Buildpack API >= 0.6 requires layer to define default process type

[samatcd]

The cnb-shim was updated from buildpack API version 0.4 to version 0.8 as part of this commit:
90c6e67

As part of the API upgrade notes from 0.5 -> 0.6, it's noted that the API will no longer set the default process type to web, and that buildpacks should contribute a default type themselves in launch.toml.
https://buildpacks.io/docs/reference/spec/migration/buildpack-api-0.5-0.6/#buildpacks-contribute-default-process-type

We use GitLab auto-build-image with cnb-shim to build PHP applications using the heroku php buildpack — the shim doesn't seem to set the default process type in launch.toml and unfortunately auto-build-image doesn't allow us to pass a --default-process argument to the pack command, so we're a little stuck.

Is there any way to update the cnb-shim to set the default process?

[samatcd]

Workaround for now:

In project.toml change your buildpack shim uri:

[[build.buildpacks]]
uri = "https://cnb-shim.herokuapp.com/v1/heroku/php?api=0.4"

Edit: this still doesn't work because of the change to:

[types]
launch = true

[edmorley]

@samatcd Thank you for filing this, and sorry for missing this breaking change during #66. I hadn't realised that this buildpack even read the procfile, since in the Heroku builders we use the actual heroku/procfile CNB.

So it seems a bit unfortunate that cnb-shim duplicates the Procfile CNB:

cnb-shim/releaser.go

Lines 25 to 45 in 90c6e67

	procfile, err := ReadProcfile(appDir)
	if err != nil {
	return err
	}
	processTypes := make(map[string]string)
	for name, command := range release.DefaultProcessTypes {
	processTypes[name] = command
	}
	for name, command := range procfile {
	processTypes[name] = command
	}
	processes := layers.Processes{}
	for name, command := range processTypes {
	processes = append(processes, layers.Process{
	Type: name,
	Command: command,
	})
	}

https://github.com/heroku/procfile-cnb/blob/cd7f0b49f5c907955f2275d9b7f2ad808e43409e/src/launch.rs#L5-L35

To resolve the default process type issue, we'd need to duplicate yet more of the implementation from the Procfile CNB. Whilst it wouldn't take long to do that, I wonder whether longer term we'd be better off removing cnb-shim's own procfile handling, and instead either:

1. Document that shimmed CNBs need to be used alongside the heroku/procfile CNB (or an equivalent CNB)
2. Or, have cnb-shim create a meta-buildpack that wraps the shimmed CNB and also adds heroku/procfile to the meta-buildpacks order?

[samatcd]

Ahh yes, interesting. It definitely doesn't make sense to maintain this in two places.

My initial thought would be to implement option 2: Have cnb-shim add heroku/procfile automatically, as that would be less of a breaking change.

[edmorley]

So looking closer, even if we removed the part of cnb-shim that duplicates the procfile CNB, we'd still need some processes handling in CNB shim, since it needs to support the default_process_types from bin/release feature of the classic buildpack API:
https://devcenter.heroku.com/articles/buildpack-api#bin-release

(Which is implemented here:

cnb-shim/releaser.go

Lines 20 to 33 in 90c6e67

	release, err := ExecReleaseScript(appDir, targetBuildpackDir)
	if err != nil {
	return err
	}
	procfile, err := ReadProcfile(appDir)
	if err != nil {
	return err
	}
	processTypes := make(map[string]string)
	for name, command := range release.DefaultProcessTypes {
	processTypes[name] = command
	}

)

Another issue I found is that cnb-shim uses libbuildpack for the Process type, and:

• cnb-shim is on a very old version of libbuildpack and there are breaking changes in order to update (Bump github.com/buildpack/libbuildpack from 1.11.0 to 1.25.11 #36)
• even if the dependency on libbuildpack is updated, the latest version of libbuildpack doesn't have the default field needed to declare the default process type
• libbuildpack itself is sunset, and consumers are supposed to migrate to libcnb (the Go library, not our Rust library) instead: https://github.com/buildpacks/libbuildpack/tree/main#deprecated---libbuildpack

This all means the time investment to resolve this issue has now increased to the point where it's not something I have time to fix now, so instead will be rolling back #66 and accepting that cnb-shim will cause deprecation warnings again (when used with lifecycle 0.16+) and is using a Buildpack API version that stops being supported upstream as of July 2023.

Longer term, we'll need to decide what the future of cnb-shim should be. The codebase is in a not so great state, it doesn't have integration tests, it allows users to customise the API version but yet doesn't have the architecture to support multiple API versions at once, the server is not properly productionised etc.

One option would be for us to write a newer, simpler implementation that leverages our existing investment in libcnb.rs.

[edmorley]

This has been resolved by reverting the change in default buildpack API version in #68.

I've filed #69 for tracking resolving the deprecation warnings (that were the reason for the update in default Buildpack API version in the first place) longer term.

In the meantime, if anyone wants to use the newer buildpack API version (with the understanding that it's not fully supported yet, per the OP here), they can override the default by using &api=0.8 etc in the cnb-shim buildpack URL.