Add helper class for usage tracking

plugins/commands/serve/util/usage_tracker.rb

@@ -0,0 +1,71 @@
+require "mutex_m"
+
+module VagrantPlugins
+  module CommandServe
+    module Util
+      # Helper class for tracking usage and handling
+      # activation and deactivation of usage based
+      # on count. Activation is only performed on
+      # the first request and deactivation is only
+      # performed on the final deactivation.
+      class UsageTracker
+        include Mutex_m
+
+        def initialize
+          super
+          @count = 0
+        end
+
+        # Activate usage. If a block is provided
+        # it will only be executed on initial
+        # activation (when count is zero).
+        #
+        # @yield Optional block to be executed
+        # @return [Boolean] true on initial activation
+        def activate
+          mu_synchronize do
+            if @count == 0 && block_given?
+              yield
+            end
+            @count += 1
+
+            return @count == 1
+          end
+        end
+
+        # Deactivate usage. If a block is provided
+        # it will only be exected on final deactivation
+        # (when count returns to zero). If the count
+        # is already zero, the block will not be
+        # executed.
+        #
+        # @yield Optional block to be executed
+        # @return [Boolean] true on final deactivation
+        def deactivate
+          mu_synchronize do
+            # If the count is already zero then
+            # we do nothing
+            return false if @count == 0
+
+            @count -= 1
+            if @count == 0 && block_given?
+              yield
+            end
+
+            return @count == 0
+          end
+        end
+
+        # @return [Boolean] usage is active
+        def active?
+          mu_synchronize { @count > 0 }
+        end
+
+        # @return [Boolean] usage is not active
+        def inactive?
+          !active?
+        end
+      end
+    end
+  end
+end