QT_QPA_PLATFORM for phantomjs headless

[gravitystorm]

While investigating #2556 I found a problem with our phantomjs config. After running vagrant up, phantomjs is in a non-working state:

vagrant@ubuntu-bionic:/srv/openstreetmap-website$ /usr/bin/phantomjs --version
qt.qpa.screen: QXcbConnection: Could not connect to display 
Could not connect to any X display.

This turns out to be expected, since the debian/ubuntu build of phantomjs doesn't use the patched+bundled version of QT that is in the official phantomjs builds.

We already have code to handle this situation in the test suite, namely:

ENV["QT_QPA_PLATFORM"] = "phantom" if IO.popen(["phantomjs", "--version"], :err => :close).read.empty?

... but this doesn't work on our vagrant boxes, because:

vagrant@ubuntu-bionic:/srv/openstreetmap-website$ QT_QPA_PLATFORM=phantom /usr/bin/phantomjs --version
This application failed to start because it could not find or load the Qt platform plugin "phantom"
in "".

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

I don't know where the "phantom" in ENV["QT_QPA_PLATFORM"] = "phantom" comes from. Should this be "offscreen"? Or will that break a different situation?

[tomhughes]

It comes from ariya/phantomjs#14376 (comment) I think but the key point is that it doesn't work with the Debian/Ubuntu packaged phantom - you need to use the upstream one.

[tomhughes]

I don't believe offscreen will work by the way because it still requires an X server I think - it just positions the window off the edge of the screen so it's not visible.

[tomhughes]

Hmm 2f342ba is odd because it was supposed to be making it conditional but also changed it from offscreen to phantom.

[tomhughes]

The core issue though is that there is no good solution for Debian/Ubuntu systems - you basically can't use the packaged phantomjs on a headless system. So you either have to use the upstream one or you have to run in an environment with an X server of some sort even if it's Xvfb or something.

[gravitystorm]

"offscreen" works on my virtual machine, without needing an X server. I suspect/guess that "phantom" is a platform that only exists on the phatomjs patched version of QT.

[tomhughes]

I mean I'm happy to change that given that I don't understand why it changed in the way it did and I suspect it may have been an accident.

There must have been a reason I made it conditional though, which suggests that there was some situation where offscreen didn't work.

[gravitystorm]

Sounds good!

[tomhughes]

Having now tried on my machine, where I use the upstream phantomjs I think I have a better handle on what happened.

The upstream phantomjs actually only has one backend, namely the phantom one, so that is why I tried to make offscreen conditional. It seems however that, probably as a result of trying various alternatives, I accidentally changed what it was setting the value to at the same time.