Symphonious

Living in a state of accord.

Benq GW2765 Monitor Display Port “No Signal Detected”

I have three Benq GW2765 monitors which periodically report “No Signal Detected” for DisplayPort even when the computer it’s attached to recognises the monitor is present (displaying it in the monitors/displays list etc). Changing  the DisplayPort cable or plugging it into a different computer doesn’t help (I tried with both Mac OS X and Linux/Fedora machines), but HDMI and D-Sub connections work perfectly (but can’t support the full screen resolution). I can even disconnect a cable from a working monitor, plug it into a non-working monitor and it will continue to complain about no signal, but plug the cable back into the working monitor and it carries on working fine.

The solution turns out to be quite simple – unplug it, wait for the power indicator light to turn off and turn it back on. It will then detect the DisplayPort signal correctly.  Unplugging the DisplayPort cable and plugging it back in will not help, nor will turning the monitor off with it’s power button. Briefly disconnecting the power cable and reconnecting it isn’t enough, you have to wait 5-10 seconds for the power indicator light to turn off.

Naturally that means that you can do all kinds of due diligence testing at home before deciding it’s a hardware problem and returning it to the shop. When you get to the shop it will work perfectly because it’s been unplugged on the car trip.

So that was fun…

Fun with Nvidia Drivers and Fedora Upgrades

After any major Fedora upgrade my system loses the proprietary nvidia drivers that make X actually work (I’ve never successfully gotten the nouveau drivers to handle my card and multi-monitor setup) so the system reboots and simply presents an “oops, something went wrong” screen.

The issue seems to be that the nvidia driver doesn’t recompile for the new kernel, despite the fact that I’m using akmod packages which should in theory automatically recompile for new kernels.

The tell-tale sign is:

[   161.484] (II) LoadModule: "nv"
[   161.484] (WW) Warning, couldn't open module nv
[   161.484] (II) UnloadModule: "nv"
[   161.484] (II) Unloading nv
[   161.484] (EE) Failed to load module "nv" (module does not exist, 0)

in the Xorg logs.

Some digging reveals that the akmod recompilation process should be triggered by /etc/kernel/postinst.d/akmodsposttrans but for whatever reason that didn’t run.

The key piece of that script was running akmods similar to:

/usr/sbin/akmods --from-kernel-posttrans --kernels 4.8.11-300.fc25.x86_64

The last argument is the current kernel version, which should match the directory name in /lib/modules/ – there will likely be a few options, either run the command for each of them or pick the latest which is likely to be the one missing the nvidia drivers.

Run that script, reboot and everything came back just fine, though there is likely a better way to do it…

Testing@LMAX – Isolate UI Tests with vncserver

One reason that automated UI tests can be unreliable is that they tend to be sensitive to what else is on screen at the time and even things like the current screen size. Developers running the tests locally also find it annoying to have windows opening and closing on their machine while the test runs and are unable to do anything else because their clicking might interfere with the test.

At LMAX we solve that by isolating tests in their own X session, created using vncserver. We simply start vncserver with:

vncserver :20 -geometry 1600x1200

Then set DISPLAY=:20 as an environment variable when starting WebDriver’s Firefox instance:

FirefoxBinary firefoxBinary = new FirefoxBinary();
firefoxBinary.setEnvironmentProperty("DISPLAY", ":20");

The Firefox window then pops up in it’s own isolated X session. We can still use a vnc client to watch as the test runs but we can also let it run in the background and continue using the machine for other things. In CI it allows us to run UI tests on a headless server.

Since we run a number of tests in parallel, in CI we start a number of vncserver instances and allocate a different one to each running test to ensure they’re completely isolated.

Simple, but incredibly effective.

Testing@LMAX – Screenshots with Selenium/WebDriver

When an automated UI test fails, it can be hard to tell exactly what went wrong just from the failure message. The failure message typically just says that some element the test was looking for wasn’t found, but it doesn’t tell you what was there.  Was there an error message displayed instead? Was the operation still executing? Did something completely unexpected happen instead?

To answer those questions our DSL automatically captures a screenshot when any UI operation fails and we include a link to it in the failure message. That way when someone reviews the test result they can see exactly what was on screen which typically makes it straight forward to identify what went wrong and fix it.

Until recently we’d been using the convenient and helpful looking TakesScreenshot.getScreenshotAs method that WebDriver provides.  For example:

((TakesScreenshot)webDriver).getScreenshotAs(
new SaveScreenshotOutputType(pngFilename));

As expected, this creates a PNG image in the specified location that looks for all the world like a screenshot of the browser content. Unfortunately, it’s lying.

WebDriver actually does something very clever and gets the browser to render the page content into a canvas element and then saves that as the PNG file. This is an extremely close approximation of what the page looks like with two important exceptions:

  1. It doesn’t respect the viewport size so body content is never scrolled off-screen.
  2. Any browser chrome or random other windows that have popped up aren’t shown.

Both of these things can be an issue – the scrolled-off-screen one being the most problematic.  Modern WebDriver quite accurately simulates a user clicking and typing keys so if somethings not on screen it can’t be clicked. When your test fails because an element was “present but not visible” and the screenshot shows it as very clearly visible, hilarity ensues. Very frustrating hilarity.

To fix this we’ve started taking honest-to-goodness screenshots. Since all our tests get their own X session (courtesy of vncserver) their windows are completely isolated from each other and a dump of the entire screen will capture precisely what a real user would see, browser chrome and scrolling included. Linux provides an entertaining array of options for capturing screenshots from the command line but the one that happened to be already installed was import, part of the ImageMagick suite. We simply execute:

import -display :20 -window root screenshot.png

where :20 is the X display this particular test has been allocated and screenshot.png is where we want the screenshot to wind up.

Since the WebDriver screenshot can be useful as well – for example finding out an error message is displayed at the top of the screen  we continue to grab that too.

Finally, for completeness we grab a dump of the DOM to a HTML file so we can later inspect what IDs, classes, attributes etc are present, including any hidden elements. webDriver.getPageSource() makes that easy and we append an extra HTML comment that includes webDriver.getCurrentUrl() for good measure.