First singularity-in-docker run fails with no loop device

[dtrudg]

The first run of Singularity inside Docker (as root in the container) with --privileged will fail with a loop device error:

$ docker run --privileged singularity:main --debug run docker://alpine
...
FATAL   [U=0,P=1]          Master()                      container creation failed: mount /proc/self/fd/3->/usr/local/var/singularity/mnt/session/rootfs error: while mounting image /proc/self/fd/3: failed to find loop device: could not attach image file to loop device: no loop devices available

The second run will complete without issue.

By examining /dev and --debug logs, it is apparent that during the first run, LOOP_CTL_ADD is called 256 times, creating /dev/loop0 -> /dev/loop255 on the host. The created loop devices are not visible in the container, however:

DEBUG   [U=0,P=136]        addLoopDev()                  LOOP_CTL_ADD for loop device 251
DEBUG   [U=0,P=136]        addLoopDev()                  Expected loop device 251 is not visible. Creating with mknod
DEBUG   [U=0,P=136]        attachLoop()                  couldn't open loop device 251: stat /dev/loop251: no such file or directory

This thread may be relevant:

https://forums.docker.com/t/can-only-create-one-loop-mount-inside-of-container/109498/6

[dtrudg]

A workaround is to bind in /dev to the Docker container with -v /dev:/dev so that the loop devices created are accessible. Alternatively they can be created on the host first. These approaches need documenting.

It would be nice to have a fall-back to using FUSE, if possible, though.