libreoffice document preview #17864

hello,
when creating previews of libreoffice documents I get the following error:
unlink(/var/local/nextcloud/data/Myname/files/myfile.png): No such file or directory at /usr/share/webapps/nextcloud/lib/private/Preview/Office.php#65

I did some digging and found two problems:

1. libreoffice needs a home folder to run, even headless, as it creates some .config files; but my http user didn't have one. I fixed it by adding
   putenv('HOME=' . $tmpDir);
   in .../lib/private/Preview/Office.php, before line 53

2. the output folder of the conversion to png is $tmpDir (see .../lib/private/Preview/Office.php, line 51) while the previewer looks for the png file in $dirname. So I changed
   $pngPreview = $dirname . '/' . $filename . '.png';
   to:
   $pngPreview = $tmpDir . '/' . $filename . '.png';
   in .../lib/private/Preview/Office.php line 59.

This solved my problem, but I am not sure if it's a good fix, especially the first part.

Cheers