When Good Code Goes Bad: Tools and Techniques for Troubleshooting Plone

When Good Code Goes Bad: Tools and Techniques for Troubleshooting Plone Plone Conference 2008 David Glick Web Developer ONE/Northwest

What we're going to cover
1. Preparation for debugging
2. Debugging tools
3. 2 case studies
4. Tips & tricks

Newbie programmer's reaction: Absolute terror

Experienced programmer's reaction: Absolute terror

Maybe it's a little bug...

...but who knows what's in there, really!

What could be wrong?
Trivial typo
Logic error
Failure to consider certain input values
Faulty assumptions about the state of the system
Deep voodoo

Mental preparation

Remember: You are in control.

Remember: You are not alone.

Use the source, Luke.

Getting ready...

Don't use debugging tools in production.
Security
Performance

Run zope in foreground mode.
Buildout:
$ bin/instance fg
Pre-buildout:
$ zopectl fg

Turn on verbose security.
Buildout:
in buildout.cfg, [instance] section:
verbose-security on
Pre-buildout:
in zope.conf:
verbose-security on
security-policy-implementation python

Stop ignoring errors in /error_log

Debugging tools

1. What is that thing?

Firebug (Firefox)
http://tinyurl.com/2z4tvg
IE Web Developer Toolbar (IE) http://tinyurl.com/iedevtoolbar
WebKit (Safari) http://webkit.org
Google Chrome inspector
Investigating HTML, CSS, and Javascript >

Investigating Plone viewlets and portlets
plone.app.gloworm http://tinyurl.com/gloworm

Investigating the current page context
Products.Clouseau http://tinyurl.com/clouseau

2. What code controls this?

Seek and ye shall find!
Grep
Multi-file search and replace feature in editor of choice
Code indexers (etags, glimpse)

Where is the code?
From a filesystem page template:
<tal:block tal:replace=”python:obj.__class__”/>
(gives you the dotted path of module + class of obj)
From any page template:
<tal:block tal:replace=”nocall:obj”/>
(gives you the class name of obj)

collective.recipe.omelette
Before :
After:
http://tinyurl.com/collective-omelette

3. What's that code doing?

Logging
context.plone_log(my_var)
Output will show up on console
Useful for when some code is being run repeatedly and you want to make sure it's operating on the correct values

pdb: the Python debugger

pdb: the Python debugger
Trigger with:
import pdb; pdb.set_trace()
Don't forget to remove the set_trace!

4. What's Zope doing right now?
DebugInfo page (built in): /ControlPanel/DebugInfo/manage_main in ZMI
DeadlockDebugger product http://tinyurl.com/deadlockdebugger

5. Why is this so slow ?
YSlow – frontend performance
http://developer.yahoo.com/yslow/
PTProfiler – page templates
http://plone.org/products/ptprofiler
ZopeProfiler – Python calls
http://pypi.python.org/pypi/Products.ZopeProfiler

What does it look like in real life?
Case #1:
plone_newbie4022's Quick Installer problem
(from #plone, 2008-09-08)

Reading a traceback module Line # method More recent
Snapshot of what code is executing
Doesn't include the values

Reading a traceback (cont'd) For page template expressions Template file location The actual TAL expression that was called Location of expression within the template The actual Error message! Bonus (but irrelevant) error The Error happened here!

Common Python exceptions
AttributeError
e.g. obj.nonexistent_attribute
ImportError
e.g. import nonexistent_module
IndexError
e.g. my_list[42] , if my_list doesn't have >=43 elements

Common Python exceptions
KeyError
e.g. my_dict['nonexistent_key']
TypeError
e.g. foo = ('tuple',) + 'string'
ValueError
e.g. int('string')

Common Zope exceptions
Unauthorized
NotFound
Redirect
TraversalError
ComponentLookupError – failed zope 3 lookup
ConflictError – ZODB conflict

Lessons from Case #1

The problem is sometimes (usually?) not where you think the problem is.

Think carefully about both:
What the code is doing
What the state is of the objects in your ZODB

Often you don't have to change any code to fix a bug, once you understand what is going on.

What does it look like in real life?
Case #2:
some (only somewhat) contrived
catalog trouble

plone.reload
Reload Python code without restarting Zope.
Just go to /@@reload.

pdb commands
l / list: print the code. l 42 == list line #42
w / where: print a traceback
n / next: run the next command
s / step: enter a method that is being called
r / return: execute until the current method completes
c / continue: continue execution indefinitely (end pdb session)

advanced pdb commands
b / breakpoint: b 42 == set a breakpoint on line 42
cl / clear: cl 1 == remove breakpoint #1
j / jump: j 42 == jump to line 42
u / up: move up in the call stack
d / down: move down in the call stack
Full pdb docs: http://www.python.org/doc/2.5.2/lib/module-pdb.html

pdb idioms
pp obj – pretty-print an object
dir(obj) – list an object's attributes and methods
obj.__dict__ -- list an object's data attributes
obj.__doc__ -- print an object's docstring
args – list the arguments passed to the current method
retval – print the value that will be returned from the current method
!expr – force expr to be treated as a Python expression rather than a pdb command

Lessons from Case #2

If you don't have a traceback, finding the source of misbehavior can be hard . Be persistent.*
* Bad Zope pun not intended.

Try the most obvious things first. (Assume there's a bug in your own code, rather than in Zope/Plone.)

Tips and Tricks

PDBDebugMode product
Automatically get a pdb whenever there is a python exception
Invoke a pdb on any context via /@@pdb view
Invoke a pdb on anything via /path/to/anything?pdb_runcall=1

Debugging python scripts
Use the zdb product http://www.simplistix.co.uk/software/zope/zdb
from Products.zdb import set_trace; set_trace()
Ensures that you can (l)ist the code of the Python script
Doesn't work for form controller Python scripts (.cpy) though
When stepping into calls in restricted python, you will first enter guarded_getattr, which you can (r)eturn out of and then the (n)ext call will be the one you want

Tab key auto-completion in pdb
Add the following to ~/.pdbrc:
import rlcompleter
pdb.Pdb.complete = rlcompleter.Completer(locals()).complete

Why aren't my changes coming alive?
template, image & stylesheet customizations:
put CSS, Javascript, KSS registries in debug mode, or at least re-save them
make sure you don't have something in portal_skins/custom or portal_view_customizations that's overriding your filesystem item
flush browser cache

changes to python code or ZCML:
restart Zope, or use plone.reload

changes to GenericSetup profiles:
re-run profile or reinstall product (don't need to restart zope)
installing a new product:
if not in the Products namespace, did you add a ZCML slug?

Debugging KSS
Use the FireKiss add-on for Firebug: http://tinyurl.com/firekiss
Configure KSS debug log at /@@kss_devel_mode/ui
Be aware that KSS will time out if you try to put a pdb set_trace in your server-side KSS action code

Common red herrings

“Not authorized to access binding.”
A deferred permission error from earlier in code execution.
See http://tinyurl.com/unauthorizedbinding

Quick Installer swallows import errors
(Fixed in Plone 3.1.2)

AttributeError: getGroups
Usually means you moved the site to a new Zope, whose acl_users needs to be converted to be PAS-enabled.
You can trigger that conversion by adding a brand new Plone site.

“No traversable adapter found.”
Usually a symptom of Five failing to load.
Start Zope in foreground mode and watch the console for the real traceback.

Credits
Cave openings, screenshots – David Glick
First computer bug – US Navy / public domain
Flickr users:
rotten apples – alisonedunn – CC Attribution
gearing up for spelunking – jarvist – CC Attribution
pause button – Jobot Da Robot – CC Attribution Share Alike
“ debug yourself” – joshlewis – CC Attribution NC Share Alike
“ no fish” – waldopepper – CC Attribution NC
These slides are licensed under the following license: Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License

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When Good Code Goes Bad: Tools and Techniques for Troubleshooting Plone

by David Glick

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