Default unspecified value added to enum in proto

chrome/browser/chromeos/policy/proto/chrome_device_policy.proto

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package enterprise_management;
 
@@ skipping 765 matching lines @@
 // The url and hash that specified in JSON format that can be used to set the
 // device-level wallpaper on the login screen before any user logs in.
 message DeviceWallpaperImageProto {
   optional string device_wallpaper_image = 1;
 }
 
 // Migration strategy for the case when ARC(N+) needs the ext4 encryption while
 // the device used ecryptfs in the past.
 message DeviceEcryptfsMigrationStrategyProto {
   enum MigrationStrategy {
+    // Default value, unspecified.
+    MIGRATION_STRATEGY_UNSPECIFIED = 0;

[Andrew T Wilson (Slow), 2017/05/22 12:30:56]

I'm not sure that's what we want. What behavior do you expect if
MIGRATION_STRATEGY_UNSPECIFIED is provided? Is the implication that DISALLOW_ARC
should not be the default for unspecified values?

[bartfab (slow), 2017/05/22 13:23:47]

We (the client) do not want or need an UNSET value. What you are seeing here are
implementation details leaking from server to client. On the server, they want
to keep track of domains/OUs that explicitly set the policy vs. ones that never
set it. They need a sepecial UNSET value for that. Since server-side storage
uses the same proto as the client-server protocol (a long-known WTF in the
design), we have to add the value here if we want to help out the server guys.

Igor, let's call this simply UNSET and note in the comment that this triggers
version- and board-specific, undefined behavior and should thus never be sent by
the server.

[igorcov, 2017/05/22 17:20:32]

Changed to UNSET. Thanks for explanations.

[Thiemo Nagel, 2017/05/23 09:27:31]

Imho this is just enforcing proto best practices (which I think is fair, BTW). 
I don't think there's a specific need for this on the server side.

     // ARC is not allowed, no data migration needed.
-    DISALLOW_ARC = 0;
+    DISALLOW_ARC = 1;
     // The data migration is allowed, opening the possibility to use ARC.
-    ALLOW_MIGRATION = 1;
+    ALLOW_MIGRATION = 2;
   };
 
   optional MigrationStrategy migration_strategy = 1;
 }
 
 message ChromeDeviceSettingsProto {
   optional DevicePolicyRefreshRateProto device_policy_refresh_rate = 1;
   optional UserWhitelistProto user_whitelist = 2;
   optional GuestModeEnabledProto guest_mode_enabled = 3;
   optional OBSOLETE_DeviceProxySettingsProto device_proxy_settings = 4
@@ skipping 46 matching lines @@
       45;
   optional DeviceLoginScreenAppInstallListProto
       device_login_screen_app_install_list = 46;
   optional NetworkThrottlingEnabledProto network_throttling = 47;
   optional DeviceWallpaperImageProto device_wallpaper_image = 48;
   optional LoginScreenLocalesProto login_screen_locales = 49;
   optional LoginScreenInputMethodsProto login_screen_input_methods = 50;
   optional DeviceEcryptfsMigrationStrategyProto
       device_ecryptfs_migration_strategy = 51;
 }