reassess DOCKER_FORCE_BUILD about linkerd2

[olix0r]

Can you explain in more detail what you think it's doing versus what it's actually doing?

[siggy]

This comment at the top of docker_maybe_build:
If the tag is 'latest', an image will always be built.

...but my reading of that function, where DOCKER_FORCE_BUILD is not set, is that it will always rely on already built images if available:

    if [ -z "${DOCKER_FORCE_BUILD:-}" ]; then
        docker pull "${repo}:${tag}" >/dev/null 2>&1 || true

        for t in $(docker_tags "${repo}:${tag}") ; do
            if [ "$t" = "$tag" ]; then
                docker tag "${repo}:${tag}" "${repo}:latest" >/dev/null

                echo "${repo}:${tag}"
                return 0
            fi
        done
    fi

[olix0r]

Ahh yeah, the latest behavior was changed without updating the comment, I believe.

Relying only on docker caching would be nice, though I think that's going to be a bit tricky.

In the short-term, i'd suggest that we ensure the scripts are commented appropriately -- I'm happy to help with that.

If we're going to retool all of the build stuff, we need to be clear in scoping/motivating that work. I don't think it should be a priority unless the current tools are actively causing problems.

[klingerf]

Am not sure @siggy is suggesting retooling all of our build stuff, but it would be nice to fix the script with regard to the latest tag. I do think it always makes sense to rebuild the latest tag, instead of potentially pulling whatever's currently tagged as latest on gcr.io.

[olix0r]

Okay, I think I understand now. Please confirm this is the problem:

When running bin/docker-build latest without DOCKER_FORCE_BUILD, we'll pull latest from gcr.io, which probably isn't what's intended.

---

The behavior changed when we stopped using latest as the default tag; and so the check was a bit pointless, since you have to try to do this.

The intended workflow is bin/docker-build && bin/docker-retag-all $(bin/root-tag) latest, which builds images with the git tag and then sets latest to that.

if you don't want to commit between builds, the recommendation would be DOCKER_FORCE_BUILD=1 bin/docker-build latest

If we find that there are larger automatable common workflows around these scripts, we should definitely script it up.

I'm not sure that we should treat latest as a special tag at this point in the code, as there can be other symbolic/floating tags that we can't account for here, and so i think we need to make those determinations in the place where we choose the tag and not down in the guts of the helpers.