The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/b2c09b131a7a7cceb1121c86a4352d7b8b9b6c32

commit b2c09b131a7a7cceb1121c86a4352d7b8b9b6c32
Author: Seth Koehler <sethkoehler@google.com>
Date: Wed Jan 03 20:53:21 2018

Fix error with stats/bot_overhead included when duration is not.

When an error occurs during swarming execution and we clear out the
exit code and duration as part of b/69462084, we must also clear out
various stats (including bot_overhead) to prevent additional swarming
server errors of the form "Failed to update task: duration must be
set when performance_stats is set".

TESTED=task_runner_test.py and verified (by forcing an error in
client/isolatedserver.py) that we no longer see errors on the
swarming server associated with task_update (the forced error itself
still shows up as a task_error correctly).

Bug: 798828
Change-Id: I46dc2dd430e3389bd1da78612752b754ad377b8d
Reviewed-on: https://chromium-review.googlesource.com/847712
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/b2c09b131a7a7cceb1121c86a4352d7b8b9b6c32/appengine/swarming/swarming_bot/bot_code/task_runner.py

 Issue 799266  has been merged into this issue.

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