x/tools/go/packages: error during the call to packages.Load with -mod other than empty.

[ldez]

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

$ go version
go version go1.15.6 linux/amd6

Does this issue reproduce with the latest release?

It's not related to a specific version of Go.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ldez/.cache/go-build"
GOENV="/home/ldez/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/ldez/sources/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/ldez/sources/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/ldez/.gvm/gos/go1.15.6"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/ldez/.gvm/gos/go1.15.6/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-build165529633=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"fmt"

	"golang.org/x/tools/go/packages"
)

func main() {
	conf := &packages.Config{
		BuildFlags: []string{"-mod=readonly"},
	}

	pkgs, err := packages.Load(conf)

	fmt.Println(err, pkgs)
}

The problem is related to the state.getGoVersion() call:

https://github.com/golang/tools/blob/929a8494cf60267d89035bc211b797a67b65a6b9/go/packages/golist.go#L848-L852

This call was introduced in golang/tools@2f4fa18

I think that the call creates this problem because of the hard-coded GO111MODULE=off

https://github.com/golang/tools/blob/929a8494cf60267d89035bc211b797a67b65a6b9/internal/gocommand/version.go#L18

Because of the hard-coded GO111MODULE=off the value of cfg.ModulesEnabled is false.

go/src/cmd/go/internal/work/init.go

Lines 271 to 274 in 4fd9455

	case "readonly", "vendor", "mod":
	if !cfg.ModulesEnabled && !base.InGOFLAGS("-mod") {
	base.Fatalf("build flag -mod=%s only valid when using modules", cfg.BuildMod)
	}

Related to golangci/golangci-lint#1502

What did you expect to see?

<nil> [.]

What did you see instead?

err: exit status 1: stderr: build flag -mod=readonly only valid when using modules
 []

[stamblerre]

Thank you for reporting this! I'm able to reproduce--the issue is that we clear out the invocation's Mod flag, but we don't remove any -mod= flag from the build flags. I'll have a fix soon.

[gopherbot]

Change https://golang.org/cl/280694 mentions this issue: go/packages: remove -mod, -modfile flags from build flags for go version

[fc]

Upgrading to go 1.16 resolved it for me.