The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/56367bbcdf2136531530145741db406d98273611

commit 56367bbcdf2136531530145741db406d98273611
Author: Marc-Antoine Ruel <maruel@chromium.org>
Date: Tue Sep 19 22:43:08 2017

swarming_bot: enables SIGUSR1/2 debugging tools.

Useful to diagnose issues with hook hang.

R=kjlubick@chromium.org
BUG= 764385 

Change-Id: I863a6be2471d785308e4ac07864f5d6df8cbfa87
Reviewed-on: https://chromium-review.googlesource.com/663210
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/56367bbcdf2136531530145741db406d98273611/appengine/swarming/server/bot_archive.py
[modify] https://crrev.com/56367bbcdf2136531530145741db406d98273611/appengine/swarming/swarming_bot/__main__.py
[add] https://crrev.com/56367bbcdf2136531530145741db406d98273611/appengine/swarming/swarming_bot/signal_trace.py
[add] https://crrev.com/56367bbcdf2136531530145741db406d98273611/appengine/swarming/swarming_bot/signal_trace_test.py

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/1829f7ad8d86624c44b01247699f8d79965f5b92

commit 1829f7ad8d86624c44b01247699f8d79965f5b92
Author: Marc-Antoine Ruel <maruel@chromium.org>
Date: Wed Sep 20 01:00:40 2017

swarming: improve the signal handling documentation

Otherwise it's not documented anywhere that it works, so nobody will ever use
it.

R=vadimsh@chromium.org
Bug:764385
Change-Id: I3e016950eb12b7e6c5d0faf01b187b523d2a2bf4
Reviewed-on: https://chromium-review.googlesource.com/673852
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/1829f7ad8d86624c44b01247699f8d79965f5b92/appengine/swarming/doc/Bot.md

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-py.git/+/f008e7266e9da37909ce40eab2c53dcff040d424

commit f008e7266e9da37909ce40eab2c53dcff040d424
Author: Marc-Antoine Ruel <maruel@chromium.org>
Date: Mon Sep 25 16:41:18 2017

swarming_bot: use logging.error instead of stderr

When printing the stack trace, stderr isn't consistently redirected across OSes
and startup environments.

In bot_main, logging is always redirected to swarming_bot.log.
On systemd, bot_main stderr can be in seen via journalctl.
On gnome startup, I have no idea where stderr goes to.
On OSX via launchd, we generally redirect bot_main stderr to bot_stderr.log.

In task_runner, it'll be sent to task_runner.log.
run_isolated is even more problematic, as it can't afford to corrupt stderr.

R=vadimsh@chromium.org
Bug:  764385 
Change-Id: I47f96df02787a843894c0426424c4f701cb746de
Reviewed-on: https://chromium-review.googlesource.com/679174
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/f008e7266e9da37909ce40eab2c53dcff040d424/appengine/swarming/swarming_bot/signal_trace.py
[modify] https://crrev.com/f008e7266e9da37909ce40eab2c53dcff040d424/appengine/swarming/swarming_bot/signal_trace_test.py