[[targets.distros]] in buildpack.toml is ignored if the run image distro name/version isn't available

[edmorley]

Summary

The Buildpack API 0.10 spec says:

Metadata specified in [[targets]] is validated against the runtime and build-time base images.

• A buildpack target satisfies a base image target when os, arch, and variant match and at least one distribution in distros (if provided) matches

(https://github.com/buildpacks/spec/blob/buildpack/v0.10/buildpack.md#targets)

As such, if a buildpack's buildpack.toml contains:

[[targets]]
os = "linux"
arch = "amd64"

[[targets.distros]]
name = "ubuntu"
version = "123"

...then I would expect the buildpack to only pass detection if the run image's distro name was "ubuntu" and version was "123".

However, it seems that if the distro name/version could not be determined by lifecycle (for example, the run image doesn't have the io.buildpacks.base.distro.* Docker labels set and the /etc/os-release fallback isn't available or is broken), then lifecycle ignores the [[targets.distros]] requirements and instead matches against any distro.

The cause of this appears to be the base.Distro == nil part of this conditional:

lifecycle/platform/target_data.go

Lines 94 to 96 in 435d226

	if base.Distro == nil || len(module.Distros) == 0 {
	return true
	}

---

Reproduction

Steps

1. pack buildpack new testcase-buildpack-targets --api 0.10 --targets "linux/amd64"
2. echo -e '\n[[targets.distros]]\nname = "ubuntu"\nversion = "123"' >> testcase-buildpack-targets/buildpack.toml
3. pack build --builder heroku/builder:22 --run-image heroku/heroku:22-cnb.v122 --buildpack testcase-buildpack-targets/ --path testcase-buildpack-targets/ testapp --verbose

Current behavior

Buildpack detection unexpectedly passes:

$ pack build --builder heroku/builder:22 --run-image heroku/heroku:22-cnb.v122 --buildpack testcase-buildpack-targets/ --path testcase-buildpack-targets/ testapp --verbose
...
===> ANALYZING
...
Run image info in analyzed metadata is: 
{"Reference":"6d5f868e91eba6a6ec22a9a9b9b31a2fd486a0f0f57082fd0864e0bf0b3e4395","Image":"heroku/heroku:22-cnb.v122","Extend":false,"target":{"os":"linux","arch":"amd64"}}
===> DETECTING
...
Checking for match against descriptor: {linux amd64  [{ubuntu 123}]}
======== Results ========
pass: testcase-buildpack-targets@1.0.0

...even though the buildpack declares that it only supports:

[[targets.distros]]
name = "ubuntu"
version = "123"

...and the run image's Ubuntu version is not "123".

Expected behavior

I would expect buildpack detection to fail, like how it fails if I use a newer version of our run image (by omitting the --run-image heroku/heroku:22-cnb.v122 argument) that has the relevant io.buildpacks.base.distro.* Docker labels set:

$ pack build --builder heroku/builder:22 --buildpack testcase-buildpack-targets/ --path testcase-b
uildpack-targets/ testapp --verbose
...
===> ANALYZING
...
Run image info in analyzed metadata is: 
{"Reference":"9e85c0884ff5a53485a1f53e4c3f958811ea755ddb24885e55b2bcdec2ddf7f1","Image":"heroku/heroku:22-cnb","Extend":false,"target":{"os":"linux","arch":"amd64","distro":{"name":"ubuntu","version":"22.04"}}}
===> DETECTING
...
Checking for match against descriptor: {linux amd64  [{ubuntu 123}]}
======== Error: testcase-buildpack-targets@1.0.0 ========
unable to satisfy target os/arch constraints; run image: {"os":"linux","arch":"amd64","distro":{"name":"ubuntu","version":"22.04"}}, buildpack: [{"os":"linux","arch":"amd64","distros":[{"name":"ubuntu","version":"123"}]}]
======== Results ========
err:  testcase-buildpack-targets@1.0.0

---

Context

lifecycle version

0.19.3

platform version(s)

$ pack report
Pack:
  Version:  0.33.2+git-f2cffc4.build-5562
  OS/Arch:  darwin/arm64

The build is using Platform API 0.12 and Buildpack API 0.10.

[natalieparellano]

@edmorley if we fix #1336 should we not expect that the base distro is never (or rarely) nil? (I am not sure that we want to fail for mismatched distros if one isn't specified)

[edmorley]

@natalieparellano Yeah once #1336 is fixed this issue should only affect:

1. Windows
2. Linux distros that don't ship with systemd (and don't otherwise add their own version of the file) - xref: https://unix.stackexchange.com/questions/351557/on-what-linux-distributions-can-i-rely-on-the-presence-of-etc-os-release

[edmorley]

(I am not sure that we want to fail for mismatched distros if one isn't specified)

So I think if the buildpack says "I'm only supported on X", then lifecycle shouldn't guess and pass detection if it can't figure out the distro name/version.

Ultimately if the buildpack author wanted to match against anything, then it feels like they should/would omit the distro name/version?