Server LTR: Why QGIS_PREFIX_PATH needs write permissions / ¿is WMS cache written there?

[lorenzogrv]

What is the bug or the crash?

I'm not sure this is a bug. Qgis server tries to cache WMS requests but fails with:

QNetworkDiskCache::prepare() unable to open temporary file

After some research this seems related to the QGIS_PREFIX_PATH env variable. There isn't too much about that variable on docs. Please see:

• https://docs.qgis.org/3.34/en/search.html?q=QGIS_PREFIX_PATH&check_keywords=yes&area=default
• QGIS server 3.0 : service configuration error (service unknown or unsupported) #26121 (considered fixed by) [bugfix][server] Fix prefix path for server #6502

Our qgis-server is running at a container which has QGIS_PREFIX_PATH=/usr. Perms inside /usr directory are drwxr-xr-x root root. That seems to have sense for me.

The qgis user runs the qgis-server because we don't want our container running its process as root.

It seems qgis user needs write permissions somewhere within /usr. That does not make sense for me.

• ¿Does WMS cache need to write files with QGIS_PREFIX_PATH?
• ¿Where does WMS cache need the write perms?

Steps to reproduce the issue

Need feedback to provide an useful reproduction

Versions

$ qgis_mapserver --version
QGIS 3.34.12-Prizren 'Prizren' (exported)
QGIS code branchRelease 3.34
Qt version 5.15.15
Python version 3.12.7
GDAL/OGR version 3.9.3
PROJ version 9.5.0
EPSG Registry database version v11.016 (2024-08-31)
GEOS version 3.13.0-CAPI-1.19.0
SQLite version 3.46.1
OS Debian GNU/Linux trixie/sid

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[kaloyan13]

I saw the same QNetworkDiskCache error in my docker image using QGIS Server 3.34.4-Prizren.
After debugging I figured out the following:

1. QGIS cache dir is properly detected from environment variable

qgis-server_1  | 11:24:55 INFO Server[16]:   - QGIS_SERVER_CACHE_DIRECTORY / '/cache/directory' (Specify the cache directory): '/var/www/cache/qgis' (read from ENVIRONMENT_VARIABLE)
qgis-server_1  | 11:24:55 INFO Server[16]:   - QGIS_SERVER_CACHE_SIZE / '/cache/size' (Specify the cache size): '268435456' (read from DEFAULT_VALUE)

2. QGIS tries to write to the wrong cache directory.

$ strace -ff -p ${QGIS_PID} -o /tmp/qgis_trace.txt
$ grep -RH 'cache' qgis_trace.txt.*
qgis_trace.txt.125421:openat(AT_FDCWD, "/.cache/QGIS/QGIS3/prepared", O_RDWR|O_CLOEXEC|O_TMPFILE, 0600) = -1 ENOENT (No such file or directory)
qgis_trace.txt.125421:statx(AT_FDCWD, "/.cache/QGIS/QGIS3/data8/8/1lpxd2j8.d", AT_STATX_SYNC_AS_STAT|AT_NO_AUTOMOUNT, STATX_ALL, 0x7f3b1eb56340) = -1 ENOENT (No such file or directory)

3. I am using Apache2 proxy_fcgi to send requests to QGIS.

ProxyPass		/cgi-bin/ fcgi://qgis-server:9000/
ProxyPassReverse	/cgi-bin/ fcgi://qgis-server:9000/

Tried setting environment inside Apache config, using SetEnv/ProxyFCGISetEnvIf, but that didn't help.

4. Workaround for the bug

mkdir -p /.cache/QGIS/
chown -R www-data:www-data /.cache/QGIS/
ln -s /var/www/cache/qgis /.cache/QGIS/QGIS3

wms_request.txt
qgis_server_1.txt