Monthly Archives: July 2012

Coders, 3 Year Olds, and Unit Tests

Anyone who has kids knows that three-year olds are starting to learn that the world is full of complex rules. Don’t go on red. Look before you cross the street. You can’t just watch TV all day. What every three year old wants to know is “why?”. I answer “why?” to my kid about twenty times a day. I’ve found that from all the teams I’ve worked on, coders are the same. Tell a coder that they need to fill out a new bug report form or write unit tests and they will ask “why”. Now, like a three year old you can threaten a coder (usually with a bad performance review rather than a timeout), you can bribe a coder (maybe a bonus or award instead of candy, or maybe just candy), or you can convince that it’s in their best interest to it. Like three year olds, it’s best to convince coders that doing something is in their best interest rather than threatening or bribing.

When it’s time to discuss unit testing, what most managers do is bring out some slides about quality and fill them with buzzwords about “quality metrics” or even worse, they bring up some industry standard that promises lots of paper work like CMM or ISO9000. I’ve got to be honest, while almost every coder I know cares about the product and the customer experience, most I know could care less about something like CMM or ISO9000.

So what is the right way to make coders care about unit tests? From my personal experience, unit tests give me four key benefits:

  1. Prove that the API is not broken
  2. Increase speed of execution
  3. Catch issues before they get shipped
  4. Allow me to really understand an API/project

Prove That the API is Not Broken

I really like writing unit tests that test an external API. I can use my tests to ensure that the API is staying stable. This is important not just to catch places where someone adds an argument to a function, but when someone makes a change that modifies the behavior of the API. Once an API ships, people start relying on the behavior of it, whether it was specified or not (wrongly or not). For example, some accomplishments-daemon code will throw a KeyError if you pass in an invalid key to a dictionary. So regardless as to whether this was specified in the documentation, someone may have code that relies on that, and so I test it. (I know that relying on implicit, undocumented behavior of an API is bad, but almost everyone does it.) If the code is changed next week to catch the KeyError instead and return None, the unit tests will at least tell us and we can decide whether at the very least the API version needs to roll.

Increase Speed of Execution

This is my favorite reason for writing unit tests. Well written tests let you isolate testing of one function all by itself and make it easier to debug an algorithm without all the setup and teardown. In a previous job I was writing software to get show listings from a TiVo and download shows. I could not make much progress unless I was in my office on the same subnet with my three TiVo devices. Also these devices were sometimes taken by the test group or marketing for various reasons, delaying my work. So what I ended up doing was writing unit tests and making a fake TiVo device, a mock, that behaved exactly like a real device (from an API stand-point). This not only allowed me to work anywhere, it let me skip the tedious setup, device discovery, and remote debugging required (my code ran on a device other than my dev box). I’d estimate that setting up each debug run would have taken me 5 minutes with a real h/w and it was more like seconds with unit tests. Over the course of three months, this saved hundreds of hours of my time.

When my first release of the TiVo code wrapped up, I was in-between assignments, so I wrote a “season subscription” feature. This feature would periodically scan the device for new shows and download them if they were part of a season that you wanted. By this time, the test team and marketing had taken my TV and all my TiVOs. I ended up spending 3 days writing all the code and only validating with new unit tests. I then just emailed the new installer to the test group and everything worked fine on the hardware.

Catch Issues Before They Get Shipped

This is the one part of unit testing that you will likely hear from a manager armed with a “quality” slide. It is in fact true. Bugs are cheaper to fix earlier, customers are happier, and better yet, bugs or quirks in your API don’t become an implicit part of the API.

Allow Me to Really Understand an API/Project

If you are new to a project and really want to understand how the API works, there’s no better way than writing unit tests. You can really learn the API, the “quirks” of the code, and help the project out at the same time.

Broken Unit Tests

Unit tests are only useful if test failures are taken seriously, and they’re only taken seriously if the build (hopefully continuous) breaks when they fail. Never underestimate the ability of people to forget (intentionally or otherwise) to run unit tests or to ignore failures. The tests I worked on for the Ubuntu Accomplishments Daemon had been broken for months and were so outdated they had to be scrapped completely. The new tests will run during every build which is run at least daily. If the tests fail, the deb is not built.

Ubuntu Accomplishments Daemon Unit Tests

I’ve spend the last week or so writing unit tests for the Ubuntu Accomplishments Daemon. You can see the pile of tests here in the file. While writing the tests we found about 5 bugs in the accomplishments-daemon, most were small fixes, but most of them also caused the API to abort, which would kill the daemon in a real environment. Making the daemon more resilient will make Ubuntu Accomplishments much friendlier to end-users.

There are still plenty of APIs that are not yet tested, so if you’d like to dive in, please pull the code and look at the to do list at the top of


There are entire books written about unit testing, about how to do it, and about why. However, if you’re like me, you’ll need to see the benefits first-hand to really want to write unit-tests. So go read the books, and then give it a try, I promise it will reduce your bug count and improve your speed of execution.

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How Does udev Know What’s a Keyboard or Mouse?

Last week, I found myself having to dive into the udev code to figure out how it determines whether what is a mouse or a keyboard. To solve the problem I was working on, I ended up having to replicate some of that logic in python, which is posted at the bottom. Let me explain how it works.

Device Info in /sys

To start, let’s look in /sys and see what input devices we have. If you look in /sys/class/input on your system, you’ll see many symlinks to devices. I’ll pick /sys/class/input/event12 to look at in more detail, so cd to event12/device

mfisch@caprica:/sys/class/input/event12/device$ ls -al
total 0
drwxr-xr-x 7 root root 0 Jul 8 16:05 .
drwxr-xr-x 3 root root 0 Jul 8 16:05 ..
drwxr-xr-x 2 root root 0 Jul 8 16:05 capabilities
lrwxrwxrwx 1 root root 0 Jul 8 20:50 device -> ../../../serio1
drwxr-xr-x 3 root root 0 Jul 8 16:05 event12
drwxr-xr-x 2 root root 0 Jul 8 16:05 id
-r--r--r-- 1 root root 4096 Jul 8 20:50 modalias
drwxr-xr-x 3 root root 0 Jul 8 16:05 mouse0
-r--r--r-- 1 root root 4096 Jul 8 16:05 name
-r--r--r-- 1 root root 4096 Jul 8 20:50 phys
drwxr-xr-x 2 root root 0 Jul 8 20:50 power
-r--r--r-- 1 root root 4096 Jul 8 20:50 properties
lrwxrwxrwx 1 root root 0 Jul 8 16:05 subsystem -> ../../../../../../class/input
-rw-r--r-- 1 root root 4096 Jul 8 16:05 uevent
-r--r--r-- 1 root root 4096 Jul 8 20:50 uniq

There are some interesting things in here, but the two I’m interested in are “name” and the “capabilities” directory. Let’s look at name first.

mfisch@caprica:/sys/class/input/event12/device$ cat name
PS/2 Generic Mouse

Using Capabilities

Okay, so this looks like a mouse, but udev doesn’t use the name to figure this out, it uses the capabilities directory. Let’s look there:

mfisch@caprica:/sys/class/input/event12/device/capabilities$ ls -al
total 0
drwxr-xr-x 2 root root 0 Jul 8 16:05 .
drwxr-xr-x 6 root root 0 Jul 8 16:05 ..
-r--r--r-- 1 root root 4096 Jul 8 16:05 abs
-r--r--r-- 1 root root 4096 Jul 8 16:05 ev
-r--r--r-- 1 root root 4096 Jul 8 20:43 ff
-r--r--r-- 1 root root 4096 Jul 8 16:05 key
-r--r--r-- 1 root root 4096 Jul 8 20:39 led
-r--r--r-- 1 root root 4096 Jul 8 20:43 msc
-r--r--r-- 1 root root 4096 Jul 8 16:05 rel
-r--r--r-- 1 root root 4096 Jul 8 20:43 snd
-r--r--r-- 1 root root 4096 Jul 8 16:05 sw

The aptly named capabilities provide information to udev on what “capabilities” the device has. udev is specifically interested in the following ones: abs (Absolute axes), ev (Event types), key (Keys and Buttons), and rel (Relative axes). Let’s examine one of these to determine what data it contains:

mfisch@caprica:/sys/class/input/event12/device/capabilities$ cat ev

So what does 7 mean? This is a bitmask (111) who’s bits are defined in /usr/include/linux/input.h. That file has bits defined for each of the types listed in the capabilities directory, the #define prefix matches the name in the capabilities directory, so we’re looking at EV_ (Note: key is represented by defines KEY_ and BTN_, since it’s for keys and buttons).

Since this mask is 0x7 or 111, we have bits 0, 1, and 2 set and the rest are false. Judging from the #defines, the following bits are set for this device:

#define EV_SYN 0x00
#define EV_KEY 0x01
#define EV_REL 0x02

This tells us that our device gets key events, relative movement event, and synchronization events. (Figuring this out required poking around the header a bit more).

A Quick Note About the ‘key’ Capability

The entry ‘key’ is much longer than ev and requires more explanation. Let’s look at what the ‘key’ capabilities shows for my keyboard:

20000 20000000020 0 0 500f02100002 3803078f900d401 feffffdfffefffff fffffffffffffffe

So that’s much larger than ev was. It’s still a bitmask, but it’s been split into words, using the word-size sense, so 8 bytes on my system. Since this mask is little endian, the ‘20000’ represents the highest set bits. This makes processing need to take place in word-size chunks, starting at the right. Another way to think of the output above is to think about it as one large mask, as long as you pad each “word” to be 16 digits, for example, what you really see above is this:

00000000000200000000020000000020000000000000000000000000000000000000500f02100002 03803078f900d401feffffdfffeffffffffffffffffffffe

To solve my problem, I split the output up on the spaces and set 64-bits at a time in my mask starting from the right-most word.

Now that we’ve read all these bitmasks, we need to do something with it.

What udev Does with this Info

If you look at the udev source code, specifically udev/udev-builtin-input_id.c, you can see how this data is used.

For mice, the code is fairly straightforward, look at the code for the test_pointers function. We really want to know where they set “is_mouse”. The code is also looking for other stuff like touchpads, so let’s ignore that. It basically boils down to this:

if EV_REL and REL_X and REL_Y and BTN_MOUSE:
you have a mouse

There’s also a similar block where you can replace REL above with ABS, which the comment claims is for VMWare’s USB mouse, but I kept it in my solution because I didn’t quite trust the comment and it seemed harmless otherwise.

For keyboards, the code is a bit more complex because it’s basically checking to see if you have a bunch of keys defined, this code is in the function test_key. then test_key looks for a keyboard it boils down to this:

If any bits are set in 'ev' and
if bits 1-31 (but not 0) are set:
you have a keyboard

Note: bits 1-31 represent the Escape key, numbers, and Q through D.

Putting it All Together

So now that we know how it works, let’s see it in action! Here it is run on my dev box:

mfisch@caprica:~/tmp/find_input$ ./
INFO:root:/dev/input/event3 is a keyboard (AT Translated Set 2 keyboard)
INFO:root:/dev/input/event12 is a mouse (PS/2 Generic Mouse)

Here it is run on another system with more devices attached:

INFO:root: /dev/input/event3 is a keyboard (AT Translated Set 2 keyboard)
INFO:root: /dev/input/event13 is a mouse (PS/2 Generic Mouse)
INFO:root: /dev/input/event15 is a keyboard (BTC USB Keyboard)
INFO:root: /dev/input/event16 is a mouse (Primax HP Wireless Laser Mini Mouse)

My python code including unit tests! is avaialble in bzr at lp:~mfisch/+junk/find_input

Comments, bugs, and fixes all gladly accepted.

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How to Unroot a Samsung Galaxy Nexus (CDMA) Without USB

This weekend the USB port on my Galaxy Nexus died. I could get it to charge, if I plugged it in about 10 times and wiggled it some, but no USB. Before I could take it back to Verizon, I needed to unroot it, reflash the stock ROM, and relock the boot loader. After digging around on various forums, here are the steps I used that worked for me.


  • You can brick your phone by doing this, so if you don’t know much about rooting, I don’t recommend these steps.
  • If USB works fine, follow these steps instead.
  • I’ve only tried this on a Verizon phone. You will certainly need a different tarball if you have a GSM device.
  • This will erase all files, texts, apps, pictures, EVERYTHING, from your phone.

Re-lock the Bootloader

If you skip this step, there is a tell-tale unlocked icon when the phone boots, so I considered this critical. Fortunately in late June 2012 there was an app published called BootUnlocker for the Galaxy Nexus posted in the app store (discussion thread).
So download and install the app, and run it. The options are simple, Lock and Unlock. You want to click Lock. Do NOT reboot at this point. I don’t know if the next steps will work if you reboot with a locked boot loader.

Install Mobile Odin Pro

From the Play Store on your phone, install Mobile Odin Pro. It costs $5, but a denied warranty on your Galaxy Nexus will cost far more.

Download the Tarball

You need to download the Mobile Odin compatible tar file. Use the PDA link for the tar file from this thread. The tar file is about 470 MB and is called VzW-PDA-ODIN-I515EL03_ICL53F_signed.tar. This tarball includes all the files you need to flash back to stock. Note: I could not get it to download directly from my phone, if this happens to you also, skip to “Wirelessly Copying ROM to Phone”

Wirelessly Copying Tarball to Phone

If you were able to download the ODIN ROM directly to your phone, you can skip this step.

Go into Settings->Developer Options and enable Remote ADB. You will need to be on wi-fi. Noting the IP of your phone, run this on your PC:

adb connect [IP of phone]

If it doesn’t connect, try a few more times. You may need to run adb kill-server and try to connect again if you still cannot connect. Once connected, you should be able to use adb push to copy the VzW tar file over. Note: It took over an hour for the copy to work for me.

There are other options for copying the file if this doesn’t work for you, uploading it to or Dropbox, samba, etc.

Flashing Back to Stock

Open Mobile ODIN Pro. Scroll down to “Open file…” and select the tarball you just copied over. It should then load a bunch of info for different components to be flashed. I also checked the Wipe Data and Wipe Dalvik cache options. You do NOT want everroot enabled! Then, just flick Flash Firmware. Your phone will flash itself back to stock. This takes about 5 minutes.

Note: “Open file…” is not shown on the screen shots

Final Update

Once the phone comes back up, sign-in and do setup as usual. Then go to Settings->About Phone->System Updates and check for an update. There is one system update you will be missing at this point (as of July 2012). I also went to the Play Store, signed-in, and retrieved my apps. While the apps were downloading the system update showed up. This was either due to a delay after checking for the system update or because I had not signed into Play. If the system update doesn’t start within 5 minutes of checking for updates, sign into the Play store, I think that will fix it. This final update takes about 5 more minutes to flash.


You now have an unrooted, stock, locked bootloader phone and are ready for warranty service.

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