[devil] Revise how EnableRoot waits for the device to come back.

devil/devil/android/device_utils.py

Index: devil/devil/android/device_utils.py
diff --git a/devil/devil/android/device_utils.py b/devil/devil/android/device_utils.py
index 99e37520200f38c48bd2cd85478f6859d40fb6dd..51f71949b770c8d1b81cf331751dca8bd2244432 100644
--- a/devil/devil/android/device_utils.py
+++ b/devil/devil/android/device_utils.py
@@ -456,7 +456,15 @@ class DeviceUtils(object):
       else:
         raise  # Failed probably due to some other reason.
 
-    self.WaitUntilFullyBooted()

[perezju, 2017/05/08 09:03:33]

Hmm, I would prefer if the fix is done _within_ WaitUntilFullyBooted; there
might be other places where that method is used, and the same kind of race might
occur?

[jbudorick, 2017/05/08 13:21:26]

I think that waiting for the device to go down is out of the scope of
WaitUntilFullyBooted. I think Reboot handles this the way I'd expect -- wait for
the device to fall offline, then call WaitUntilFullyBooted.

+    def device_online_with_root():
+      try:
+        self.adb.WaitForDevice()
+        return self.GetProp('service.adb.root', cache=False) == '1'
+      except (device_errors.AdbCommandFailedError,
+              device_errors.DeviceUnreachableError):
+        return False
+
+    timeout_retry.WaitFor(device_online_with_root, wait_period=1)
 
   @decorators.WithTimeoutAndRetriesFromInstance()
   def IsUserBuild(self, timeout=None, retries=None):