core: more intuitive logic around default providers

lib/vagrant/environment.rb

@@ -249,16 +249,20 @@ module Vagrant
     end
 
     # This returns the provider name for the default provider for this
-    # environment. The provider returned is currently hardcoded to "virtualbox"
-    # but one day should be a detected valid, best-case provider for this
     # environment.
     #
     # @return [Symbol] Name of the default provider.
     def default_provider(**opts)
+      opts[:force_default] = true if !opts.has_key?(:force_default)
+
       default = ENV["VAGRANT_DEFAULT_PROVIDER"]
       default = nil if default == ""
       default = default.to_sym if default
 
+      # If we're forcing the default, just short-circuit and return
+      # that (the default behavior)
+      return default if default && opts[:force_default]
+
       ordered = []
       Vagrant.plugin("2").manager.providers.each do |key, data|
         impl = data[0]

test/unit/vagrant/environment_test.rb

@@ -717,7 +717,7 @@ VF
       end
     end
 
-    it "is the default provider if it is usable" do
+    it "is the default provider set if 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 }]
@@ -728,6 +728,26 @@ VF
       end
     end
 
+    it "is the default provider set even if unusable" do
+      plugin_providers[:baz] = [provider_usable_class(false), { priority: 5 }]
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
+      plugin_providers[:bar] = [provider_usable_class(true), { priority: 7 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => "baz") do
+        expect(subject.default_provider).to eq(:baz)
+      end
+    end
+
+    it "is the usable despite default if not forced" do
+      plugin_providers[:baz] = [provider_usable_class(false), { priority: 5 }]
+      plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]
+      plugin_providers[:bar] = [provider_usable_class(true), { priority: 7 }]
+
+      with_temp_env("VAGRANT_DEFAULT_PROVIDER" => "baz") do
+        expect(subject.default_provider(force_default: false)).to eq(:bar)
+      end
+    end
+
     it "is VirtualBox if nothing else is usable" do
       plugin_providers[:foo] = [provider_usable_class(false), { priority: 5 }]
       plugin_providers[:bar] = [provider_usable_class(false), { priority: 5 }]