global.json file should be considered as an SDK version source

[sophiewigmore]

What happened?

• What were you attempting to do?

Trying to pack build an application that contains a global.json file that looks like this:

{
      "sdk": {
            "version": "2.1.0"
      }
}

In the dotnet-core-sdk buildpack, the global.json file is a part of the plan entry resolver priority list, so we should be looking at this file as a version source.

• What did you expect to happen?

Expected the app to build properly, and to see "2.1.0" with the version source global.json in the SDK buildpack logs.

• What was the actual behavior? Please provide log output, if possible.

Logs from SDK that do not include global.json

Paketo .NET Core SDK Buildpack 0.1.4
  Resolving .NET Core SDK version
    Candidate version sources (in priority order):
      RUNTIME_VERSION     -> "2.1.812"
      simple_brats.csproj -> "2.1.*"
      <unknown>           -> "*"

The pack build failed during publish:

Paketo .NET Publish Buildpack 0.1.3
  Executing build process
    Running 'dotnet publish /workspace --configuration Release --runtime ubuntu.18.04-x64 --self-contained false --output /tmp/dotnet-publish-output047634497'
failed to execute 'dotnet publish': exit status 145
      Failed after 2.741886ms
        A compatible SDK version for global.json version: [2.1.0] from [/workspace/global.json] was not found
        Did you mean to run dotnet SDK commands? Please install dotnet SDK from:
          https://go.microsoft.com/fwlink/?LinkID=798306&clcid=0x409
ERROR: failed to build: exit status 1

Build Configuration

Trying to build this app: https://github.com/cloudfoundry/dotnet-core-buildpack/tree/v2.3.22/fixtures/source_2.1_global_json_templated

With the global.json changed to have version 2.1.0 inside instead of templating.

• What platform (pack, kpack, tekton buildpacks plugin, etc.) are you
  using? Please include a version.

pack v0.15.1+git-79adc30.build-1660

• What buildpacks are you using? Please include versions.

• What builder are you using? If custom, can you provide the output from pack inspect-builder <builder>?

• Can you provide a sample app or relevant configuration (buildpack.yml,
  nginx.conf, etc.)?

Checklist

• I have included log output.
• The log output includes an error message.
• I have included steps for reproduction.

[fg-j]

According to the .NET docs:

The global.json file allows you to define which .NET SDK version is used when you run .NET CLI commands. Selecting the .NET SDK is independent from specifying the runtime your project targets. The .NET SDK version indicates which versions of the .NET CLI is used.

Given the intended purpose of the global.json file, I think it should be considered as an SDK version source. In fact, I think the version source should probably overrule the version determined by RUNTIME_VERSION. A global.json file is the canonical manner of indicating what version of the .NET CLI to use for an app, and I think it should be respected even if it results in a "no available compatible version" error. This way, a developer will be alerted to a difference between their local build environment and the environment of the buildpack build.

[fg-j]

It may be easier to implement this now that .NET RFC 004 has been accepted.