Bug: sam local start-api not working in a docker-out-of-docker setup, due to an empty /var/task directory. about aws-sam-cli

[hawflau]

Hey @git-noise thanks for raising the issue. After some digging, it seems the limitation is that, in the docker-in-docker scenario, the path (/tmp/tmpqn5dh6cm in your example) mounted to /var/task is relative to the host but not to the container. /tmp/tmpqn5dh6cm does not exist in the host but only in the container. That's why /var/task is empty. (reference: https://forums.docker.com/t/mounting-a-volume-not-working-with-running-docker-in-docker/25775/3)

SAM CLI does not support "Docker-in-Docker" right now. I'll mark this issue as a feature request for support for "Docker-in-Docker" and bring that up to the team.

[git-noise]

Hello, thanks a lot for looking into it @hawflau.

Based on your insight, I was able to confirm that it is indeed an issue relating to volume mounting.
A quick test check that consists in mounting the full /tmp in the docker container, ensures that the temporary directory - here /tmp/tmpqn5dh6cm - is available within both the host and the container, and then /var/task properly populated.

Maybe an "easy" fix would be to adjust the function that creates these temporary directory, to take an optional base directory parameter? It would then allow for these temporary directory to be still created on the host, but in a specific directory that could be mounted on the sam-cli container?

Thanks for your help,