storage managers: add mirror storage manager.

Add a storage manager that allows mirroring files to multiple storage
backends.

test/unit/storage_manager_test.rb

@@ -38,4 +38,48 @@ class StorageManagerTest < ActiveSupport::TestCase
       end
     end
   end
+
+  context "StorageManager::Mirror" do
+    setup do
+      @temp_dir1 = Dir.mktmpdir("danbooru-temp1-")
+      @temp_dir2 = Dir.mktmpdir("danbooru-temp2-")
+
+      @storage_manager = StorageManager::Mirror.new([
+        StorageManager::Local.new(base_dir: @temp_dir1, base_url: "/data"),
+        StorageManager::Local.new(base_dir: @temp_dir2, base_url: "/data")
+      ])
+    end
+
+    teardown do
+      FileUtils.rm_rf(@temp_dir1)
+      FileUtils.rm_rf(@temp_dir2)
+    end
+
+    context "#store method" do
+      should "store the file on both backends" do
+        @storage_manager.store(StringIO.new("data"), "test.txt")
+
+        assert("data", File.read("#{@temp_dir1}/test.txt"))
+        assert("data", File.read("#{@temp_dir2}/test.txt"))
+      end
+    end
+
+    context "#delete method" do
+      should "delete the file from both backends" do
+        @storage_manager.store(StringIO.new("data"), "test.txt")
+        @storage_manager.delete("test.txt")
+
+        assert_not(File.exist?("#{@temp_dir1}/test.txt"))
+        assert_not(File.exist?("#{@temp_dir2}/test.txt"))
+      end
+    end
+
+    context "#open method" do
+      should "open the file from the first backend" do
+        @storage_manager.store(StringIO.new("data"), "test.txt")
+
+        assert_equal("data", @storage_manager.open("test.txt").read)
+      end
+    end
+  end
 end