Fails in Docker for Mac 2.4?

[dankegel]

The script I'd been happily using was broken by the Docker for Mac 2.4.0.0 update that was pushed out today. The symptom was:

mkfs.ext4: I/O error while writing out and closing file system

Downgrading to Docker 2.3.0.5 worked around the problem.

I tried to come up with a minimal reproducer to submit to the Docker guys, but so far failed. This is where I left it:

# Run with 'docker build -t nbdbug . && docker run --privileged -t nbdbug'
FROM alpine:3.12
RUN apk update && apk add qemu-img e2fsprogs
CMD \
   find /dev -maxdepth 2 -name 'nbd[0-9]*' && \
   qemu-img create -f qcow2 dummy.img 500M && \
   qemu-nbd --connect=/dev/nbd1 --cache=writeback --format=qcow2 dummy.img && \
   mkfs.ext4 -L root -O ^64bit -E nodiscard /dev/nbd1 && \
   qemu-nbd --disconnect /dev/nbd1

but that also succeeds on the new docker. /me is perplexed.

This also affected a coworker who was using docker's Edge version, so he saw it before I did.

[jirutka]

I don’t understand what do you think that I’m supposed to do about it. alpine-make-vm-image doesn’t use Docker and it’s not supposed to be used in this way. Why do you even run it inside Docker?

[dankegel]

Alas, you cannot escape Docker these days. Any tool you write will be used in a script that some poor slob has to run in Docker sometime. There is a whole world of developers who wouldn't dream of running anything *not" in Docker, I kid you not.

I filed the report for the benefit of anyone else in the same boat. I do think it's really a bug in Docker for Mac, which comes with its own VM and copy of Linux to act as a host; their kernel probably dropped an important configuration flag in the update.

Under the older Docker, your script works a treat. Thanks!