Hi! I have the following

BUILD

file that reproduces the issue I'm having.

docker_image(
    name="img-base",
    instructions=[
        "FROM python:3.11.10-slim-bookworm",
    ],
    registries = [
        "some-registry"
    ],
    image_tags=["{build_args.TAG_DST}"],
)

docker_image(
    name="img-code",
    dependencies=[
        ":img-base",
    ],
    instructions=[
        "ARG TAG_SRC",
        "FROM some-registry/img-base:${TAG_SRC}",
    ],
    registries = [
        "some-registry"
    ],
    image_tags=["{build_args.TAG_DST}"],
)

Now I'm not sure whether my whole thinking is wrong and I perhaps shouldn't expect

pants

to work like this, but here we go ... I would first run the following:

pants package //:img-base --docker-build-args="['TAG_DST=abc']"
pants package //:img-code --docker-build-args="['TAG_SRC=abc', 'TAG_DST=abc']"

Now when I list docker images, I see what I expect:

╰─❯ docker images --filter=reference='some-registry/*'
REPOSITORY               TAG       IMAGE ID       CREATED        SIZE
some-registry/img-base   abc       80abb94774d6   2 months ago   129MB
some-registry/img-code   abc       80abb94774d6   2 months ago   129MB

Now when I want to build

img-code:def

from

img-base:abc

, pants still goes building

img-base:def

, even if it doesn't need it:

╰─❯ pants package //:img-code --docker-build-args="['TAG_SRC=abc', 'TAG_DST=def']"
14:05:11.31 [INFO] Completed: Building docker image some-registry/img-base:def
14:05:11.31 [INFO] Completed: Building docker image some-registry/img-code:def
14:05:11.31 [INFO] Wrote dist/img-code.docker-info.json
Built docker image: some-registry/img-code:def
Docker image ID: sha256:80abb94774d660831663290fe0b208b3e067b88647902f0f9e1d3a331a997afd

Docker images now look like this:

╰─❯ docker images --filter=reference='some-registry/*'
REPOSITORY               TAG       IMAGE ID       CREATED        SIZE
some-registry/img-base   abc       80abb94774d6   2 months ago   129MB
some-registry/img-base   def       80abb94774d6   2 months ago   129MB
some-registry/img-code   abc       80abb94774d6   2 months ago   129MB
some-registry/img-code   def       80abb94774d6   2 months ago   129MB

Should

pants

skip building

img-base:def

here?

1. Since

img-code

depends on

img-base

, pants has determined that it needs to build

img-base

before building

img-code

. 2. Because you're passing `TAG_DST`(I know its intended for

img-code

, but still), pants rebuilt (probably pulled from cache and re-tagged)

img-base

as well. 3. If you use different vars for the tags in each image, what you'll probably see here is that the unexpected

img-base

gets tagged with a null value (or is this an error), because its own specific var didn't get passed 4. If you remove the explicit `dependencies=[":img-base"]`from

img-code

, I expect this would not occur at all, as there would be no dependency link causing pants to build

img-base

However, the best way to fix this would be to do something like:

instructions=[
    "ARG BASE_IMAGE=:img-base",
    "FROM $BASE_IMAGE",
]

Notice that I set the var to supply the whole image (instead of just the tag), and I actually provide the default value as the "pants name" of the target, rather than the built image as it ends up in docker. Then, you can remove the explicit dependency on

img-base

since pants will be able to infer it by reading that ARG. Pants will do all the necessary substitution to make this work correctly at build time. Now, you can just run

pants package //:img-code

and pants will automatically build the base image if necessary, or pull it from cache if it is already there. Also, instead of trying to pass TAG_DST as a build arg (since its not actually part of the docker build), you can use pants' own env substitution.

image_tags=[env("TAG_DST", "default-tag")]

and then run

TAG_DST=abc pants package //:img-code

Thanks a lot for the explanation and the hints! Will try it out.