cmd/go: document that go env GOMOD can return /dev/null #36052
Description
What version of Go are you using (go version)?
$ go version go version devel +da4d58587e Sat Dec 7 15:57:30 2019 +0000 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/myitcv/.cache/go-build" GOENV="/home/myitcv/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOINSECURE="" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/myitcv/gostuff" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/home/myitcv/gos" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="" 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 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build492256999=/tmp/go-build -gno-record-gcc-switches"
What did you do?
The docs for GOMOD state:
$ go help environment
...
GOMOD
The absolute path to the go.mod of the main module,
or the empty string if not using modules.
For some time I have (incorrectly) been advising people that the way to determine if you are in a module context or not (i.e. modules not off and you are in the context of a go.mod) is by testing whether the output from go env GOMOD is the empty string or not.
However this fails when you consider:
$ cd $(mktemp -d)
$ GO111MODULE=on go env GOMOD
/dev/null
It's also not clear the docs are quite correct when it comes to modules being not off (i.e. auto or on):
$ cd $(mktemp -d)
$ go env GOMOD
(i.e. empty string, but we aren't not using modules, we are in the auto mode. Granted this is a bit less clearly incorrect)
I realise that this way go env GOMOD can be used to determine whether modules are off/not off, and whether you are in a module context or not assuming that they are not off. But documenting this a bit more clearly would, I think, help (selfishly me at least).
Because the actual logic for correctly determining that modules are not off and you are in a module context is:
cmd := exec.Command("go", "env", "GOMOD")
out, _ := cmd.CombinedOutput()
gomod := strings.TrimSpace(string(out))
if gomod != "" and gomod != os.DevNull {
// ....Thoughts?
FYI @mvdan @kortschak