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Debugging an application that freezes the X server

kinow @ Jan 02, 2017 00:39:03

Some time ago, I decided to start writing C++ again, and picked Krita for that. Krita is written in C++ and Qt. The bug I was working on involved a memory issue when changing the UI.

The problem was that this UI change would result in, not only Krita, but the whole X server freezing. My set up was basically Ubuntu 16.04.1 LTS, with Eclipse, Qt5, and the latest version of Krita checked out via git.

Initially I spent some time looking at the logs, tracing the binary, even downloaded some Qt utility tools to look at the events and what was happening with the application when it froze.

However, eventually I realized I was going to spend more time on this part of the issue, rather than on the memory bug. So decided to look for a work-around.

Thankfully someone else blogged about a similar issue and saved me a lot of time :-) Here’s how I did it, following the instructions in this blog post.

$ sudo apt-get install xnest -y
$ cd krita_install/
$ Xnest :10
$ twm -display :10
$ export DISPLAY=:10
$ ./krita

That’s it. You should have Krita running in a separate window, with Xnest, and within this window the twm window manager running. So when it freezes, at least you can still debug your application in Eclipse or whatever IDE you prefer.

Happy hacking!

Drawing sketch: Uh!

kinow @ Dec 19, 2016 19:40:03


Uh! progress
Uh! progress

And final work here.


For redditgetsdrawn

PermGen errors and java.lang.ClassCastException: com.sun.crypto.provider.AESCipher cannot be cast to javax.crypto.CipherSpi running Jenkins plug-in tests with PowerMock

kinow @ Dec 19, 2016 19:20:03

Recently while working on a Jenkins plugin some tests were failing with PermGen errors. Even though it worked in my notebook at home (with Java 8, thus no Permgen), it failed in a CloudBees hosted Jenkins job, and also on my Mac (with Java 7) at work.

As I could not change the settings in the CloudBees hosted Jenkins, I decided to spent some time investigating why these tests would require so much memory. Then I found this blog post about PowerMock.

I was not using the @PrepareForTest annotation, but after adding it the issue was gone. So I assume it either prevents PowerMock from trying to dynamically load several classes, or instructs it to unload classes after the tests. But in anyway after adding it the issue was gone.

Then I got the following exception.

WARNING: Failed to instantiate Key[$DescriptorImpl, annotation=[none]]; skipping this component Guice provision errors:

1) Error injecting constructor, java.lang.ClassCastException: com.sun.crypto.provider.AESCipher cannot be cast to javax.crypto.CipherSpi

1 error

And then thanks to this issue I understood that PowerMock was mocking javax.crypto classes. Turns out it is quite easy to tell PowerMock to ignore certain classes from being mocked, with the @PowerMockIgnore annotation.

// snip
@PowerMockIgnore({"javax.crypto.*" })
public class TestAbstractUnoChoiceParameter {
// snip

Added a couple of notes to this Jenkins Wiki page so that users facing similar issues can try these possible workarounds.

Hope that helps someone!

Using Requests Session Objects for web scraping

kinow @ Nov 25, 2016 01:24:03

I had to write a Python script some months ago, that would retrieve Solar energy data from a web site. It was basically a handful of HTTP calls, parse the response that was mainly in JSON, and store the results as JSON and CSV for processing later.

Since it was such a small task, I used the Requests module instead of a complete web scraper. The HTTP requests had to be made with a time out, and also pass certain headers. I started customizing each call, until I learned about the Requests Session Objects.

You create a session, as in ORM/JPA, where you can define a context, with certain properties and control an orthogonal behavior.

import requests

def get(session, url):
    """Utility method to HTTP GET a URL"""
    response = session.get(url, timeout=None)
    return response

def post(session, url, data):
    """Utility method to HTTP POST a URL with data parameters"""
    response =, data=data, timeout=None)
    if response.status_code !=
    return response

with requests.Session() as s:
    # x-test header will be sent with every request
    s.headers.update({'x-test': 'true'})

    data = {'user': user, 'password': password, 'rememberne': 'false'}
    r = post(s, 'https://portal.login.url', data)

    r = get(s, 'https://portal.home.url')

Besides the session object, that gives you ability to add headers to all requests, you won’t have to worry about redirects. The library by default takes care of that with a default limit of up to 30.

Happy hacking!

Changing Spring Boot environment variables in the command line

kinow @ Nov 21, 2016 21:26:03

This week while helping developers and testers to experiment with a backend application, some of them found useful to learn a simple trick to change Spring Boot properties when you can run the application locally (our testers build, compile, change the code, how cool is that?).

Here’s how it works. Say you have the following settings in your application’s


And that you want to change these parameters in order to, for instance, create an application error, so that you can code and test what happens to the frontend application in that situation.

You replace dots by underscores, and put all your words in upper case. So the variables above would be: MY_APPLICATION_DATABASE_USERNAME and MY_APPLICATION_DATABASE_PASSWORD.

Furthermore, you do not need to edit your file, if you are on Linux or Mac OS. You can start the application and override environment variables at the same time with the following syntax.


This way your application will start with the new values.

Happy hacking!


As pointed by Stéphane Nicoll (thanks!), you could change the property values without having to use the upper case syntax.

mvn -Dmy.application.database.username=anotheruser clean spring-boot:run

And he even included a link to docs! ♥ the Internet and Open Source!