cmd/go: run gives conflicting advice when using path@version syntax

[davecheney]

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

% go version
go version devel +97d5cb24b1 Sat Dec 22 09:37:04 2018 +0000 linux/amd64

Does this issue reproduce with the latest release?

unknown

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

go env Output

% go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dfc/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GOPROXY=""
GORACE=""
GOROOT="/home/dfc/go"
GOTMPDIR=""
GOTOOLDIR="/home/dfc/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build125128327=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Go 1.12beta1 (tip) does not permit @Version syntax unless modules explicitly enabled, even if GOPATH is unset

% unset GOPATH && go run -v github.com/davecheney/httpstat@1.0.0
package github.com/davecheney/httpstat@1.1.0: cannot use path@version syntax in GOPATH mode

What did you expect to see?

env GO111MODULE=on go run -v github.com/davecheney/httpstat@1.0.0

would download and build httpstat v1.0.0

What did you see instead?

% unset GOPATH && env GO111MODULE=on go run -v github.com/davecheney/httpstat@1.0.0
missing $GOPATH

Running without modules enabled tells me I must enable modules even thought I don't have GOPATH set. Running with modules enable tells me I must set GOPATH.

lucky(/tmp) % env GO111MODULE=on GOPATH=/tmp/ go run -v github.com/davecheney/httpstat@1.0.0
package github.com/davecheney/httpstat@1.0.0: can only use path@version syntax with 'go get'

Setting both tells me that this isn't supported.

a. Can go run path@version be supported? If not, why not?
b. Can the error messages be improved make it clearer sooner that the operation will not succeed once the correct permutation of environment variables is discovered.

[odeke-em]

Thank you for this report @davecheney!

Kindly paging @bcmills @rsc.

[jadekler]

Seems to not be working for me, either:

$ cd /tmp/whatever && unset GOPATH && go get cloud.google.com/go@v0.30.0
go: cannot use path@version syntax in GOPATH mode

[ahmetb]

Hit this as well in #29575. Closing that as duplicate.

[rsc]

Only go get and module-manipulation commands take path@version.
go build, go run, etc do not.

[jadekler]

Go get does not take path@version for me. I've filed #29659, since that appears to be different from the go run case that this issue was describing. (happy to dedupe if you prefer the conversation to happen here)

[ahmetb]

Can someone take a look at this please?

Example from https://golang.org/cmd/go/#hdr-Module_aware_go_get which is go get golang.org/x/text@v0.3.0 doesn't work and fails with the same error when GOPATH environment variable is not set.

This is breaking go-get’s ability as a package manager as the support for the go get -v github.com/rakyll/hey@v0.1.1 syntax is currently not working.

[bcmills]

@ahmetb, this issue is about the diagnostic messages. If go get is not actually behaving as documented, please file that as a separate issue.

That said, from the detail in #29575 it appears that you are, in fact, running in GOPATH mode.
Per https://golang.org/cmd/go/#hdr-Preliminary_module_support:

If GO111MODULE=auto or is unset, then the go command enables or disables module support based on the current directory. Module support is enabled only when the current directory is outside GOPATH/src and itself contains a go.mod file or is below a directory containing a go.mod file.

[ahmetb]

@bcmills I'm still seeing this despite I'm not running in GOPATH mode. go get or go build uses modules just fine when I don't specify a @version syntax.

Here's a minimal repro with go1.12

dir=$(mktemp -d)

cd $dir

# write a go.mod to indicate go111module=on
echo 'module foo' > go.mod

go get module syntax uses go.mod file:

# unset all env vars, with the exception of $HOME (mock it) and $PATH (inherit it)

env -i HOME="$PWD/mock-home" PATH="$PATH" go build -o ./httpstat github.com/davecheney/httpstat

    go: finding github.com/davecheney/httpstat v1.0.0
    go: downloading github.com/davecheney/httpstat v1.0.0
    go: extracting github.com/davecheney/httpstat v1.0.0
    ...
    (works)

go get module@1.0.0 syntax is not working in the same setup:

env -i HOME="$PWD/mock-home" PATH="$PATH" go build -o ./httpstat github.com/davecheney/httpstat@1.0.0

can't load package: package github.com/davecheney/httpstat@1.0.0: can only use path@version syntax with 'go get'

Would you still suggest I'm in "GOPATH mode"?

[bcmills]

@ahmetb, this issue is about the go command giving conflicting advice.

The advice from the commands you posted above does not conflict: it correctly indicates that, in module mode (which you are clearly in), you “can only use path@version syntax with 'go get'”.

[probonopd]

With Go 1.13, also running into

imports github.com/diskfs/go-diskfs/filesystem@squashfs: cannot use path@version syntax in GOPATH mode

This is so annoying. Especially because it does not tell one what to do.

[smikulcik]

From what I can tell, you just have to be in a package that has a go.mod file. Then you can get a particular version.