cmd/go: go list fails when run in $GOPATH/pkg/mod

[pmer]

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

$ go version
go version go1.11.5 darwin/amd64

$ go version
go version go1.12rc1 darwin/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
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/pdm/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/pdm/example"
GOPROXY=""
GORACE=""
GOROOT="/Users/pdm/SourceRef/go-go1.12rc1"
GOTMPDIR=""
GOTOOLDIR="/Users/pdm/SourceRef/go-go1.12rc1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/pdm/example/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a/go.mod"
CGO_CFLAGS="-I/usr/local/include"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/ml/qr7r1km5315bsm79wn1q60c00000gn/T/go-build465168117=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Tried to go list a module in $GOPATH/pkg/mod.

mkdir example && cd example
export GOPATH="$PWD"
mkdir main && cd main
cat > main.go <<EOF
package main

import "github.com/gocql/gocql"

func main() {
    gocql.NewCluster("192.168.1.1", "192.168.1.2", "192.168.1.3")
}
EOF
cat > go.mod <<EOF
module main

require github.com/gocql/gocql v0.0.0-20190208221138-c53c3654dc8a
EOF
go mod download
cd "$GOPATH/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a" && go list

What did you expect to see?

$ go list
github.com/gocql/gocql

What did you see instead?

In go1.11.5:

$ cd "$GOPATH/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a" && go list
go: writing go.sum: open $GOPATH/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a/go.sum: permission denied

In go1.12rc1:

$ cd "$GOPATH/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a" && go list
go: writing go.sum: open $GOPATH/pkg/mod/github.com/gocql/gocql@v0.0.0-20190208221138-c53c3654dc8a/go.sum389063318.tmp: permission denied

Related

#27161

[bcmills]

Dup of #30185.