The problem

My existing script for publishing my blog has Pelican run on the web server and generate the static site directly into the directory served by nginx. This has the effect that while the blog is being published, it is inaccessible or some of the pages or styles are missing. The publish takes well under a minute, so this isn't a big issue, but there's no reason for any downtime at all.

The solution

Instead of serving the output/ directory, instead generate it and then copy it over by changing the make publish line in schedule_publish.sh to the following:

make publish || exit 1
if [ -L output_dir ]
then
    cp -r output output_dir/
    rm -rf output_dir/html.old
    mv output_dir/html output_dir/html.old
    mv output_dir/output output_dir/html
fi

where output_dir/ is a symbolic link to the parent of the directory actually being served and html/ is the directory actually being served (which output/ previously was a symbolic link to).

The details

The problem

While in my last post, I said serverless WebRTC was too cumbersome, I wanted to try to see how streamlined I could make the process. While researching, I did find a variant of serverless-webrtc called serverless-webrtc-qrcode as well as a similar demo, webrtc-qr that both use QR codes as an easy way to transmit the offer strings. But both require that both sides have a camera to scan QR codes, while my use case is a WebRTC connection between my desktop without a camera and my smartphone.

The solution

minimal-webrtc now has a checkbox to enable serverless mode. In that mode, the QR code shown by the host is a much longer URL that includes the initial WebRTC offer. Opening that URL on another device (or in another browser window) will show another QR code along with a "Copy" button. With the first device, either press the "Scan QR code" button and point it at the QR code or use some other mechanism to copy the text and paste it into the text area labeled "Paste offer here:".

To run it locally, download the source code and run a web server to serve the wwwroot/ directory. If both devices can run a web server, then you can just access it via localhost on each, but, as before, because WebRTC requires HTTPS, to run it on your local network, you may need to set up a self-signed certificate.

The details

The problem

I wanted to stream video of myself and my screen at the same time. My plan was to put the video of myself on my screen and stream the entire screen, but I do not have a camera on my desktop. On the other hand, I do have a smartphone with a camera, so I needed a way to show the video from my phone's camera on my desktop's screen.

There are a few Android apps that promise to do so, but none of the ones I tried worked very well. But I know that video chat works just fine on my phone, including in a web browser using WebRTC which supports peer-to-peer video chat between two web browsers, so it should be easy to show the video from my phone's camera in a browser window on my desktop. Unfortunately, I couldn't find any straightforward solution for setting up just a peer-to-peer video link.

The solution

Open minimal-webrtc on the computer you want the video streamed to. A QR code will appear; use your smartphone to read it and after approving access to the camera, the video should appear in the first browser window. This is intended to be used for local connections, so it may not work if the two devices are not on the same network. Only the signaling to set up the connection goes to the minimal-webrtc server, the actual video will be sent peer-to-peer over the local network.

To get just the video as a bare window with no decorations, use chromium --app=uri to get rid of the address bar, etc., and this script to remove the rest.

To host it yourself, download the source code¹ and use the included run_daphne.sh script (which assumes daphne is installed) and nginx configuration. As WebRTC requires HTTPS, to run it on your local network, you may need to set up a self-signed certificate.

The details

The problem

Washington state has been holding a lot of press conferences with updates about the COVID-19 situation recently. The information has always been summarized in a few slides during the video, but those slides and explanatory text are only posted separately several hours to a day later.

The solution

youtube-dl will download videos off Twitter just given the URL of the tweet like this one. Then clone and run slide-detector:

./slide-detector.py video.mp4 473 105 727 397

(requires opencv-python) where video.mp4 is the filename of the video and the relevant section of the video is a 727x397 rectangle whose top-left corner is at the coordinates (473, 105), which is the correct rectangle to crop the linked video to just the main video section (i.e. omitting the ASL interpreter who is always on screen). Omit the numbers to not crop the video.

The script will output the slides as image files in the current directory with names like static_at_3:55.jpg for the slide that appears on the screen 3 minutes and 55 seconds into the video.

The details

The problem

Every game, no matter how old and simple, I run on my computer constantly uses an entire CPU thread even when idling at a menu. (Except for some newer multi-threaded games that do the same with multiple threads!) To raise this from a curiosity to a problem, this means that my computer's fans are on at full blast whenever I have a game going, so I notice.

The solution

To be clear, that symptom could be the result of many different possible causes, others of which I may explore in future blog posts.¹ But specifically for systems with Nvidia GPUs using the Nvidia proprietary driver (as opposed to nouveau), setting the environmental variable __GL_YIELD to USLEEP fixed the issue in some games for me. To do so when running a single game, run __GL_YIELD="USLEEP" /path/to/game or to do so permanently, add the line

export __GL_YIELD="USLEEP"

to ~/.profile and restart X.

The details

The problem

In the computer game Keep Talking and Nobody Explodes, the "bomb expert" players are looking at a fictional "bomb manual", often frantically searching for the right page. While the intention is for this document to be printed out—and physical paper makes it relatively easy to keep the headings at the top of each page visible—there is also a web version if you prefer to view it on a screen (or don't have access to a printer). Scrolling through the web version feels a lot more awkward than flipping through the paper version; one workaround I found was to open different pages in different browser tabs or windows, but then identifying which page is in which window is still awkward.

The solution

I created a userscript, "BombManual.com Tab Title and TOC" that automatically updates the tab title to match the title of the currently visible page. It additionally adds a table of contents to make it easy to quickly open all of the pages in separate tabs or windows.

If you do not already have one, you will need to install a userscript manager extension for your browser to use it. Alternatively, you could bookmark this bookmarklet, but as you would have to click that bookmark on every instance of the page you opened, that's less convenient.

The details

The problem

In a web app where the display should be constantly up-to-date, the client needs some way to get up-to-date information from the server. One of the simplest ways to do so is to regularly (every few seconds) query the server asking if there is new information. This involves making a lot of requests and is wasteful of bandwidth and processor time on both the client and server (the latter can be improved with caching).

If updates are rare, it makes much more sense for the server to notify the client when they occur, but HTTP is designed around the client making requests to the server, not the other way around. And, furthermore, the Django web framework (like many web frameworks) is built around that model.

The solution

Of course, this is a well-understood problem and there are a wide variety of APIs and libraries to solve it discussed on the Wikipedia page for Comet. The main workarounds are WebSockets which is a very flexible technology for two-way communication in a web browser and long polling which is a simpler technique which involves merely having the server not answer a request immediately and instead wait until it actually has an update to reply with.

In the rest of this blog post, I discuss the changes I made to convert a Django-based web app that I originally wrote to use a basic polling pattern and hosted using uWSGI to instead use long polling and be hosted using Gunicorn/Uvicorn. I also cover nginx configuration including hosting the app in a subdirectory.

The details

The problem

Compiling the Haskell package language-python (a dependency of xcffib), I got the following warning stating that the typeclass Error is deprecated:

language-python/src/Language/Python/Common/ParseError.hs:25:10: warning: [-Wdeprecations]
    In the use of type constructor or class ‘Error’
    (imported from Control.Monad.Error.Class, but defined in Control.Monad.Trans.Error):
    Deprecated: "Use Control.Monad.Trans.Except instead"
   |                                 
25 | instance Error ParseError where 
   |          ^^^^^                  

I wasn't sure how to "Use Control.Monad.Trans.Except instead", as Except is not a drop-in replacement for Error.

The solution

As this StackOverflow answer recommended,

Short answer is: Replace Error by nothing at all

The code used throwError, which I replaced with

throwError = lift . Left

Other than that, I just removed the imports of Control.Monad.Error and the typeclass instance of Error. The full diff is in this pull request.

The details

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

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