Pack silently fails to use project.toml if the schema-version is not specified

[edmorley]

Summary

If a project.toml file does not contain an appropriately set schema-version, Pack silently ignores values set in the project.toml file, leading to a frustrating to debug experience.

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Reproduction

Steps

1. mkdir testcase && cd $_
2. echo -e "[io.buildpacks]\nbuilder = \"some-builder\"" > project.toml
3. pack build testcase

Current behavior

The pack build tries to perform a build using whatever is set as the default builder.

The cause of this is that the builder property is only supported as of project.toml schema version 0.2, but Pack CLI silently defaults to v0.1 if no schema version was specified, and ignores knowns keys rather than failing with a schema validation error.

If I add this, then the expected behaviour occurs:

[_]
schema-version = "0.2"

Expected behavior

For either:

1. Pack to default The build to be performed using the specified builder (which would fail with an invalid builder error, but this is just a testcase)
2. Pack to output a warning or error saying "no schema-version specified, defaulting to schema v0.1", along with perhaps a schema v0.1 validation error about the builder property not being recognised.

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Environment

pack info

$ pack report
Pack:
  Version:  0.31.0+git-3a994bd.build-5086
  OS/Arch:  darwin/arm64

Default Lifecycle Version:  0.17.1

Supported Platform APIs:  0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 0.11, 0.12

Config:
  default-builder-image = "[REDACTED]"
  experimental = true
  
  [[trusted-builders]]
    name = "[REDACTED]"
  
  [[trusted-builders]]
    name = "[REDACTED]"

[edmorley]

I think that either Pack should either:

1. Reject project.toml files without an explicit schema-version entirely
2. Always warn if an explicit schema-version isn't set (and state what it is defaulting to)

If the decision is made to allow implicit schema-version (ie option 2), I also think that the default version should be raised from v0.1 to whatever the latest version is. (This will be a breaking change, but it could be mitigated by adding the warning the release prior etc.)

Lastly, wrt schema validation, IMO Pack should either:

• Reject unknown fields (within tables specified explicitly in the spec) entirely
• Warn if an unknown field is found during parsing (and for bonus points, hint that maybe the schema-version needs changing)

[edmorley]

In fact, the spec states:

It MUST contain schema-version to denote which schema version the descriptor is using.

(https://github.com/buildpacks/spec/blob/main/extensions/project-descriptor.md#_)

So I guess option (1) above is the right choice here.

The docs here also need fixing, since they say the [_] table is optional, when it's not according to the spec:
https://buildpacks.io/docs/reference/config/project-descriptor/#_-_table-optional_

[edmorley]

This caused confusion for another user recently in:
https://cloud-native.slack.com/archives/C033DV9EBDF/p1701906234287769