Repository URL in appendix is no longer substituted - #2129

• I am the author of this work
• Bug fix (non-breaking change which fixes an issue)

Problem

I run a few public BinderHubs (https://terrarium.evidencepub.io, https://live.evidencepub.io). Recently, I switched my registry (https://registry.evidencepub.io) to zot as part of an infrastructure migration, and while I was at it, I wanted to use BuildExecutor.appendix to stamp OCI labels onto built images, so that it is more aligned with the spirit of " reproducible publishing":

LABEL org.opencontainers.image.source="{repo_url}"
LABEL org.opencontainers.image.url="{binder_url}"

After a build I went looking for those labels in my registry and found them set to the literal strings. The build log confirms:

Step 51/57 : LABEL org.opencontainers.image.title="{repo_url}"

Why?

BinderHub.appendix documents the value as a string template:

This should be a Python string template. It will be formatted with at least the following names available: binder_url, repo_url.

and examples/appendix/binderhub_config.py still depends on that:

ENV BINDER_URL={binder_url}
ENV REPO_URL={repo_url}

But nothing really formats it. build.py hands the value straight to repo2docker:

if self.appendix:
    r2d_options.extend(["--appendix", self.appendix])

Tracing it back, the formatting used to live in builder.py:

appendix = self.settings['appendix'].format(
    binder_url=self.binder_launch_host + self.binder_request,
    persistent_binder_url=...,
    repo_url=repo_url,
    ref_url=self.ref_url,
)

Relevance

#1518 moved build parameters to traitlets and left that line commented out on the new code path.Later on, 451059c deleted the commented line as dead code.

How can this PR be tested?

I have not added an automated test, but this reproduces in a couple of minutes with the local dev config, I ran it with enable_api_only_mode:

Adding to binderhub_config.py:

c.BinderHub.build_class = LocalRepo2dockerBuild
c.BinderHub.use_registry = False
c.BinderHub.enable_api_only_mode = True
c.BuildExecutor.appendix = 'LABEL binder.repo="{repo_url}"\nLABEL binder.url="{binder_url}"'

Then:

curl -N -H 'Accept: text/event-stream' \
  'http://localhost:8585/build/gh/binder-examples/requirements/master?build_only=true'
docker inspect <built image> --format '{{json .Config.Labels}}'

Before:

binder.repo = '{repo_url}'
binder.url  = '{binder_url}'

After:

binder.repo = 'https://github.com/binder-examples/requirements'
binder.url  = 'http://localhost:8585/v2/gh/binder-examples/requirements/HEAD'    

Note: use a repo that does NOT contain a Dockerfile. I realized that repo2docker only supports the appendix in the template-generating buildpacks...

I know @rgaiacs is working hard to make it all wasm ready, but we still rely heavily on Binderhub and containers :)

Thank you so much!