core: Environment#default_provider can look into machines

lib/vagrant/environment.rb

@@ -308,10 +308,19 @@ module Vagrant
       # that (the default behavior)
       return default if default && opts[:force_default]
 
+      # Determine the config to use to look for provider definitions. By
+      # default it is the global but if we're targeting a specific machine,
+      # then look there.
+      root_config = vagrantfile.config
+      if opts[:machine]
+        machine_info = vagrantfile.machine_config(opts[:machine], nil, nil)
+        root_config = machine_info[:config]
+      end
+
       # Get the list of providers within our configuration and assign
       # a priority to each in the order they exist so that we try these first.
       config = {}
-      vagrantfile.config.vm.__providers.each_with_index do |key, idx|
+      root_config.vm.__providers.each_with_index do |key, idx|
         config[key] = idx
       end
 

lib/vagrant/vagrantfile.rb

@@ -114,6 +114,11 @@ module Vagrant
           name: name, provider: provider
       end
 
+      provider_plugin  = nil
+      provider_cls     = nil
+      provider_options = {}
+      box_formats      = nil
+      if provider != nil
         provider_plugin  = Vagrant.plugin("2").manager.providers[provider]
         if !provider_plugin
           raise Errors::ProviderNotFound,
@@ -133,6 +138,7 @@ module Vagrant
             provider: provider.to_s,
             message: e.to_s
         end
+      end
 
       # Add the sub-machine configuration to the loader and keys
       vm_config_key = "#{object_id}_machine_#{name}"
@@ -153,7 +159,7 @@ module Vagrant
         local_keys = keys.dup
 
         # Load the box Vagrantfile, if there is one
-        if config.vm.box
+        if config.vm.box && boxes
           box = boxes.find(config.vm.box, box_formats, config.vm.box_version)
           if box
             box_vagrantfile = find_vagrantfile(box.directory)

test/unit/vagrant/environment_test.rb

@@ -822,7 +822,7 @@ VF
       end
     end
 
-    it "is the provider in the Vagrantfile that is usable" do
+    it "is the highest usable provider outside the Vagrantfile" do
       subject.vagrantfile.config.vm.provider "foo"
       subject.vagrantfile.config.vm.finalize!
 
@@ -835,6 +835,23 @@ VF
         expect(subject.default_provider).to eq(:bar)
       end
     end
+
+    it "is the provider in the Vagrantfile that is usable for a machine" do
+      subject.vagrantfile.config.vm.provider "foo"
+      subject.vagrantfile.config.vm.define "sub" do |v|
+        v.vm.provider "bar"
+      end
+      subject.vagrantfile.config.vm.finalize!
+
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
+      plugin_providers[:bar] = [provider_usable_class(true), { priority: 7 }]
+      plugin_providers[:baz] = [provider_usable_class(true), { priority: 2 }]
+      plugin_providers[:boom] = [provider_usable_class(true), { priority: 3 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => nil) do
+        expect(subject.default_provider(machine: :sub)).to eq(:bar)
+      end
+    end
   end
 
   describe "local data path" do

test/unit/vagrant/vagrantfile_test.rb

@@ -149,6 +149,21 @@ describe Vagrant::Vagrantfile do
       expect(results[:provider_cls]).to equal(provider_cls)
     end
 
+    it "configures without a provider or boxes" do
+      register_provider("foo")
+
+      configure do |config|
+        config.vm.box = "foo"
+      end
+
+      results = subject.machine_config(:default, nil, nil)
+      box     = results[:box]
+      config  = results[:config]
+      expect(config.vm.box).to eq("foo")
+      expect(box).to be_nil
+      expect(results[:provider_cls]).to be_nil
+    end
+
     it "configures with sub-machine config" do
       register_provider("foo")