update_engine: Process Omaha response for rollback images

Omaha returns whether the image returned is a rollback in the
_rollback="true" argument. If this is set, the client has to check
whether it's OK to apply the rollback image (policy is specifically
requesting a rollback and verified boot will accept the image based
on its kernel and firmware key versions).

In addition to this, the device has to do a safe powerwash if the
image is a rollback. (We're not supporting rollbacks with partial
or no powerwash yet.)

We're also setting the rollback_happened preference to avoid force
updates happening before the policy is available again.

Chromium CL adding the error code: http://crrev.com/c/1047866

BUG=chromium:840432
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: I1436ca96211b2a8523e78bf83602ef8b6b525570
Reviewed-on: https://chromium-review.googlesource.com/1047610
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/common/error_code_utils.cc b/common/error_code_utils.cc
index 6b72eee8..a0e75f0e 100644
--- a/common/error_code_utils.cc
+++ b/common/error_code_utils.cc
@@ -150,6 +150,8 @@ string ErrorCodeToString(ErrorCode code) {
       return "ErrorCode::kUpdatedButNotActive";
     case ErrorCode::kNoUpdate:
       return "ErrorCode::kNoUpdate";
+    case ErrorCode::kRollbackNotPossible:
+      return "ErrorCode::kRollbackNotPossible";
       // Don't add a default case to let the compiler warn about newly added
       // error codes which should be added here.
   }