cmd/go: vgo run|build *.go fails for module versions >=2 where module is in GOPATH

[mwf]

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

go version go1.10.1 darwin/amd64 vgo:2018-02-20.1

Latest vgo:

commit 0f3e556044ecbd8a0805d098e52a743a1d0e3566 (HEAD -> master, origin/master, origin/HEAD)
Author: Tim Cooper <tim.cooper@layeh.com>
Date:   Fri Jun 22 17:51:20 2018 -0300

    cmd/go/internal/modcmd: fix sorting

What did you do?

Please refer to https://github.com/mwf/goplay/tree/master/vgo/hello_world

I check a case when we've got a binary with major version v2.0.0 and there is no v2 subdirectory or branch, major version is listed in go.mod instead.
It should be OK for vgo if I'm not mistaken, because you didn't want to force all go users to use a single workflow with branches or subdirectories. So we assume master branch is our current stable version, tags correspond with major version set in module directive.

go.mod:

module github.com/mwf/goplay/vgo/hello_world/v2

main.go:

package main

import (
	"fmt"
)

const Version = "V2"

func main() {
	fmt.Println("Hello world!")
	fmt.Println("Version:", Version)
}

vgo run . or vgo build . work without errors.

What did you expect to see?

vgo run main.go should run without errors

What did you see instead?

$ vgo run main.go
build github.com/mwf/goplay/vgo/hello_world: cannot find module for path github.com/mwf/goplay/vgo/hello_world

[mwf]

@bcmills @myitcv
Sorry for bothering guys, but have you looked at this issue?

I think it's rather important, because we usually use a command like go build main.go. Would be pity if it breaks in case of v2+ modules after the go1.11 release.

[myitcv]

@mwf is this still an issue? I don't see any problems as of 6cd5a417451d8ee907692eded07ef1b6b53825b1:

cd `mktemp -d`
export GOPATH=$PWD
mkdir hello
cd hello
vgo mod -init -module example.com/hello/v2
cat <<EOD > main.go
package main

import (
        "fmt"
)

const Version = "V2"

func main() {
        fmt.Println("Hello world!")
        fmt.Println("Version:", Version)
}
EOD
vgo run main.go
vgo build
./v2

(note: the last step needing to be ./v2 is covered by #24667)

[mwf]

@myitcv yeap, I checked it with the same version. And there is some crazy thing about it.

Compare this (failing):

cd `mktemp -d`
export GOPATH=$PWD
git clone https://github.com/mwf/goplay src/github.com/mwf/goplay
cd src/github.com/mwf/goplay/vgo/hello_world
vgo build main.go

Output: build github.com/mwf/goplay/vgo/hello_world: cannot find module for path github.com/mwf/goplay/vgo/hello_world

And this (OK):

cd `mktemp -d`
git clone https://github.com/mwf/goplay
vgo build main.go

It's weird, but it builds OK if outside of GOPATH and fails if inside.
And the problem is only with v2+ modules, v1 works well in both cases.

[myitcv]

@mwf nicely tracked down. Here's a standalone repro:

cd `mktemp -d`
export GOPATH=$PWD
mkdir -p src/example.com/hello
cd src/example.com/hello
vgo mod -init -module example.com/hello/v2
cat <<EOD > main.go
package main

import (
        "fmt"
)

const Version = "V2"

func main() {
        fmt.Println("Hello world!")
        fmt.Println("Version:", Version)
}
EOD
vgo run main.go

gives

build example.com/hello: cannot find module for path example.com/hello

[rsc]

Reproduced, thanks for these great scripts @myitcv.

[rsc]

Related: #25281. Both are about incorrectly guessing the import path of the current directory because it happens to be in GOPATH.

[rsc]

This script works for me now (updated to use go instead of vgo).
Please reopen if you can still reproduce the problem some other way.

cd `mktemp -d`
export GOPATH=$PWD
mkdir -p src/example.com/hello
cd src/example.com/hello
go mod init example.com/hello/v2
cat <<EOD > main.go
package main

import (
        "fmt"
)

const Version = "V2"

func main() {
        fmt.Println("Hello world!")
        fmt.Println("Version:", Version)
}
EOD
go run main.go

[myitcv]

@rsc I think this issue remains.

The point I missed when I was when retesting a couple of days ago is that since this issue was opened we have introduced the concept of GO111MODULE.

A slight tweak to the above shows the issue remains:

export GO111MODULE=on
cd `mktemp -d`
export GOPATH=$PWD
mkdir -p src/example.com/hello
cd src/example.com/hello
go mod init example.com/hello/v2
cat <<EOD > main.go
package main

import (
        "fmt"
)

const Version = "V2"

func main() {
        fmt.Println("Hello world!")
        fmt.Println("Version:", Version)
}
EOD
go run main.go

gives

build example.com/hello: cannot find module for path example.com/hello

[gopherbot]

Change https://golang.org/cl/129798 mentions this issue: cmd/go: allow 'go run x.go' to use nearby internal imports in module mode