Add option to output device status to a JSON file.

build/android/buildbot/bb_device_status_check.py

Index: build/android/buildbot/bb_device_status_check.py
diff --git a/build/android/buildbot/bb_device_status_check.py b/build/android/buildbot/bb_device_status_check.py
index 1a238de9a054febd12cfde75b76b53d6016eaf5e..8b4018f74bbde31b75fc099528565a6ba86eb5ba 100755
--- a/build/android/buildbot/bb_device_status_check.py
+++ b/build/android/buildbot/bb_device_status_check.py
@@ -5,6 +5,7 @@
 # found in the LICENSE file.
 
 """A class to keep track of devices across builds and report state."""
+import json
 import logging
 import optparse
 import os
@@ -275,6 +276,8 @@ def main():
                     help='Output device status data for dashboard.')
   parser.add_option('--restart-usb', action='store_true',
                     help='Restart USB ports before running device check.')
+  parser.add_option('--json-output',
+                    help='Output JSON information into a specified file.')

[tonyg, 2014/07/11 02:19:15]

We should document the format somewhere.

 
   options, args = parser.parse_args()
   if args:
@@ -359,6 +362,13 @@ def main():
                                                 [battery], '%',
                                                 'unimportant')
 
+  if options.json_output:
+    with open(options.json_output, 'wb') as f:
+      f.write(json.dumps({
+        'online_devices': devices,

[tonyg, 2014/07/11 02:19:15]

Let's add expected_devices too.

Also, prior to recipes we got some other helpful info in the build step such as
the android version and device type.

"Online devices: 1. Device types ['mako'], builds ['KOT49H']"
http://chromegw/i/chromium.perf/builders/Android%20Nexus4%20Perf/builds/983

Should we add more info to this json so that could be restored?

[zty, 2014/07/11 18:58:30]

Acknowledged.

+        'offline_devices': offline_devices
+      }))
+
   if False in fail_step_lst:
     # TODO(navabi): Build fails on device status check step if there exists any
     # devices with critically low battery. Remove those devices from testing,