cmd/go: '$/' is an invalid environment variable name on plan 9

[oridb]

When running the tests on plan 9, there are currently a number of failures when attempting to set
the environment, with the message:

can't open: '/env//' create without DMDIR

This is because src/cmd/go/script_test.go atempts to set '$/' to indicate the path separator.

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

commit e822b1e

Does this issue reproduce with the latest release?

The issue was added with commit 742dcba

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

Plan 9 (9front, amd64)

go env Output

% go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/usr/orib/lib/cache/go-build'
GOENV='/usr/orib/lib/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='plan9'
GOINSECURE=''
GOMODCACHE=''
GONOPROXY=''
GONOSUMDB=''
GOOS='plan9'
GOPATH=''
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/orib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLDIR='/usr/orib/go/pkg/tool/plan9_amd64'
GOVCS=''
GOVERSION='VERS'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='6c'
CXX='g++'
CGO_ENABLED='0'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-g -O2'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-g -O2'
CGO_FFLAGS='-g -O2'
CGO_LDFLAGS='-g -O2'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build388427748=/tmp/go-build -gno-record-gcc-switches'

What did you do?

./run.rc

What did you expect to see?

More tests passing than I did

What did you see instead?

script_test.go failing with

can't open: '/env//' create without DMDIR

[oridb]

Removing setting the environment variable fixes some of the script tests, but others fail with a missing path separator in paths. Currently, I'm having trouble finding where $/ is getting used to create the paths.

[ianlancetaylor]

CC @bcmills

I see $/ used, as ${/}, in a number of tests in cmd/go/testdata/script.

Perhaps we should just omit / and : when setting the background process environment in testScript.startBackground.

[oridb]

Got it, I swapped the generated/golden output when I was reading it, and was looking for a place $/ was used in the go commands to generate the paths.

Need to run some tests, but patch on the way.

[gopherbot]

Change https://go.dev/cl/415681 mentions this issue: cmd/go: replace ${slash} with ${/} for plan9 compatibility

[mvdan]

For some context, the testscript fork of script_test.go reached a similar conclusion: rogpeppe/go-internal#115

With the caveat that we want to keep ${/} working where possible, for the sake of backwards compatibility.

[bcmills]

@oridb, I'm trying to better understand the nature of the problem. cmd/go uses os/exec under the hood, so if these environment variables are not valid on Plan 9, does that imply that some behavior in exec.Cmd also needs to change, or at least have its documentation clarified?

For example, should (*exec.Cmd).Start fail explicitly on Plan 9 if the Env slice contains an entry beginning with /? (Or should it ignore such entries entirely?)

[gopherbot]

Change https://go.dev/cl/416554 mentions this issue: cmd/go: avoid setting variables for '/' and ':' in TestScript subprocess environments