I had accidentally filled my /var partition. This caused backlight to fail to
start even with disk space, because the brightness files were truncated to zero
size:
systemd-backlight[624]: acpi_video0: Failed to parse brightness "": Invalid argument
systemd-backlight[624]: acpi_video0: Failed to write system 'brightness' attribute: No such device or address
systemd[1]: systemd-backlight@backlight:acpi_video0.service: Main process exited, code=exited, status=1/FAILURE
systemd[1]: systemd-backlight@backlight:acpi_video0.service: Failed with result 'exit-code'.
systemd[1]: Failed to start Load/Save Screen Backlight Brightness of backlight:acpi_video0.
Handle a truncated file like non-existent file.
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