SYNC_PREFER defaults back to /source after Docker restart

[normanlolx]

Taking the basic docker-compose.yml from the README but SYNC_PREFER: newer and restart: always set. Everything works fine except after you restart the Docker for Mac app (or restart you Mac). From then on the sync is unidirectional again as if SYNC_PREFER=newer never was set.

Steps to reproduce:

1. $ git clone https://gist.github.com/leymannx/5d27bd6f0f0b08b13e58baa4bcd866ae.git bg-sync
2. $ cd bg-sync
3. $ docker-compose up -d (wait until done)
4. $ touch hello.txt
5. $ docker exec -it -w /var/www/myapp bgsync_web_1 bash -c "touch world.txt"
6. Confirm both files exist in both locations:
   1. $ ls
   2. $ docker exec -it -w /var/www/myapp bgsync_web_1 bash -c "ls"

Restart Docker for Mac app (or restart Mac).

1. Again inside the bg-sync folder from step 1. on the host: $ touch alpha.txt
2. $ docker exec -it -w /var/www/myapp bgsync_web_1 bash -c "touch beta.txt"
3. Confirm there's no alpha.txt in the container:
   1. $ ls
   2. $ docker exec -it -w /var/www/myapp bgsync_web_1 bash -c "ls"

---

macOS 10.13.3
Docker for Mac (stable) 17.12.0-ce-mac49 (21995)

Workaround is to simply leave restart: always out and/or always (re-)start the containers manually $ docker-compose start.

Although the Dockerfile and the sync script is kept brilliantly small I don't have a clue how to fix this.

[dj95]

It doesn't default back to SYNC_PREFER=newer. Please check the environment-variables in your sync-container with docker exec -it -w /var/www/myapp bgsync_bg-sync_1 env and the unison config file with docker exec -it -w /root/.unison bgsync_bg-sync_1 cat default.prf.

I think it is a problem with docker on macOS(and maybe Windows?) in general, not only with docker bg-sync. Under Linux this issue doesn't exist. Unison triggers on inotify events. If you measure inotify events with the following procedure, you will see, that those are not occuring, if a file is created on the host side of the source-volume after restarting the docker host and starting the containers with their restart: always behaviour.

1.) Get the docker-bg-sync container in the state where it doesnt sync from the host, like @leymannx described in his issue.

2.) Start the inotifywatcher with docker exec -ti bgsync_bg-sync_1 inotifywatch -r /source/

3.) Create a file on your docker host machine in the folder, that is mounted to /source

4.) Ctrl-C to exit the inotifywatcher and display it stats.

5.) Restart docker containers with docker-compose stop and docker-compose up -d

6.) Start the inotifywatcher in the bg-container again

7.) Create another file on your docker host machine, like the one before

8.) Ctrl-C to exit the inotifywatcher.

In step 8 you should see at least one inotify event when the file is created. After step 4 you shouldn't be able to see any inotify events in this folder even if the file was created. This explains why unison is not recognizing the recently created file.

After a short research in docker for mac issues on Github I found this one:

docker/for-mac#681

It is exactly the problem, which explains the behaviour of this issue. I tested the inotify propagation and the behaviour of docker-bg-sync after a simple docker-compose restart and it falls back to the state described in this issue. So I think we need to wait for a fix from the docker team.