cmd/go: go get -u && go mod tidy behaves differently starting with Go 1.21.0, leading to unknown directive toolchain errors

[gaby]

What version of Go are you using (go version)?

$ go version
go version go1.21.0 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/ubuntu/.cache/go-build'
GOENV='/home/ubuntu/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/ubuntu/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/ubuntu/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/snap/go/10319'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/snap/go/10319/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/ubuntu/Desktop/git/fix/contrib/websocket/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2504710228=/tmp/go-build -gno-record-gcc-switches'

What did you do?

This is a continuation of #57001

Lets say, I have a library made with Go that supports go version 1.17, 1.18, 1.19, and 1.20.

My go.mod file looks as follow:

module github.com/my/lib

go 1.18

require (
	github.com/some/other/lib v1.0.0
)

Normally when getting updates, you would do go get -u -v then go mod tidy. Since go1.21 running go mod tidy now changes your go.mod file to the following:

module github.com/my/lib

go 1.21

toolchain go1.21.0

require (
	github.com/some/other/lib v1.0.0
)

This a breaking change behavior, given that now if I commit that code? Any previous CI/CD process, or any other developer in the team that's using a different version of go can no longer build/test/run the code because of the toolchain directive.

What did you expect to see?

I expected that running go mod tidy would only cleanup imports and the go.sum file.

What did you see instead?

Running go mod tidy changes the go.mod go version and adds a toolchain directive that's not available in any previous go version.

Suggested solution

There's two possible solutions here:

• Make go commands use the local install by default instead of auto.
• The go mod tidy command shouldn't change the version/add a directive to go.mod

Example running with go 1.20.7 in Docker:

root@31579b87a9bd:/data# cat go.mod 
module github.com/my/lib

go 1.21

toolchain go1.21.0

require (
	github.com/some/other/lib v1.0.0
)

root@31579b87a9bd:/data# go mod tidy
go: errors parsing go.mod:
/data/go.mod:5: unknown directive: toolchain
root@31579b87a9bd:/data# go version 
go version go1.20.7 linux/amd64

This bring the question, how can we write libraries that are backward compatible with previous go versions?

[dmitshur]

Thanks for reporting this. CC @bcmills, @matloob.

I wanted to clarify a part of the "What did you do?" section. Specifically, when you start out with a go.mod file with support for Go 1.17 through 1.20 and a go 1.18 directive, how does it end up with go 1.21?

I tried to reproduce it as follows (using go1.21.0):

$ cd $(mktemp -d)
$ cat <<EOF >go.mod
module github.com/my/lib

go 1.18

require github.com/google/uuid v1.2.0
EOF
$ cat <<EOF >p.go
package p

import _ "github.com/google/uuid"
EOF

At that point, doing go get -u -v updated the uuid to its latest version v1.3.1, but left the go directive at its starting 1.18 value:

$ go get -u -v
go: trying upgrade to github.com/google/uuid@v1.3.1
go: upgraded github.com/google/uuid v1.2.0 => v1.3.1

$ cat go.mod
module github.com/my/lib

go 1.18

require github.com/google/uuid v1.3.1

Similarly, running go mod tidy did clean up unused entries from go.sum, but otherwise left go.mod as it was before, with go 1.18:

$ go mod tidy
$ echo $?
0

$ cat go.mod
module m

go 1.18

require github.com/google/uuid v1.3.1

Can you clarify what the reproduction steps are missing that's causing the problem? Thanks.

[gaby]

@dmitshur Interesting, could this be triggered by a dependency having a toolchain directive on their go.mod file?

Example CI that is currently failing using 1.21: https://github.com/gofiber/contrib/actions/runs/6047961443/job/16412414494

The CI file is located here: https://github.com/gofiber/contrib/blob/main/.github/workflows/govulncheck.yml

The go.mod for when the failure happens is here: https://github.com/gofiber/contrib/blob/main/paseto/go.mod

This also happens with all the go.mod files in each directory of https://github.com/gofiber/contrib/

[dmitshur]

Yes, that appears to be what's happening.

As far as I can see, the root problem is that as of gofiber/fiber#2588, the go.mod of the module github.com/gofiber/fiber/v2 states "go 1.21", which means that module requires Go language 1.21 or higher to be used. So when go mod tidy is run in the github.com/gofiber/contrib/paseto module which requires github.com/gofiber/fiber/v2@v2.49.0, it upgrades said module's go line to be 1.21 too. Notably, if you run go get go@1.18 in paseto, it downgrades to github.com/gofiber/fiber/v2@v2.48.0.

(Please refer to https://go.dev/ref/mod#go-mod-file-go and https://go.dev/blog/toolchain for more detailed information on this topic.)

[gaby]

@dmitshur I just literally realize that too... Sending a message to the Fiber maintainers as I am typing this.

My guess is that the only fix is to roll back those changes in gofiber/fiber.

[gaby]

@dmitshur Thanks for your help, this can be closed. It's a Fiber issue, not Go.

[dmitshur]

Glad that we figured out what was causing this. Thanks for the detailed report.