qt.qpa.xcb: could not connect to display

overfl0w

Failing to start QT5 application from a privileged process. The application starts normally if it is started from a bash shell.

To be more precise, we are using AWS Greengrass Edge runtime environment that runs "components" (apps, that can be any language). We have a service script that usually starts the QT5 application when the environment initially boots. The service script is the following:

#!/bin/sh

if test -z "$XDG_RUNTIME_DIR"; then
    export XDG_RUNTIME_DIR=/run/user/`id -u`
    if ! test -d "$XDG_RUNTIME_DIR"; then
        mkdir --parents $XDG_RUNTIME_DIR
        chmod 0700 $XDG_RUNTIME_DIR
    fi
fi
# wait for weston
while [ ! -e  $XDG_RUNTIME_DIR/wayland-0 ] ; do sleep 0.1; done
sleep 1
cd

until pids=$(pidof weston)
do   
    sleep 1
done

echo "Starting Dash Display"
export QT_DEBUG_PLUGINS=1 
QT_IM_MODULE=qtvirtualkeyboard /usr/bin/dashV3 --fullscreen &
unset QT_DEBUG_PLUGINS

NOTE: I've done the QT_DEBUG_PLUGINS=1 to see the output, and it fails when it attempts to load xcb plugin:

Got keys from plugin meta data ("xcb").
QFactoryLoader::QFactoryLoader() checking directory path "/usr/bin/platforms"
loaded library "/usr/lib/plugins/platforms/libqxcb.so".
qt.qpa.xcb: could not connect to display.
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found..

If I run the application normally in an opened bash environment, it spins it up, loading up all of the plugins available, but when ran within another process (in a privilege mode, from Greengrass runtime environment, which is still ROOT privilege uid/gid of 0), it fails about the 3rd of 4th plugin loading (XCB)

(I've omitted the other plugins as part of the QT_DEBUG_PLUGINS=1, since the logs in Greengrass environment come out funny prefixed and suffixed with Greengrass-specific log format, and it is notoriously hard for me to "edit" it).

I guess, the key question is: why does it run normally when the same command is ran from BASH, but fails on the above, when another process kicks it off.

I thought I'd provide some additional details, how the script is run, from Greengrass component (via, what they call "recipes"):

RecipeFormatVersion: "2020-01-25"
ComponentName: "{COMPONENT_NAME}"
ComponentVersion: "{COMPONENT_VERSION}"
ComponentDescription: "Savic MC Dash App"
ComponentPublisher: "{COMPONENT_AUTHOR}"
Manifests:
  - Platform:
      os: linux
    Lifecycle:
      Run:
        RequiresPrivilege: true
        script: bash {artifacts:decompressedPath}/Dash/run.sh
    Artifacts:
      - URI: "s3://BUCKET_NAME/COMPONENT_NAME/COMPONENT_VERSION/Dash.zip"
        Unarchive: ZIP
        Permission:
          Execute: OWNER

The key thing here to note is:

Run:
        RequiresPrivilege: true
        script: bash {artifacts:decompressedPath}/Dash/run.sh

Nothing special about this part of the config, other than it tells Greengrass to run run.sh as root.

Are there some env variables that may be missing?
Do I need to run it in some other "mode"?

Thanks for your help.

Additional notes:
The OS that our QT5 runs on is YOCTO Linux (Embedded Linux), on a Toradex IMX8 board.

The reason why we've moved from running it as a system process, to a Greengrass runtime environment is that Greengrass has an entire ecosystem surrounding over-the-air updates, lifecycle management of apps/components/services, visibility, access to MQTT etc. etc.

jsulm

@overfl0w said in qt.qpa.xcb: could not connect to display:

which is still ROOT privilege uid/gid of 0), it fails about the 3rd of 4th plugin loading (XCB)

Usually, root cannot access X server when running from other user account. Do you really have to start your app as root?

Regarding plug-ins: set QT_DEBUG_PLUGINS=1 in a terminal and start your app there: there should be much more output providing more details why exactly the plug-in isn't loaded, please post it.