mount under /mnt made by snap apps is invisible outside snap

[Woodrow Shen]

There is a test case regarding automount when inserting sd card. I did something like `mount -t ext4 -o $mopts /dev/mmcblk1 /mnt/sdcard` inside the test snap command, triggered by udev rule, for example, put a new rule as below:

$ cat /etc/udev/rules.d/11-sd-cards-auto-mount-linuxrc.rules
KERNEL=="mmcblk[0-9]p[0-9]", GOTO="sd_cards_start_mount"
KERNEL!="mmcblk[0-9]", GOTO="sd_cards_auto_mount_end"

LABEL="sd_cards_start_mount"
ACTION=="add", IMPORT{program}="/sbin/blkid -o udev -p %N"
ACTION=="add", ENV{ID_FS_TYPE}=="", GOTO="sd_cards_auto_mount_end"
ACTION=="add", RUN+="/usr/bin/snap run test-snap.media-automount %k"
ACTION=="remove", RUN+="/usr/bin/snap run test-snap.media-automount %k"
KERNEL=="mmcblk[0-9]", ACTION=="change", RUN+="/usr/bin/snap run test-snap.media-automount %k"
LABEL="sd_cards_auto_mount_end"

So when the sd card is detected, the script media-automount tries to mount it into /mnt/sdcard in the snap namespace, but it's invisible in the system:

$ mountpoint /mnt/sdcard
/mnt/sdcard is not a mountpoint

Ian's comment: /mnt should be shared such that mounts to /mnt made inside the snap mount namespace will propagate to the root mount namespace

Note:the test snap I did was installed with devmode.

[Maciej Borzecki (maciek-borzecki)]

/media is set up with shared propagation, so mount events done under /media within the snap mount namespace will be visible outside. /mnt is not. Can the rule be updated to use a path under /media instead?