Set `GOTOOLCHAIN=local` if `go-version[-file]` is set and `GOTOOLCHAIN` is not already present

[Frederick888]

Description:
When a user sets go-version[-file] explicitly, they'd often expect the version (range) to be enforced.

So I think we should set GOTOOLCHAIN=local to disable automatic toolchain switching in this case.

This is similar to #420 but with a bit more details.

Justification:
We maintain a few libraries where we want to make sure they are compatible with the last two Go major version releases (1.X and 1.X-1 to be clear, i.e. minor versions in SemVer).

We don't dictate which Go version contributors use, especially patch versions, but we have set up a GHA matrix like:

jobs:
  test:
    name: Test
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        go_version:
          - 1.21.x
          - 1.22.x
    steps:
      - uses: actions/setup-go@v5
        with:
          go-version: ${{ matrix.go_version }}

Now for example, someone who uses Go 1.22.x may accidentally put a go 1.22.5 line in go.mod, and both jobs in the testing matrix will still pass. If it isn't caught in review, due to the changes since Go 1.21, AFAIU the library will require its consumers to upgrade to Go 1.22.5+, which is apparently unexpected.

So I think when a user specifies a Go version, we should set GOTOOLCHAIN=local to disable this new Go behaviour. If they do want it to happen, they can set an Action or job level GOTOOLCHAIN=auto (or don't set go-version[-file] if they don't care about it at all).

Are you willing to submit a PR?

Yes.

[priya-kinthali]

Hello @Frederick888 👋
Thank you for this feature request. We will investigate it and get back to you as soon as we have some feedback.

[codyoss]

Setting GOTOOLCHAIN=local is the choice made in the official golang container images: docker-library/golang#472

That issue mentions CI for one of the reasons why they wanted to make this change. I personally believe this is likely the behaviour most would expect when declaring a specific go version to set up in a CI environment as well. This would be a large behaviour change though over the default of auto and should perhaps be made as a major version change if this request is accepted.

[matthewhughes934]

there's some similar discussion in #457 (see also #460)

[myitcv]

@Frederick888 - completely agree with your logic here. The setting of a specific version of Go expresses intent: the default behaviour of GOTOOLCHAIN=auto subverts that intent, by silently switching to other toolchain versions.

We are, for now, setting GOTOOLCHAIN=local in all our CI workflows.