Tracker supports volume management under a minimal environment

While Nemo Mobile OS doesn’t ship with udisks2 nor with the GLib/GIO GVfs2 modules that interact with it, we still wanted removable volume management working with the file indexer.

It means that types like GVolume and GVolumeMonitor in GLib’s GIO will fall back to GUnixVolume and GUnixVolumeMonitor using GUnixMount and GUnixMounts instead of using the more competent GVfs2 modules.

The GUnixMounts fallback uses the _PATH_MNTTAB, which generally points to /proc/mounts, to know what the mount points are.

Removable volumes usually aren’t configured in the /etc/fstab file, which would or could affect /proc/mounts, plus if you’d do it this way the UUID label can’t be known upfront (you don’t know which sdcard the user will insert). Tracker’s FS miner needs this label to uniquely identify a removable volume to know if a previously seen volume is returning.

If you look at gunixvolume.c’s g_unix_volume_get_identifier you’ll notice that it always returns NULL in case the UUID label isn’t set in the mtab file: the pure-Unix fall back implementations aren’t fit for non-typical desktop usage; it’s what udisks2 and GVfs2 normally provide for you. But we don’t have it on the Nemo Mobile OS.

The mount_add in libtracker-common/tracker-storage.c luckily has an alternative that uses the mountpoint’s name (line ~592). We’ll use this facility to compensate for the lacking UUID.

Basically, we add the UUID of the device to the mountpoint’s directory name and Tracker’s existing volume management will generate a unique UUID using MD5 for each unique mountpoint directory. What follows is specific for Nemo Mobile and its systemd setup.

We added some udev rules to /etc/udev/rules.d/90-mount-sd.rules:

SUBSYSTEM=="block", KERNEL=="mmcblk1*", ACTION=="add", MODE="0660", TAG+="systemd", 
  ENV{SYSTEMD_WANTS}="mount-sd@%k.service", ENV{SYSTEMD_USER_WANTS}="tracker-miner-fs.service

We added /etc/systemd/system/mount-sd@.service:

Description=Handle sdcard
After=init-done.service dev-%i.device

ExecStart=/usr/sbin/ add %i
ExecStop=/usr/sbin/ remove %i

And we created

if [ "$ACTION" = "add" ]; then
    eval "$(/sbin/blkid -c /dev/null -o export /dev/$2)"
    test -d $MNT/${UUID} || mkdir -p $MNT/${UUID}
    chown $DEF_UID:$DEF_GID $MNT $MNT/${UUID}
    touch $MNT/${UUID}
    mount ${DEVNAME} $MNT/${UUID} -o $MOUNT_OPTS || /bin/rmdir $MNT/${UUID}
    test -d $MNT/${UUID} && touch $MNT/${UUID}
    DIR=$(mount | grep -w ${DEVNAME} | cut -d \  -f 3)
    if [ -n "${DIR}" ] ; then
        umount $DIR || umount -l $DIR

Now we just have to configure Tracker right:

gsettings set org.freedesktop.Tracker.Miner.Files index-removable-devices true

Let’s try that:

# Insert sdcard
[nemo@Jolla ~]$ mount | grep sdcard
/dev/mmcblk1 on /media/sdcard/F6D0-FC42 type vfat (rw,nosuid,nodev,noexec,...
[nemo@Jolla ~]$ 

[nemo@Jolla ~]$ touch  /media/sdcard/F6D0-FC42/test.txt
[nemo@Jolla ~]$ tracker-sparql -q "select tracker:available(?s) nfo:fileName(?s) \
     { ?s nie:url 'file:///media/sdcard/F6D0-FC42/test.txt' }"
  true, test.txt

# Take out the sdcard

[nemo@Jolla ~]$ mount | grep sdcard
[nemo@Jolla ~]$ tracker-sparql -q "select tracker:available(?s) nfo:fileName(?s) \
     { ?s nie:url 'file:///media/sdcard/F6D0-FC42/test.txt' }"
  (null), test.txt
[nemo@Jolla ~]$