Debugging Ansible for fun and no profit

28 Apr 2014

A colleague reported some strange behaviour regarding Ansible, in particular with pgrep and pkill in the shell module.

I created a simplish test-case (I could have made it simpler but wanted to make it safish for other people to use)

- hosts: localhost
connection: local

tasks:
- name: generate random process string
action: shell openssl rand 15 | base64
register: process_string

- name: warn user
pause: prompt="About to kill processes matching with signal SIGIO. Hit Return to continue or Ctrl-C then a to abort"

- name: show bad effects of pkill -f
action: shell pkill -IO -f || true

Running this with ansible-playbook pkilldemo.yml -v shows the problem:

PLAY [localhost] ************************************************************** 

GATHERING FACTS ***************************************************************
ok: [localhost]

TASK: [generate random process string] ****************************************
changed: [localhost] => {"changed": true, "cmd": "openssl rand 15 | base64 ", "delta": "0:00:00.011483", "end": "2014-05-28 15:46:36.753195", "rc": 0, "start": "2014-05-28 15:46:36.741712", "stderr": "", "stdout": "dkfCGgWBFr2ntK9uhSaw", "warnings": []}

TASK: [warn user] *************************************************************
[localhost]
About to kill processes matching dkfCGgWBFr2ntK9uhSaw with signal SIGIO. Hit Return to continue or Ctrl-C then a to abort:

ok: [localhost] => {"changed": false, "delta": 9, "rc": 0, "start": "2014-05-28 15:46:36.770642", "stderr": "", "stdout": "Paused for 0.16 minutes", "stop": "2014-05-28 15:46:46.223047", "user_input": ""}

TASK: [show bad effects of pkill] *********************************************
failed: [localhost] => {"changed": true, "cmd": "pkill -IO -f dkfCGgWBFr2ntK9uhSaw || true ", "delta": "0:00:00.010195", "end": "2014-05-28 15:46:46.379405", "rc": -29, "start": "2014-05-28 15:46:46.369210", "warnings": []}

FATAL: all hosts have already failed -- aborting

PLAY RECAP ********************************************************************
to retry, use: --limit @/home/will/pkilldemo.retry

localhost : ok=3 changed=1 unreachable=0 failed=1

So running pkill -IO -f pattern_to_search_for || true returns -29 (where SIGIO is signal 29 on my machine).

Adding the following to the above playbook allows us to see different success and failure scenarios:

    ignore_errors: True

- name: works ok without -f
action: shell pkill -IO || true

- name: works ok with pgrep -f
action: shell pgrep -f || true

- name: this is supposed to fail as we don't handle the error with true
action: shell pgrep -f
ignore_errors: True

- name: what we should actually run
action: raw pkill -f
ignore_errors: True

After running the resulting playbook (that playbook and output are available as a gist), the known facts are these:

Setting up debugging in Ansible

I tend to use pdb for debugging with the python script that actually gets generated by Ansible. The creator of Ansible, Michael Dehaan (@laserllama) suggests epdb for Ansible debugging.

As Michael suggests in that blogpost, it’s very important to set forks to 1. You can do this temporarily using --forks=1 on the command line but I tend to do so little stuff that needs parallelism that I have forks = 1 in my ansible config file!

To obtain the module python script, you can set the ANSIBLE_KEEP_REMOTE_FILES environment variable, either using export ANSIBLE_KEEP_REMOTE_FILES=1 or

[will@fedora pkilldemo]$ ANSIBLE_KEEP_REMOTE_FILES=1 ansible-playbook pkilldemo.yml -vvv

PLAY [localhost] **************************************************************

GATHERING FACTS ***************************************************************
<snip>

TASK: [generate random process string] ****************************************
<snip>

TASK: [warn user] *************************************************************
<snip>

TASK: [show bad effects of pkill -f] ******************************************
<localhost> REMOTE_MODULE command pkill -IO -f Ki5ViZNY4EIRaf2JDqvQ || true #USE_SHELL
<localhost> EXEC ['/bin/sh', '-c', 'mkdir -p $HOME/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374 && chmod a+rx $HOME/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374 && echo $HOME/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374']
<localhost> PUT /tmp/tmp39O8iv TO /home/will/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374/command
<localhost> EXEC ['/bin/sh', '-c', u'LC_CTYPE=en_US.UTF-8 LANG=en_US.UTF-8 /usr/bin/python /home/will/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374/command']
failed: [localhost] => {"changed": true, "cmd": "pkill -IO -f Ki5ViZNY4EIRaf2JDqvQ || true ", "delta": "0:00:00.017252", "end": "2014-05-28 16:01:58.302777", "rc": -29, "start": "2014-05-28 16:01:58.285525", "warnings": []}

So I can then run python /home/will/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374/command to repeat the task

[will@fedora pkilldemo]$ python  /home/will/.ansible/tmp/ansible-tmp-1401256918.15-50781745619374/command
{"changed": true, "end": "2014-05-28 16:03:49.657242", "stdout": "", "cmd": "pkill -IO -f Ki5ViZNY4EIRaf2JDqvQ || true ", "start": "2014-05-28 16:03:49.646183", "delta": "0:00:00.011059", "stderr": "", "rc": -29, "warnings": []}

At this point it’s probably worth a brief exposition of how the shell module works in Ansible. Really it just kicks off the command module with a USE_SHELL=1 module. So I’m going to be looking at the command module source from the latest commit at time of writing.

What Ansible does with that module is to replace line 162 with the contents of the module_utils/basic code and then swap a few template tokens (e.g. "<<INCLUDE_ANSIBLE_MODULE_ARGS>>" gets replaced with 'pkill -IO -f Ki5ViZNY4EIRaf2JDqvQ || true #USE_SHELL'). Anyway, the end result is that the 213 line command module gets expanded to 1419 lines.

The line I’m interested is the same either way - it’s line 140, where the command gets executed. So I open up the script and add

import pdb
pdb.set_trace()

just before line 140, and run it again.

The python docs have some good instructions on how to use the debugger. After stepping through the debugger enough, I realise the eventual result is that python is going to kick off a shell that looks like

sh -c 'pkill -IO -f Ki5ViZNY4EIRaf2JDqvQ || true'

And, sure enough, that matches itself, and kills itself before it gets to the true:

[will@fedora ansible (devel)]$ sh -c 'pkill -IO -f bobbins || true' 
I/O possible
[will@fedora ansible (devel)]$ echo $?
157

where 157 is 128 + 29.

When a command terminates on a fatal signal N, bash uses the value of 128+N as the exit status

man bash

This explains why pgrep does not fail, and why pkill without -f does not fail. It’s not really an Ansible bug (one could argue that maybe process management is a missing module), but it was instructive to use Ansible debugging techniques to get to the cause.

And the actual solution to the problem was to use ignore_errors rather than using true in the shell to hide the error.

  name: kill process if running
action: command pkill -f bobbins
ignore_errors: True