A Weird Imagination

Troubleshooting python-xcffib

The problem

The monitor-lock.py script in my previous blog post uses python-xlib, which currently mainly relies on manually porting Xlib functions to Python. This is why it is missing the barrier-related functions I needed in that post. There is work on automating this process, but it appears to be abandoned. I started trying to pick up where they had left off before finding the python-xcffib project which provides auto-generated bindings for libxcb and therefore gives full support for interacting with X at a low level from Python.

python-xcffib (named after the cffi library it uses for binding to the C XCB library) gives a slightly lower-level API than python-xlib, but they are both fairly thin wrappers over the X protocol, so the differences are minor. It was fairly straightforward to port my script from the previous post to use python-xcffib, available as monitor-lock-xcb.py.

Unfortunately, I ran into a bug in python-xcffib:

Traceback (most recent call last):
...
  File "./monitor-lock-xcb.py", line 38, in main
    devices = conn.xinput.XIQueryDevice(xcffib.xinput.Device.AllMaster).reply().infos
...
  File "/usr/lib/python3/dist-packages/xcffib/__init__.py", line 139, in _resize
    assert self.size + increment <= self.known_max
AssertionError

The solution

I've submitted the fix upstream, so most likely you will not encounter this error. Updating to the latest version (after v0.8.1) should be sufficient to fix the problem.

The fix I applied was to modify the module's __init__.py (the location, which may be different on your machine, is in the stack trace). Specifically, on line 108 in the function Unpacker.unpack(), in the call to struct.calcsize(), change fmt to "=" + fmt.

The details

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Lightweight multiseat X

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The problem

I hosted a LAN party1 a little while ago and ended up needing to loan out multiple computers to guests in the interest of having no one try to lug their desktop over. As it turns out, I don't keep multiple of spare gaming-ready laptops around, so I needed to get more computers somehow.

The solution

My desktop has three screens attached to it (two monitors plus a projector), so given an extra keyboard and mouse (or two), it should be possible to run multiple instances of the game on it at the same time to let multiple people play using the same computer.

The script from this forum post makes it easy to set up multi-pointer X so a second keyboard and mouse will get its own mouse cursor. Then each keyboard and mouse pair can interact with its own instance of the game.

As an additional aid, I wrote monitor-lock.py which allows you to assign a mouse to a monitor, so it cannot be moved off that monitor to prevent accidentally interacting with the other player's instance of the game.

The basic usage is that you first run it with no arguments to get the available screens and pointers getting an output something like this:

$ ./monitor-lock.py 
...
Available screens:
screen 0: {'x': 0, 'y': 0, 'width': 3840, 'height': 2160}
screen 1: {'x': 3840, 'y': 0, 'width': 1920, 'height': 1200}
screen 2: {'x': 3840, 'y': 1200, 'width': 1920, 'height': 1080}

Available pointers:
device 2: Virtual core pointer
device 17: second pointer

USAGE: ./monitor-lock.py [device] [screen]

and then in a screen session (so you don't have to worry about accidentally doing this on a monitor you've locked your pointer away from), run

./monitor-lock.py 2 0

and

./monitor-lock.py 17 1

to lock the primary pointer to the first screen and the second pointer to the second screen.

Just use Ctrl+C to kill the process when you want the pointer to be able to move freely again.

The details

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Emulating Xbox controllers using GameCube controllers

The problem

I previously wrote about making different controllers act like Xbox 360 controllers. While it's a useful general-purpose solution, it's can be a bit clunky to have to explicitly set the mappings for each controller. More importantly, the remapping leaves the original controller entries in /dev/input/, although they don't do anything, and some games1 assume that the four players are controlled by the first four controllers. This is no longer true if js0 is the real first controller and js1 is the copy made by xboxdrv to look like an Xbox 360 controller. Or, worse, if js0-js3 are the four real controllers and js4-js7 are the ones we want the game to actually use.

The specific reason I'm remapping the controllers, is that the gamepads I'm actually using are GameCube controllers connected via the Nintendo GameCube controller Adapter for Wii U, which connects up to four GameCube controllers to a USB port. wii-u-gc-adapter makes them usable as controllers, but they appear different enough from Xbox 360 controllers that remapping them is necessary for most games.

The solution

Just build and use the version of wii-u-gc-adapter in my feature/mimic-xpad branch and your GameCube controllers will show up as Xbox controllers.

The details

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User scripts on iPad

The problem

Google Forms is a tool that allows for easily setting up simple structured data entry. But it's designed to make it easy to analyze a lot of data that has been entered, not to view a single entry. There is a view to show individual entries, but it's very cluttered due to including all of the options that were not selected as well as those that were selected. A display that showed only the entries that were selected could be used as a quick and dirty way to make a form letter-like website.

To make this problem harder, the solution has to run on iPad, a platform not exactly known for its user programmability.

The solution

Bookmark this link: hide unselected items in Google Form. Then select that bookmark when on the appropriate Google Forms page. Note that in addition to hiding unselected entries, if the entry that is not select has a value of "Yes", then its entire section will be hidden. If you don't want that behavior, bookmark this variant of the script instead.

The details

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Encrypted files in Vim

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The problem

There's a handy Vim plugin openssl.vim that allows you to easily edit encrypted files with Vim simply by giving the file an extension like .aes. Then Vim will ask for a password upon loading and saving the file in order to decrypt and encrypt it with openssl.

Unfortunately, the plugin was last updated in 2008 and makes some assumptions about openssl's defaults which are no longer valid. The most pressing issue is that the plugin now outputs a warning message when encrypting. By itself, that's worrisome, but, worse, that warning message gets output into the file along with the ciphertext. Needless to say, the resulting file cannot be decrypted without manually removing the warning text.

The solution

Simply fixing the options the script passes to openssl is a good start, but I also wanted to make sure any files encrypted with the old settings could be decrypted. My updated openssl.vim1 does both in addition to fixing some other annoyances.

The details

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Devlog: Anagram Bagels: Part 2

There were two non-trivial aspects of the design of Anagram Bagels: puzzle generation, which I discussed in my last post, and how to handle saving and sharing puzzles, which I will discuss in this post. I wanted an intuitive design that satisfied the following constraints:

  1. It should be possible to easily share a puzzle with another person in the form of a link.

  2. The difference between a link to the game and a link to a specific puzzle should be clear. (So the user doesn't accidentally bookmark a link to a specific puzzle when meaning to bookmark the game.)

  3. The game should gracefully handle the common mobile browser behavior of reloading the page if it hasn't been viewed in a while.

  4. Opening multiple instances of the game in separate tabs shouldn't break anything. (This is the default for web sites, so it's true unless doing something to actively break this assumption.)

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Devlog: Anagram Bagels: Part 1

Introduction

I have a friend who plays a lot of simple puzzle games on their phone. One of them is this word puzzle, which is variant of Bagels (also known as Bulls and Cows or by the trademarked name Mastermind) where the secret is an English word and the guesses must be valid words. Additionally, the alphabet of the guesses is limited to a set selected for the puzzle, and the feedback is given for specific letters as opposed to giving just a count of the correct letters.

While playing the game, my friend would often find that it would be useful to type letters out of order. For example, once determing that the word ends in "ing", it would be easier to simply write that in at the end and then fill out the beginning. As the feedback means the player often knows exactly what they want to write in the middle of the word, typing each word in order from the start to end can be awkward.

As the game seems quite simple, I decided to reimplement it and improve upon the UI. My implementation is in HTML5/JavaScript and should work in any modern browser. Play Anagram Bagels or view the source.

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Timezones and scheduling tasks with at

The problem

My system for automatically posting future-dated blog posts mysteriously stopped working recently. The posts would appear if I manually published the blog, but not with the automatic scheduling mechanism.

The solution

In schedule_publish.sh, I changed the line

echo "$0" | at -q g $time

to

if [ "$(date -d "$time PST" +'%s')" -ge "$now" ]
then
    echo "$0" | at -q g -t "$(date +'%Y%m%d%H%M' -d "$time PST")"
fi

(where "PST" is the timezone of this blog; adjust as appropriate for your blog). $now is initialized with

now="$(date +'%s')"

before the call to make publish to avoid a race condition.

The details

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PulseAudio headphone jack troubles

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The problem

Since I got a new motherboard (and therefore new audio hardware as I'm using the basic one built into the motherboard) sometimes after I unplugged my headphones, my speakers would not output any sound.

pavucontrol showed the only available output as "Built-in Audio Digital Stereo" with a port of "S/PDIF", which does not describe any audio device I had ever used. If I plugged my headphones back in, they would work fine, and usually after unplugging and plugging back in my headphones enough times, my computer would eventually acknowledge that my speakers were connected by showing the expected "Built-in Audio Analog Stereo" with a port of "Line Out".

The solution

In /usr/share/pulseaudio/alsa-mixer/paths/analog-output-lineout.conf change

[Jack Front Headphone]
state.plugged = no
state.unplugged = unknown

to

[Jack Front Headphone]
state.plugged = no
state.unplugged = yes       # changed from unknown

This forces PulseAudio to consider there to be speakers plugged into the "Line Out" port, so it may cause strange behavior if that is not the case.

To apply the change, run

pulseaudio --kill
pulseaudio --start

to restart PulseAudio.

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Nvidia GLX not working

The problem

I recently replaced my old Nvidia graphics card with a newer one. Upon booting up, I ran glxgears to test that 3D graphics were working properly and got an error like

X Error of failed request:  BadWindow (invalid Window parameter)
 Major opcode of failed request:  155 (NV-GLX)
 Minor opcode of failed request:  4 ()
 Resource id in failed request:  0x1200003
 Serial number of failed request:  34
 Current serial number in output stream:  34

The solution

Either delete /etc/X11/xorg.conf or edit it and remove (or comment out) the "Files" section; that is, the lines

Section "Files"
    ...
EndSection

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