Make memory erasure feature compatible with overlayfs

Originally created by @intrigeri on #15146 (Redmine)

The “Erasure of the overlayfs read-write branch on shutdown” test scenario fails. But interestingly, “Tails erases memory on DVD boot medium removal: overlayfs read-write branch” passes. Looks like live-boot behaves slightly differently on overlayfs than on aufs, which seems to explain why mountpoints are not visible and can’t be unmounted and thus cleaned.

We need to update the design doc (wiki/src/contribute/design/memory_erasure.mdwn) to document the overlayfs implementation instead of the aufs one.

Feature Branch: bugfix/15146-overlayfs-memory-erasure+force-all-tests

Attachments

• aufs.png
• overlayfs.png
• exposed-upper-dir-log-2.png
• exposed-upper-dir-log-1.png
• exposed-upper-dir-log-3.png
• exposed-upper-dir-log-4.png
• exposed-upper-dir-log-5.png
• exposed-upper-dir-log-6.png
• 02_10_33_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.png
• 02_10_33_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.mkv
• 00_21_12_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.png
• 00_21_12_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.mkv
• 03_25_07_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.png
• 03_25_07_Tails_erases_memory_on_DVD_boot_medium_removal__overlayfs_read-write_branch.mkv
• with-util-linux-mount-umount.png

Related issues

• Related to #8415 (closed)