Shallow Thoughts : tags : debugging

Akkana's Musings on Open Source Computing, Science, and Nature.

Sun, 09 Jun 2013

Debugging a Firefox freeze on Ringtail and Sid

I recently went on an upgrading spree on my main computer. In the hope of getting more up-to-date libraries, I updated my Ubuntu to 13.04 "Raring Ringtail", and Debian to unstable "Sid". Most things went fine -- except for Firefox.

Under both Ringtail and Sid, Firefox became extremely unstable. I couldn't use it for more than about fifteen minutes before it would freeze while trying to access some web resource. The only cure when that happened was to kill it and start another Firefox. This was happening with the exact same Firefox -- a 21.0 build from mozilla.org -- that I was using without any problems on older versions of Debian and Ubuntu; and with the exact same profile. So it was clearly something that had changed about Debian and Ubuntu.

The first thing I do when I hit a Firefox bug is test with a fresh profile. I have all sorts of Firefox customizations, extensions and other hacks. In fact, the customizations are what keep me tied to Firefox rather than jumping to some other browser. But they do, too often, cause problems. I have a generic profile I keep around for testing, so I fired it up and used it for browsing for a day. Firefox still froze, but not as often.

Disabling Extensions

Was it one of my extensions? I went to the Tools->Add-ons to try disabling them all ... and Firefox froze. Bingo! That was actually good news. Problems like "Firefox freezes a lot" are hard to debug. "Firefox freezes every time I open Tools->Add-ons" are a whole lot easier. Now I needed to find some other way of disabling extensions to see if that helped.

I went to my Firefox profile directory and moved everything in the extensions directory into a new directory I made called extensions.sav. Then I started moving them back one by one, each time starting Firefox and calling up Tools->Add-ons. It turned out two extensions were causing the freeze: Open in Browser and Custom Tab Width. So I left those off for the time being.

Disabling Themes

Along the way, I discovered that clicking on Appearance in Tools->Add-ons would also cause a freeze, so my visual theme was also a problem. This wasn't something I cared about: some time back when Mozilla started trumpeting their themeability, I clicked around and picked up some theme involving stars and planets. I could live without that.

But how do you disable a theme? Especially if you can't go to Tools->Add-ons->Appearance?

Turns out everything written on the web on this is wrong. First, everything on themes on mozilla.org assumes you can get to that Appearance tab, and doesn't even consider the possibility that you might have to look in your profile and remove a file. Search further and you might find references to files named lightweighttheme-header and lightweighttheme-footer, neither of which existed in my profile.

But I did have a directory called lwtheme. So I removed that, plus four preferences in prefs.js that included the term "lightweightThemes". After a restart, my theme was gone, I was able to view that Appearance tab, and I was able to browse the web for nearly 4 hours before firefox hung again. Darn! That wasn't all of it.

Debugging the environment

But soon after that I had a breakthrough. I discovered a page on my bank's website that froze Firefox every time. But that was annoying for testing, since it required logging in then clicking through several other pages, and you never know what a bank website might decide to do if you start logging in over and over. I didn't want to get locked out.

But then I was checking an episode in one of the podcasts I listen to, which involved going to the link http://downloads.bbc.co.uk/podcasts/radio4/moreorless/rss.xml -- and Firefox froze, on a simple RSS link. I restarted and tried again -- another freeze. I'd finally found the Rosetta stone, something that hung Firefox every time. Now I could do some serious testing!

I'd had friends try this using the same version of Firefox and Ubuntu, without seeing a freeze. Was it something about my user environment? I created a new user, switched to another virtual console (Ctrl-Alt-F2) and logged in as my new user, then ran X. This was a handy way to test: I could get to my normal user's X session in Ctrl-Alt-F7, while the new user's X session was on Ctrl-Alt-F8. Since I don't have Gnome or KDE installed on this machine, the new user came up with a default Openbox session. It came up at the wrong resolution -- the X11 in the newest Linux distros apparently doesn't read the HDMI monitor properly -- but I wasn't worried about that.

And when I ran Firefox as the new user (letting it create a new profile) and middlemouse-pasted the BBC RSS URL, it loaded it, without freezing.

Now we're getting somewhere. Now I knew it was something about my user environment.

I tried copying all of ~/.config from my user to the new user. No hang. I tried various other configuration files. Still no hang.

The X initialization

I'll skip some steps here, and just mention that in trying to fix the resolution problem, so I didn't have to do all my debugging at 1024x768, I discovered that if I used my .xinitrc file to start X, I'd get a freezy Firefox. If I didn't use my .xinitrc, and defaulted to the system one, Firefox was fine. Even if I removed everything else from my .xinitrc, and simply ran openbox from it, that was enough to make Firefox hang.

Okay, what was the system doing? I poked around /etc/X11: it was running /etc/X11/Xsession. I copied that file to my .xinitrc and started X. No hang.

Xsession does a bunch of things, but one of the main things it does is run every script in the /etc/X11/Xsession.d directory. So I made a copy of that directory inside my home directory, and modified .xinitrc to execute those files instead. Then I started moving them aside to see which ones made a difference.

And I found it. /etc/X11/Xsession.d/75dbus_dbus-launch was the file that mattered.

75dbus_dbus-launch takes the name of the program that's going to be executed -- in this case that was x-session-manager, which links to /etc/alternatives/x-session-manager, which links to /usr/bin/openbox-session -- and instead runs /usr/bin/dbus-launch --exit-with-session x-session-manager.

Now that I knew that, I moved everything aside and made a little .xinitrc that ran /usr/bin/dbus-launch --exit-with-session openbox-session. And Firefox didn't crash.

Dbus

So it all comes down to dbus. I was already running dbus: ps shows /usr/bin/dbus-daemon --system running -- and that worked fine for everything dbussy I normally do, like run "gimp image.jpg" and have it open in my already running GIMP.

But on Ringtail and Sid, that isn't enough for Firefox. For some reason, on these newer systems, Firefox requires a second dbus daemon -- it shows up in ps as /usr/bin/dbus-daemon --fork --print-pid 5 --print-address 7 --session -- for the X session. If it doesn't have that, it's fine for a while, and then, hours later, it will mysteriously freeze while waiting for a network resource.

Why? I have no idea. No one I've asked seems to know anything about how dbus works, the difference between system and session dbus daemons, or why any of it it would have this effect on Firefox.

I filed a Firefox bug, Bug 881122, though I don't have much hope of anyone being interested in a bug that only affects Linux users using nonstandard X sessions. But maybe I'm not the only one. If your Firefox is hanging and you found your way here, I hope I've given you some ideas. And if anyone has a clue as to what's really happening and why dbus would have that effect, I'd love to hear from you.

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[ 19:08 Jun 09, 2013    More linux | permalink to this entry | comments ]

Thu, 09 Dec 2010

Article: Troubleshooting, part II: /proc and Python

My article this week on Linux Planet concerns Advanced Linux Server Troubleshooting (part 2).

It's two loosely related topics: exploring the /proc filesystem, and how to use it to find information on a running process; and several ways to get stack traces from Python programs.

This (as well as Troubleshooting part I) arose from a problem we had at work, where we use Linux plug computers (ARM-based Linux appliances) running Python scripts. It's not uncommon for Python networking scripts to go into never-never-land, waiting forever on a network connection without timing out. Since plug computers tend not to be outfitted with the latest and greatest tools like gdb and debug versions of libraries, we've needed to find more creative ways of figuring out what processes are doing to make sure our programs are ready for anything.

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[ 10:44 Dec 09, 2010    More linux | permalink to this entry | comments ]

Wed, 24 Nov 2010

Article: Troubleshooting running processes, part I

How do you troubleshoot a process that's running away, sucking up too much CPU, or not doing anything at all? Today on Linux Planet: Troubleshooting Linux Servers: top and Other Basic System Tools.

This is part I, covering basics like top, strace and gdb. Part II will get into hairier stuff and tips for debugging Python applications.

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[ 20:06 Nov 24, 2010    More linux | permalink to this entry | comments ]

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