_override_key])
+      end
+
       # Get the provider configuration from the final loaded configuration
       provider_config = config.vm.get_provider_config(provider)
 

plugins/kernel_v2/config/vm.rb

@@ -33,6 +33,7 @@ module VagrantPlugins
 
         # Internal state
         @__compiled_provider_configs = {}
+        @__compiled_provider_overrides = {}
         @__defined_vm_keys = []
         @__defined_vms = {}
         @__finalized = false
@@ -196,17 +197,32 @@ module VagrantPlugins
 
         # Compile all the provider configurations
         @__providers.each do |name, blocks|
+          # If we don't have any configuration blocks, then ignore it
+          next if blocks.empty?
+
           # Find the configuration class for this provider
           config_class = Vagrant.plugin("2").manager.provider_configs[name]
-          next if !config_class
+          config_class ||= Vagrant::Config::V2::DummyConfig
 
           # Load it up
-          config = config_class.new
-          blocks.each { |b| b.call(config) }
+          config    = config_class.new
+          overrides = []
+
+          blocks.each do |b|
+            b.call(config, Vagrant::Config::V2::DummyConfig.new)
+
+            # If the block takes two arguments, then we store the second
+            # one away for overrides later.
+            if b.arity == 2
+              overrides << b.curry[Vagrant::Config::V2::DummyConfig.new]
+            end
+          end
+
           config.finalize!
 
           # Store it for retrieval later
-          @__compiled_provider_configs[name] = config
+          @__compiled_provider_configs[name]   = config
+          @__compiled_provider_overrides[name] = overrides
         end
 
         # Flag that we finalized
@@ -235,6 +251,17 @@ module VagrantPlugins
         return result
       end
 
+      # This returns a list of VM configurations that are overrides
+      # for this provider.
+      #
+      # @param [Symbol] name Name of the provider
+      # @return [Array<Proc>]
+      def get_provider_overrides(name)
+        (@__compiled_provider_overrides[name] || []).map do |p|
+          ["2", p]
+        end
+      end
+
       # This returns the list of networks configured.
       def networks
         @__networks.values

test/unit/vagrant/environment_test.rb

@@ -550,6 +550,29 @@ VF
       machine.config.ssh.port.should == 100
     end
 
+    it "should load the provider override if set" do
+      register_provider("bar")
+      register_provider("foo")
+
+      isolated_env = isolated_environment do |e|
+        e.vagrantfile(<<-VF)
+Vagrant.configure("2") do |config|
+  config.vm.box = "foo"
+
+  config.vm.provider :foo do |_, c|
+    c.vm.box = "bar"
+  end
+end
+VF
+      end
+
+      env = isolated_env.create_vagrant_env
+      foo_vm = env.machine(:default, :foo)
+      bar_vm = env.machine(:default, :bar)
+      foo_vm.config.vm.box.should == "bar"
+      bar_vm.config.vm.box.should == "foo"
+    end
+
     it "should reload the cache if refresh is set" do
       # Create a provider
       foo_provider = register_provider("foo")