Remove apk cache

[mtsmfm]

$ docker image ls ruby:alpine
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ruby                alpine              e8e61f5c55cb        35 hours ago        62MB

$ docker run --rm -it ruby:alpine ls -la /var/cache/apk
total 1204
drwxr-xr-x    1 root     root          4096 Sep  5 05:55 .
drwxr-xr-x    1 root     root          4096 Jul  5 14:46 ..
-rw-r--r--    1 root     root        451901 Sep  5 05:55 APKINDEX.5022a8a2.tar.gz
-rw-r--r--    1 root     root        769090 Sep  5 05:55 APKINDEX.70c88391.tar.gz

$ docker image ls my-ruby
REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
my-ruby             latest              ca40c914bcb6        About a minute ago   60.8MB

$ docker run --rm -it my-ruby ls -la /var/cache/apk
total 8
drwxr-xr-x    2 root     root          4096 Jul  5 14:46 .
drwxr-xr-x    1 root     root          4096 Jul  5 14:46 ..

[yosifkit]

From docker-library/python#330 (comment):

This is actually a (recent) bug, and they're planning to fix it: https://bugs.alpinelinux.org/issues/9063

(https://git.alpinelinux.org/cgit/apk-tools/commit/?id=e1a05c74cb8ae6ab41ee960343cc64a0a9af95a5)

https://git.alpinelinux.org/cgit/apk-tools/commit/?id=8a28c6d0d4f81406c67e76b2bcdbe491aa734717 appears to be the commit which introduced this "autoupdate" behavior.

On the other hand, it does affect alpine 3.7 based images (and current 3.8) since the Alpine bug is only being updated in 3.8. 😢

Now every apk add|del will need to download the cache and delete it, even if it could've been done without hitting the network (and --no-network doesn't help).

So, I guess LGTM cc @tianon.

[yosifkit]

cc @ncopa, is there any other way to get apk to not download the cache on every add/del?

This bug, https://bugs.alpinelinux.org/issues/9063, shows that only 3.8 is getting the fix for del, could that be backported to 3.7 as well or is that outside of what can be backported?

I'm not sure if you have seen an increase in bandwidth on your cdn, but this issue could have a large impact as we fix base images to not have cache files and every user starts downloading them in each del and add. It was nice it 3.6 when we could add a --virtual package without pulling down cache.

Related issues: docker-library/python#330, docker-library/php#708

[ncopa]

I guess it makes sense to fix it in v3.7 as well.

[tianon]

So if I'm reading right, we should be using --no-network on apk add --virtual ... (things that are already installed) and apk del? 🎉