cmd/go: toolchain escape via PATH in covermode when GOTOOLCHAIN auto-switches

[zccold]

Go version

go1.25.x linux/amd64 (but 1.26.0 in go.mod)

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/cold/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/cold/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build264210646=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/cold/projects/hello/go.mod'
GOMODCACHE='/home/cold/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/cold/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/cold/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.0.linux-amd64'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/cold/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/cold/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.26.0.linux-amd64/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

1. Have a host system with an older Go version installed (e.g., go1.25.5).
2. Have a project where go.mod specifies a newer version like go 1.26.0 (which triggers the GOTOOLCHAIN auto-switch mechanism).
3. Run a test command that invokes the covdata tool, for example:
   go test -covermode=count -coverpkg=./... ./...

What did you see happen?

The build fails with a toolchain version mismatch error, looking something like this:
compile: version "go1.26.0" does not match go tool version "go1.25.5 (xxx)"

What did you expect to see?

The tests should run successfully, using the automatically downloaded 1.26.0 toolchain to handle the coverage data.

[gopherbot]

Change https://go.dev/cl/749360 mentions this issue: cmd/go: fix toolchain escape via PATH in covermode

[gabyhelp]

Related Issues

• cmd/go: test -cover can't find covdata tool with switched toolchain and empty tests #75031
• cmd/go: GOTOOLCHAIN=version+auto doesn't use the named version in go install package@version #69051 (closed)
• cmd/go: go toolchain version switch doesn't work when only language version is set in go.mod #65580 (closed)
• go run not adhering to toolchain of go.mod when specifying package version #69817 (closed)
• cmd/go: TestScript/gotoolchain_path fails if golang.org/dl/go1.21.1 is installed in the user's $PATH #62709 (closed)
• cmd/go: GOTOOLCHAIN=<name>+[auto|path] is incorrectly rejected as "invalid minimum toolchain" #61068 (closed)
• runtime/coverage: counter mode clash when testing with instrumented go tools #57924 (closed)
• cmd/go: covermode always results in build ID mismatch #68319 (closed)
• cmd/go: reject to download toolchain when gotoolchain < go directive #72003 (closed)

Related Code Changes

• cmd/go: fix toolchain escape via PATH in covermode

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[dr2chase]

@golang/command-line

[dmitshur]

This may be the same issue as #75031, which is fixed by now. @zccold, does this still reproduce for you with go1.27rc1+, or go1.26.3 or go1.25.10?