core: allow IO redirection of UI for testing

Use of $stdin, $stdout, and $stderr globals makes testing difficult. By
exposing the IO objects as writable attributes, input/output can be more
easily simulated using StringIO or doubles.

lib/vagrant/ui.rb

@@ -21,9 +21,22 @@ module Vagrant
       # specific. See the implementation for more docs.
       attr_accessor :opts
 
+      # @return [IO] UI input. Defaults to `$stdin`.
+      attr_accessor :stdin
+
+      # @return [IO] UI output. Defaults to `$stdout`.
+      attr_accessor :stdout
+
+      # @return [IO] UI error output. Defaults to `$stderr`.
+      attr_accessor :stderr
+
       def initialize
         @logger   = Log4r::Logger.new("vagrant::ui::interface")
         @opts     = {}
+
+        @stdin  = $stdin
+        @stdout = $stdout
+        @stderr = $stderr
       end
 
       def initialize_copy(original)
@@ -132,7 +145,7 @@ module Vagrant
         super(message)
 
         # We can't ask questions when the output isn't a TTY.
-        raise Errors::UIExpectsTTY if !$stdin.tty? && !Vagrant::Util::Platform.cygwin?
+        raise Errors::UIExpectsTTY if !@stdin.tty? && !Vagrant::Util::Platform.cygwin?
 
         # Setup the options so that the new line is suppressed
         opts ||= {}
@@ -144,11 +157,11 @@ module Vagrant
         say(:info, message, opts)
 
         input = nil
-        if opts[:echo]
+        if opts[:echo] || !@stdin.respond_to?(:noecho)
-          input = $stdin.gets
+          input = @stdin.gets
         else
           begin
-            input = $stdin.noecho(&:gets)
+            input = @stdin.noecho(&:gets)
 
             # Output a newline because without echo, the newline isn't
             # echoed either.
@@ -206,7 +219,7 @@ module Vagrant
 
         # Determine the proper IO channel to send this message
         # to based on the type of the message
-        channel = type == :error || opts[:channel] == :error ? $stderr : $stdout
+        channel = type == :error || opts[:channel] == :error ? @stderr : @stdout
 
         # Output! We wrap this in a lock so that it safely outputs only
         # one line at a time. We wrap this in a thread because as of Ruby 2.0

test/unit/vagrant/ui_test.rb

@@ -40,23 +40,37 @@ describe Vagrant::UI::Basic do
       subject.output("foo", new_line: false)
     end
 
-    it "outputs to stdout" do
+    it "outputs to the assigned stdout" do
+      stdout = StringIO.new
+      subject.stdout = stdout
+
       expect(subject).to receive(:safe_puts).with { |message, **opts|
-        expect(opts[:io]).to be($stdout)
+        expect(opts[:io]).to be(stdout)
         true
       }
 
       subject.output("foo")
     end
 
-    it "outputs to stderr for errors" do
+    it "outputs to stdout by default" do
+      expect(subject.stdout).to be($stdout)
+    end
+
+    it "outputs to the assigned stderr for errors" do
+      stderr = StringIO.new
+      subject.stderr = stderr
+
       expect(subject).to receive(:safe_puts).with { |message, **opts|
-        expect(opts[:io]).to be($stderr)
+        expect(opts[:io]).to be(stderr)
         true
       }
 
       subject.error("foo")
     end
+
+    it "outputs to stderr for errors by default" do
+      expect(subject.stderr).to be($stderr)
+    end
   end
 
   context "#detail" do