Desi Penguin’s Blog » Mandar Vaze

Mystery of jsonrpc call that never completes

Mandar Vaze — Thu, 12 Jan 2012 18:16:35 +0000

Image via Wikipedia

Recently I came across an interesting problem (to say the least) where jsonrpc call would execute the error call back function, but did not provide the details. Additionally (as the title suggests) Firebug showed that the call never completed. Firebug continues to show rotating ring next to this call. But server logs suggested that server not only received the call, but it also processed the request completely.

I googled for “jsonrpc never completes” (and other similar terms) But no one had faced the problem. Strange

Normally, when jsonrpc call executes error callback, there would be some error on the server (serving the rpc call) But in this case, the call went thru fine. No error on server.

So I started debugging. First place to look, check server logs. No error there. Added additional debug statements on server. Still no help.

Then I modified the client-side code. Checked if there were additional information in the response sent back. Nothing.

May be there was something wrong with this specific RPC. Just to isolate, I made another RPC, which was known to work at other places. I was so sure that problem would go away, that I was shocked that problem remained.

So clearly it wasn’t the RPC, but the way it was called. So I started looking at surrounding code. Wasn’t any different. So I went further up the stack. It turns out later in the chain – unconditional window.location.reload() was called. This was causing the browser to reload the page even before the jsonrpc would complete.

Looking back – it was obvious. So why did I write this post ? So that someone else who may come across such symptom in future, may have some tips to troubleshoot.

In case you are interested, this was part of this project

Debugging Javascript applications (markos.gaivo.net)
Firebug Guide for Web Designers (sixrevisions.com)

Installing gevent on shared hosting server

Mandar Vaze — Tue, 03 Jan 2012 06:18:50 +0000

Recently, I had to install “gevent” python module on webfaction. Normally, I would install it using “pip install gevent” – Very straight forward. But python gevent module requires libevent library, specifically the .so file. This file normally resides in /usr/lib, but on shared host you do not have permissions to write to this location, hence the normal method wouldn’t work. The trick is to install the .so in custom location, but more importantly, ask pip to use this non-standard location using –install-option command line option

Here are the steps I followed :

wget https://github.com/downloads/libevent/libevent/libevent-1.4.13-stable.tar.gz

tar xvzf libevent-1.4.13-stable.tar.gz

mkdir ~/externals

cd libevent-1.4.13-stable

./configure –prefix=$HOME/externals/

make

make install

pip install –install-option=”-I$HOME/externals/include” –install-option=”-L$HOME/externals/lib” gevent

In the above steps $HOME is taken only as an example, feel free to install it at other locations, and pass the appropriate path to -I and -L as arguments.

While the example above talks about libevent, This method works well for other software as well.

Django 1.2.1 : Page Not Found :/

Mandar Vaze — Sat, 18 Sep 2010 17:05:54 +0000

This was my first instance of installing Django and deploying a third party app on it. After deploying the Django-App, I started the app using “python manage.py runserver” It seemed to have started cause I got the following message :

Validating models…
0 errors found

Django version 1.2.1, using settings ‘xxx.settings’
Development server is running at http://127.0.0.1:8000/
Quit the server with CTRL-BREAK.

But when I visited the page http://127.0.0.1:8000/ (or http://127.0.0.1:8000/admin) I would get “Page Not Found :/” (or “Page Not Found :/admin” – depending on which page I tried to launch) Since I was trying this app for the first time, I suspected that this must be the problem with the app. I checked with other folks who worked on this project, but I couldn’t get any help. While reading the Django documentation further – I came across an option to “runserver” command where one can start the server on non-default port. As part of troubleshooting, I just tried it, not hoping for much success, but it worked.

Now I am thinking why doesn’t the app work on default port, but works on non-default port. So I stopped the server (running on non-default port) and tried the URL again. I still got the “Page not found” error, and this made it clear that, some other server was listening on port 8000. Eventually I was find out the process that was listening on port 8000 using the following command :

netstat -ao | findstr LISTENING

Option ‘0′ prints the process ID – I was able to find out the eecutable using ProcExp (which has replaced my Task Manager) On linux – use “grep” in place of “findstr” and “ps” can be used to find out the executable

What surprised me was even though some process is already listening on default port 8000, How did Django “runserver” did not give an error when it tries to bind to port 8000. This can be reproduced easily by running two instances of same app (or two different apps) from two different terminals. Second instance should fail while binding to default port – and I expect it to show error – But it doesn’t. If it matters I was using Django 1.2.1 on Windows XP (Python 2.6)

All in all, It was very interesting to troubleshoot the problem.

Installing Lucid Lynx

Mandar Vaze — Tue, 29 Jun 2010 16:38:10 +0000

For what its worth, Lucid Lynx didn’t generate positive initial buzz, and for the first time since Ubuntu 5.04, I didn’t download and install latest version of Ubuntu. I had read comments that there are problems with latest version (that too LTS – Long Term support) of Ubuntu, so I decided to hold back.

Then couple days ago, I got hold of an ISO someone had downloaded, and installed successfully. (They came to me asking about why ssh wouldn’t work for them – As you might already know, Desktop edition doesn’t ship ssh server by default. It was matter of minutes to download ssh via apt-get, and they were up and running – and now to your regularly scheduled program) So I decided to give it a shot. Soon I realized, my decision to hold off was correct.

Attempt to boot from LiveCD

First of all, it took really long time for LiveCD to boot. I mean it did show the GUI splash screen quite quickly, but then for a really long time, it kept showing Ubuntu, and a progress bar (as dots/circles) None of the terminals were accessible via Alt+Fn.

Unable to recognize the existing Partitions

When it did boot, it was unable to recognize the partition table. Further troubleshooting showed that “Disk Utility” program under System->Administrator was able to read the partition table correctly, and identified the partitions correctly, but GParted (and embedded Partition Manager for the installer) could not. They showed entire disk as available. But a picture is worth a thousand words. See this image (I’ve not captured this, several others are facing the same problem) .

To be fair, this issue is well documented in Release notes here. But it still doesn’t change the fact that there was no way to preserve existing partitions and install Lucid on existing empty partition.

Some link of the internet asked to uninstall “dmraid” package, but it didn’t help. I didn’t have RAID anyway.
Next, back to the person who had successfully installed it from the same ISO. Turns out he installed it via Wubi. So I decided to try that.

Wubi woes

Now I ran into “permission denied” issue. After copying the files in target folder from CD, it would exit. Unhiding AppData folder didn’t help. Finally I downloaded wubi.exe, and copied an ISO and Wubi.exe in same newly created empty folder. This seemed to have worked – but make sure that if you want Wubi to use local ISO – Disable the internet or better still, remove the ethernet cable (as I did) else Wubi will go out and try to download the entire image before it starts installing.

This time it went much further – installed Ubntu successfully (or so I thought) and provided me a choice to reboot now, or later. After I reboot, I selected Ubuntu from Windows Boot loader list. It “continued” the installation, but at he stage where it gets time from network time server, it was stuck in a loop “No root partition defined” there were no option other than OK. Interestingly, the installation progress bar showed 272% (Yes more than 100%)

Finally … Success (sort of)

Luckily I didn’t have a precious data, and machine was my secondary machine, so I could afford to repartition the entire disk. That is what I did. I already had brand new ISO, This time I booted from USB drive, so that I didn’t have to worry about bad CD. I selected to repartition the disk from the Lucid Installer. Rest of the install went very smooth.

Conclusion

It is really sad that all three issues (permission denied, unable to recognize partition table, and no root partition defined) are widely reported. Only the first one (permission denied) has a work around that worked.

Image courtesy : jeffpro57

I can’t take it anymo….

Mandar Vaze — Fri, 12 Feb 2010 17:26:17 +0000

I was wide awake when I heard the above words around midnight yesterday (it was 21 minutes past midnight to be precise)

For a while I didn’t realize what is going on, or who is saying this … Then I realized that it was my cell phone saying this. (No, I wasn’t hallucinating.) Recently I bought a Nokia 5800 xPressMusic phone. I downloaded this software called “Green Charging Save Energy“.

Green Charging Save Energy

The Description reads :

Green Charging is an application that will assist you in saving power and hence, assists in curbing Global Warming. This unique application will be useful when the user will charge his/ her phone. The application will notify the user (with sound effects) when the battery is fully charged. Green charging will also share other important information like the battery status, stand by time and talk-time (in %). Why Green Charging? Saves Money Prevents Global Warming For a Greener Planet

So what had happened was that battery meter on my cell phone was showing only single bar since the morning. Thanks to this application, I knew that even if the indicator said low battery, it would definitely last me the entire day (and it did). So finally when I was about to sleep, I hooked the cell phone to the charger. Generally I turn it off just when I am about to fall asleep, but yesterday I forgot. So after say about an hour or so, the battery was fully charged, and I was told “with the sound effects” that I need to turn off the charger.

Here are some of my views of the whole thing:

I think it is a great application. Even if I am not convinced if leaving the charger ON even when phone is not connected, consumes/wastes electricity, I think it is generally great idea, not to overcharge the cellphone battery.
It is said that one should charge the battery only after it is fully (or near fully) drained. This helps prolong the battery life, and avoid “memory effect“. The app is very useful is helping me decide whether I really need to charge the battery immediately or can I wait.
While “I can’t take it anymo…” has a dramatic effect, it may not be appropriate sound effect for all the cases. There isn’t any obvious way to turn off the sound effects or to change it to something less dramatic. (Is someone from Impetus Infotech listening ? )

Four essential tips for extending the battery life of your computer, cell phone, and every other gadget. (slate.com)