dl: No longer able to download a new Go toolchain from an old Go version

[punya]

As a result of https://go-review.googlesource.com/c/dl/+/540221, (golang/dl@2a66dfb)

go get golang.org/dl/go1.19.13 run from any Go version <= 1.16 now fails with:

# golang.org/dl/internal/version
go/src/golang.org/dl/internal/version/signal_unix.go:14: signalsToIgnore redeclared in this block
	previous declaration at go/src/golang.org/dl/internal/version/signal_notunix.go:13
go/src/golang.org/dl/internal/version/version.go:151: undefined: os.WriteFile
go/src/golang.org/dl/internal/version/version.go:322: undefined: io.ReadAll

because //+build was removed and //go:build is not supported on those versions.

My CI pipeline run from an old Debian that tries to download a newer Go version now fails because of this change.

Originally posted by @delthas in #60268 (comment)

[seankhliao]

as noted in the original issue, those versions are outside of our support window (2 most recent major releases)

[punya]

You're right that the original issue calls out the support window. However, I think it's worth making a distinction between the golang.org/dl package which is often used to install a desired version of Go, and the other packages which are used as libraries.

If this is working as intended, please consider adding a prominent note to https://go.dev/doc/manage-install documenting the minimum version.

[delthas]

Agreed: while supporting only the 2 latest major versions for Go in general makes sense; for dl in particular this means that the only remaining use cases are:

• upgrade from the N-1 latest major version to the latest major version (because N-2 is out of support by the time N comes out)
• upgrade from a minor version to another minor version (probably not the main use case)

(In other words, go dl stops working after 1 year (2 releases) / no upgrade on a system for one year means you have to reinstall Go.)

Which is much more specific and reduced in scope than the previous behavior (effectively) where you could start from any old install and upgrade to latest.

Maybe it could be worth maintaining Go compatibility for dl/ specifically for more versions, so that even users on older machines can profit from dl. The tradeoff being to have to keep a somewhat larger compatability for dl/ (not that many files?)

[punya]

Perhaps the team's intent is that dl is really for installing older versions of Go, not newer ones? It does seem worth clarifying.

[ianlancetaylor]

CC @golang/release

[seankhliao]

consider #44329 (comment) where the use of GOVERSION may obsolete the use golang.org/dl (assuming a new enough starting version)