core: default provider chosen by usability and prority

lib/vagrant/action/builtin/mixin_synced_folders.rb

@@ -22,7 +22,7 @@ module Vagrant
             ordered << [priority, key, impl]
           end
 
-          # Order the plugins by priority. Higher is tries before lower.
+          # Order the plugins by priority. Higher is tried before lower.
           ordered = ordered.sort { |a, b| b[0] <=> a[0] }
 
           # Find the proper implementation

lib/vagrant/environment.rb

@@ -254,8 +254,35 @@ module Vagrant
     # environment.
     #
     # @return [Symbol] Name of the default provider.
-    def default_provider
-      (ENV['VAGRANT_DEFAULT_PROVIDER'] || :virtualbox).to_sym
+    def default_provider(**opts)
+      default = ENV["VAGRANT_DEFAULT_PROVIDER"]
+      default = nil if default == ""
+      default = default.to_sym if default
+
+      ordered = []
+      Vagrant.plugin("2").manager.providers.each do |key, data|
+        impl = data[0]
+        opts = data[1]
+
+        ordered << [opts[:priority], key, impl, opts]
+      end
+
+      # Order the providers by priority. Higher values are tried first.
+      ordered = ordered.sort do |a, b|
+        # If we see the default, then that one always wins
+        next -1 if a[1] == default
+        next 1  if b[1] == default
+
+        b[0] <=> a[0]
+      end
+
+      # Find the matching implementation
+      ordered.each do |_, key, impl, _|
+        return key if impl.usable?(false)
+      end
+
+      # If all else fails, return VirtualBox
+      return :virtualbox
     end
 
     # Returns the collection of boxes for the environment.

test/unit/support/shared/capability_helpers_context.rb

@@ -7,6 +7,14 @@ shared_context "capability_helpers" do
     end
   end
 
+  def provider_usable_class(result)
+    Class.new do
+      define_singleton_method(:usable?) do |*args|
+        result
+      end
+    end
+  end
+
   def cap_instance(name, options=nil)
     options ||= {}
 

test/unit/vagrant/environment_test.rb

@@ -699,15 +699,32 @@ VF
   end
 
   describe "default provider" do
-    it "is virtualbox without any environmental variable" do
+    let(:plugin_providers) { {} }
+
+    before do
+      m = Vagrant.plugin("2").manager
+      m.stub(providers: plugin_providers)
+    end
+
+    it "is the highest matching usable provider" do
+      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).to eq(:virtualbox)
+        expect(subject.default_provider).to eq(:bar)
       end
     end
 
-    it "is whatever the environmental variable is if set" do
-      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => "foo") do
-        expect(subject.default_provider).to eq(:foo)
+    it "is the default provider if it is usable" do
+      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" => "baz") do
+        expect(subject.default_provider).to eq(:baz)
       end
     end
   end