This functions detects whether the code runs in a "headless" environment, i.e., with no graphical server available.
Details
The draw.io application requires a graphical server to work, this is a known limitation. However, headless environments such as Docker containers, CI/CD pipelines, remote server (through SSH), etc., do not have a display.
It is thus important to be able to detect such environments, in order to use workarounds that prevent draw.io from crashing.
We use 2 methods:
If the
xrandrtool is available, we use it to query the configuration of the current display. Ifxrandrdoes not find any display, we assume to be in a headless environment.Otherwise, we resort to a simpler test: on Linux, the
$DISPLAYenvironment variable controls the display server which should be used. If it is empty, or not set, we assume to be in a headless environment.
Note
In both methods, it is possible that the system has an available
display, which is simply not recognized at the time, e.g., because of an
incorrect configuration. This is especially the case if $DISPLAY is
set to "". In this case, this functions will incorrectly believe to
be in a headless environment. Users should make sure that their system is
properly configured to avoid this.
These 2 proposed ways do not cover Windows nor Mac OS X. On such systems, the headless detection will certainly fail. By default, we consider we are not in a headless environment: the knitrdrawio engine will thus invoke draw.io normally. Maybe, by chance, it will work. The worst possible outcome is that draw.io crashes, and the document rendering will fail as well.