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This page collects tips that might make debugging Launchpad a little easier. They're presented in no particular order -- please add yours! All pages dedicated to testing can be found in CategoryTesting.

Debugging stories/pagetests

Debugging stories (a.k.a. pagetests) can be kind of a pain because they build up state as they go. So if a test fails down deep in the doctest, and you want to see what the page really looks like at that point, you'd normally have to manually click through each step in the doctest until you get to the place that's broken.

But there's an easier way!

Just add this at the point where your pagetest is failing:

    >>> stop()

This is just a convenience wrapper around pdb.set_trace() except that it also redirects stdout back to your console. When your pagetest hits this line, you'll be dropped into the debugger. Now, go to a different shell and type:

    % make LPCONFIG=testrunner run

(Note from deryck: To get the above make run command working, I had to use the value in os.environ['LPCONFIG'] obtained at the pdb prompt, rather than just "testrunner")

This starts up the appserver, except that it will use the testrunner configuration. What's cool about that is that this configuration attaches to the same database as the pagetest you're now stopped inside of. Meaning, all that state your doctest has built up, is available to your browser. So just hit the URL of the page that your doctest is failing on, and see it in all it's wondrous, albeit borked, glory.

Additionally, it is also possible to connect directly to the database the page test is using.

    % psql -d launchpad_ftest

However, uncommitted transactions will not be visible in the database. So if there is something particular you are trying to query, like a bug subscription, you may need to add the following to the page test.

    >>> transaction.commit()

Debugging memory leaks

The app servers and the Librarian install a signal handler to dump their memory using meliae. To make use of that you just have to send the 44 signal to the appropriate process. This will create a file named /tmp/launchpad-memory.dump (or /tmp/librarian-memory.dump, for the Librarian), which you can debug later.

In general, though, the leaks will happen in production, so the LOSAs will give us a memory dump. Assuming you've got a memory.dump file, this is how you load it:

    $ ./bin/py
    >>> from meliae import loader
    >>> om = loader.load('memory.dump')
    >>> s = om.summarize(); s

This will give you a nice summary of the kinds of objects that consume the most memory. Something like

Total 704144 objects, 768 types, Total size = 129.5MiB (135823229 bytes)
 Index   Count   %      Size   % Cum     Max Kind
     0  108751  15  68190568  50  50 3146008 dict
     1  192992  27  16136424  11  62    6904 tuple
     2  162136  23  14850362  10  73   11103 str

You can then use that information to navigate the object tree as described in John Meinel's excellent post, and hopefully find what's leaking.

Debugging With GDB

Some kinds of bugs (segfaults, hung processes, out-of-control daemons) are hard to debug from within Python. See Debugging/GDB for how to debug them.

There's also a doc on Debugging Core Dumps.

Debugging Buildd with the Visual Studio Code IDE

Although we set up Buildd in a VM, Unit tests can be debugged in a visual & interactive way with Visual Studio Code with below setup for remote ssh&debug.

sudo snap install --classic code

Launch VSCode and install the "Python" and "Remote - SSH" extensions. For the Python extension perform the install on the remote host (your Buildd VM) as well.

F1 to open the command palette and type Remote-SSH: Open SSH Configuration file. From the drop-down choose your home ssh cofing file (~/.ssh/config - if you don't have one create it and add your configuration). Add the following entry to it (for Host I have the name of my Buildd VM and for HostName the IP of my Buildd VM):

Host buildd
    User ubuntu
    IdentityFile ~/.ssh/

F1 to open the command palette and type "Remote-SSH: Connect to Host..." and choose "buildd" from the drop-down. You might be prompted for the password for the "ubuntu" user.

Once the ssh connection is established go to the Explorer and "Open Folder" -> launchpad-buildd (the git clone of the buildd repo on your VM).

Configure the test framework (visual examples - for buildd choose unittests for the lpbuildd folder and the test*.py file pattern.


1. Command palette opens with F1.

2. When the workspace is large and contains many files VS Code file watcher is running out of handles (ENOSPC Error visible is you start VSCode in terminal with code --verbose). Solution to this is: to see your current limit:

cat /proc/sys/fs/inotify/max_user_watches

Add this line:


to /etc/sysctl.conf and then  sudo sysctl -p .

Special URLs

Launchpad provides special URLs that can be used to help with debugging.

URL element




show the TAL declarations in the HTML source code

developer box

Example: https://launchpad.test/++debug++tal


show path to templates for a given view in the HTML source code

developer box

Example: https://launchpad.test/++debug++source


Get help on how to use the ++profile++ option.

developer box, [qa]staging

Example: https://launchpad.test/++profile++ or


See SQL queries used by the page.

developer box, [qa]staging

Example: https://launchpad.test/++profile++sql or


See SQL queries and Python stack traces that led to them.

developer box, [qa]staging

Example: https://launchpad.test/++profile++sqltrace or


Show Python profile data and OOPS data, including SQL queries and timing.

developer box, [qa]staging

Example: https://launchpad.test/++profile++show or


Generate a pstats (Python standard library) profile file on the file system. Browser page gives you full path to generated file. Note that, on [qa]staging, you will need to ask LOSAs to get you the file.

developer box, [qa]staging

Example: https://launchpad.test/++profile++pstats or


Generate a KCacheGrind profile file on the file system. Browser page gives you full path to generated file. Note that, on [qa]staging, you will need to ask LOSAs to get you the file.

developer box, [qa]staging

Example: https://launchpad.test/++profile++callgrind or


record an OOPS report while still rendering the page correctly. The OOPS id is provided in the HTML source code


Example: https://launchpad.test/++oops++ or


Not a debug tool. Used for JS. Gives inner form HTML.


Example: https://launchpad.test/~/+edit/++form++

Some of those can combined, like: ++debug++tal,source or ++profile++show,pstats.

++debug++errors is not working currently, probably because of Launchpad customizations. It is supposed to show tracebacks of errors handled in the template.

Tracing SQL statements through STORM

These can be useful when optimising pages to run fewer queries, as you can see exactly when and what is executed rather than pulled from cache.

Tracing a full request

Set LP_DEBUG_SQL=1 environment variable before running make harness or make run to get the SQL statements as they are run, along with the start and stop times and the name of the database on which the statement was run. Note that in a request the times are relative to the start of the request. For scripts and make harness, the start time is always 0 and the stop time is the duration of the SQL call.

Set LP_DEBUG_SQL_EXTRA=1 to get all of the above, plus tracebacks for every query execution, including template and traversal information.

When using make run, these affect all requests while the server is running, and output the value in the console.

Alternatively, to only look at a single request's values in the browser, use ++profile++sql instead, which includes the information equivalent to LP_DEBUG_SQL=1; or use ++profile++sqltrace, which gives you all of the information equivalent to LP_DEBUG_SQL_EXTRA=1. These are described above in the "Special URLs" section.

Tracing a part of a request

from storm.tracer import debug; debug(True) will cause all statements run by Storm to be written to stderr. debug(False) disables this behaviour.

Alternatively, if you find LP_DEBUG_SQL=1 and/or LP_DEBUG_SQL_EXTRA=1 handy but want more control turning it on and off within a request, in the debugger you can make sure the LaunchpadStatementTracer is the first in the results of get_tracers and modify as needed. For instance, you can do the following.

This gives output equivalent to LP_DEBUG_SQL=1 but for only as long as _debug_sql = True.

from storm.tracer import get_tracers
get_tracers()[0]._debug_sql = True

This gives output equivalent to LP_DEBUG_SQL_EXTRA=1 but for only as long as _debug_sql_extra = True.

from storm.tracer import get_tracers
get_tracers()[0]._debug_sql_extra = True

Tracing a code snippet

Similar to the previous section, sometimes you want to look at the SQL of just a certain slice of code, such as within make harness. The StormStatementRecorder can be a useful tool for this.

Basic usage will get you the SQL run while the recorder is used:

from lp.testing import StormStatementRecorder

with StormStatementRecorder() as recorder:
    ...code that touches the DB goes here...

print recorder

Printing the recorder gives you a full output of what happened. You can also look at .statements, .count, and so on (use dir!).

You can get all tracebacks by passing True when you instantiate the recorder (StormStatementRecorder(True)). Again, print the recorder to see the results.

You can conditionally get tracebacks by passing a callable that receives a SQL query string and returns a boolean True if a traceback should be collected, and False if it should not. The SQL will be normalized to capitalization and space normalized. For example, StormStatementRecorder(lambda sql: 'STRUCTURALSUBSCRIPTION' in sql) would get you tracebacks when the SQL has something to do with structural subscriptons.

Getting more information in your tracebacks

The tracebacks from LP_DEBUG_SQL_EXTRA=1 and ++profile++sqltrace include extra information from page templates and traversals to tell you the expressions and values being processed. If you have functions or loops for which you'd also like to add your own extra debugging information to the tracebacks, here is how.

If you don't plan on checking the change in, or if the string you want already exists and does not need to be generated, just assign the string with the extra information you want to the variable name __traceback_info__. That string will then be included in the information for that frame in tracebacks generated by this machinery, as well as in renderings of tracebacks from the appserver.

If you plan on checking the change in, you should be more careful: we only want to do the work if a traceback is rendered, not every time the code path is traveled. Then you have two options. The first is to create an object that will do the work only when it is cast to a string (in __str__) and assign it to a variable named __traceback_info__, as above.

The second, more involved option is to assign a two-tuple to __traceback_supplement__. The first element of the tuple should be a factory, and the second argument should be an iterable that is passed to the factory as *args. The factory should produce an object with any or all of the following attributes:

Some string that represents a source. For page templates, this is the path to the template file.
value castable to str that is presented as a line number.
value castable to str that is presented as a column number.
value castable to str that is presented as an expression currently being processed (like a TALES expression).
an iterable of strings that represent some warning to communicate.
a callable that returns some extra string.

Tracing SQL statements with PostgreSQL

Statement logging can be configured in postgresql.conf, by setting log_statement to one of none, ddl, mod or all (docs). The server needs to be reloaded (by SIGHUP or pg_ctl reload) for changes to take effect.

It can also be set for a session, user or database:

  SET log_statement TO 'all'; -- (docs)

  ALTER ROLE launchpad SET log_statement TO 'all'; -- (docs)

  ALTER DATABASE launchpad_dev SET log_statement TO 'all'; -- (docs)

Once enabled, statements will be logged to /var/log/postgresql/postgresql-*-main.log.

Getting past "LocationError: 'json'" in TAL template tracebacks

If you're testing with a new TAL template (.pt file) and you get nasty-looking tracebacks that says something about

  LocationError: (<lazr.restful.tales.WebLayerAPI object at 0xd932ccc>, 'json')

then try visiting the corresponding URL in the web services API. For example, if https://bugs.launchpad.test/redfish gets an unwieldy traceback, then try https://launchpad.test/api/beta/redfish instead; you'll often get a much more comprehensible error trace that way.

Using iharness for digging error tracebacks

If you are reading this, most probably you have noticed that when things get wrong, ZOPE and TAL will rather give you a pointless LocationError without to much information about what is causing it.

To find out what exactly went wrong you can use make iharness and investigate that specific LocationError

Let's say that you got this error for language_translation_statistics:

LocationError: (<zope.browserpage.metaconfigure.SimpleViewClass
from PATH_TO_TEMPLATE/ object at 0xcf60fec>,

To start the testing/debugging environment (the harness) run:

make iharness

Next you will have to import your classed and get your object. In our example we were trying to get the PerLanguageStatisticsView for ubuntu['hoary'] series.

from canonical.launchpad.webapp.servers import LaunchpadTestRequest
from lp.our.module import  PerLanguageStatisticsView

#create and initialize the view
ubuntu = getUtility(ILaunchpadCelebrities).ubuntu
view = PerLanguageStatisticsView (ubuntu['hoary'], LaunchpadTestRequest())

#request the view key
key = view.language_translation_statistics

Now you should see a more meaningful message.

Profiling page requests

You can generate KCacheGrind and pstats (Python standard library) profiles of requests on your local system.

On your developer machine, try going to https://launchpad.test/++profile++ or https://launchpad.test/++profile++/~mark/+archive/ppa . Inserting ++profile++ in the URL like this will give you instructions on how to use the feature.

The ++profile++ mechanism has a number of features now, as described in the "Special URLs" section above. For Python profiling, it can generate immediate profiles in the browser (++profile++show), profiles on the filesystem ready for kcachegrind (++profile++callgrind), profiles on the filesystem ready for pstats (++profile++pstats),or combinations (such as ++profile++show,pstats).

If you want to use this on staging or qastaging, this is already set up for you. You may need to ask a LOSA to temporaily increase the timeout for the page that you want to analyze using the feature flags mechanism (e.g., if you want to profile BugTask:+index pages, you'll need to ask LOSAs to add something like

hard_timeout pageid:BugTask:+index   2       30000

to That sets a timeout of 30 seconds (30000 milliseconds).

You can also turn on a configuration variable to profile every request. Edit configs/development/launchpad-lazr.conf and add the following section:

profile_requests: True

Then start the development server and make ONE request to the URL you wish to profile (in order to make a single request on pages that make subsequent JS calls immediately on load, you may need to use wget or similar):

$ make run
... server starts...
$ curl -k https://launchpad.test/ -o /dev/null
# or
$ wget --no-check-certificate https://launchpad.test

You can now load the resulting *.prof file into KCacheGrind

$ kcachegrind 2010-07-20_10\:01\:46.680-RootObject\

The doc for these features is lib/canonical/launchpad/doc/profiling.txt , but you may find that the ++profile++ overlay gives you sufficient instructions, if you use that approach.

Profiling one part of your page

If you are working on a developer instance of Launchpad, you can also insert calls directly in your code to profile certain code paths when viewing pages. This will aggregate profiling calls within the request, so you can do this around code that is called multiple times in the request. Try something like this:

from import profiling
with profiling():
    # Do the work that you want profiled here!

This will then generate a pstats file for you on the filesystem at the end of the request, and give you the data in the browser as well.

Debugging production OpenID problems

You can use the production OpenID provider to debug problems that can't be reproduced with the test provider by changing configs/development/launchpad-lazr.conf thusly:


Debugging security proxy problems

Ever wondered which attributes are protected on an instance and by which permission? You can use debug_proxy to get the information you need.

Example make harness session:

francis@Casteneda:~/canonical/launchpad/bug-365098$ make harness
Initializing storm...
Creating the factory...

>>> from lp.registry.interfaces.distribution import IDistributionSet
>>> ubuntu = getUtility(IDistributionSet).getByName('ubuntu')
>>> evolution = ubuntu.currentseries.getSourcePackage('evolution')
>>> from lazr.restful.debug import debug_proxy
>>> debug_proxy(evolution)
' (using\n    
public: __eq__, __getitem__, __hash__, __ne__, _getOfficialTagClause, 
all_bugtasks, bug_reported_acknowledgement, bug_reporting_guidelines, 
bugtargetdisplayname, bugtargetname, bugtasks, closed_bugtasks, createBug, 
critical_bugtasks, currentrelease, deletePackaging, development_version, 
direct_packaging, displayname, distinctreleases, distribution, 
distribution_sourcepackage, distroseries, enable_bugfiling_duplicate_search, 
format, getBranch, getBranches, getBugCounts, getBugTaskWeightFunction, 
getBuildRecords, getCurrentTemplatesCollection, getCurrentTranslationFiles, 
getCurrentTranslationTemplates, getFirstEntryToImport, 
getLatestTranslationsUploads, getMergeProposals, getPocketPath, 
getSharingDetailPermissions, getSharingPartner, getSuiteSourcePackage, 
getTemplatesAndLanguageCounts, getTemplatesCollection, 
getTranslationImportQueueEntries, getTranslationTemplateByName, 
getTranslationTemplateFormats, getTranslationTemplates, getUsedBugTags, 
getUsedBugTagsWithOpenCounts, get_default_archive, has_bugtasks, 
has_current_translation_templates, has_obsolete_translation_templates, 
has_sharing_translation_templates, has_translation_files, 
has_translation_templates, high_bugtasks, id, inprogress_bugtasks, 
latest_published_component, latest_published_component_name, linkedBranches, 
linked_branches, max_bug_heat, name, newCodeImport, new_bugtasks, 
official_bug_tags, open_bugtasks, packaging, path, product, productseries, 
published_by_pocket, recalculateBugHeatCache, releases, searchTasks, 
setBranch, setMaxBugHeat, setPackaging, 
setPackagingReturnSharingDetailPermissions, shouldimport, sourcepackagename, 
summary, title, unassigned_bugtasks\n'

CategoryTipsAndTricks CategoryTesting

Debugging (last edited 2021-09-14 10:44:48 by ilasc)