cmd/go: malformed module path with retract v2+

[seankhliao]

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

go version go1.16 linux/amd64
go version devel +0f66fb7b85 Sun Feb 21 02:25:41 2021 +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/arccy/.cache/go-build"
GOENV="/home/arccy/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/arccy/.data/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/arccy/.data/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/arccy/sdk/gotip"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/arccy/sdk/gotip/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel +0f66fb7b85 Sun Feb 21 02:25:41 2021 +0000"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/arccy/testrepo-291/go.mod"
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-build1689737487=/tmp/go-build -gno-record-gcc-switches"

What did you do?

add a retract directive to a v2+ module and run go mod tidy

module go.seankhliao.com/testrepo-291/v2

go 1.16

retract v2.0.0

What did you expect to see?

no error

What did you see instead?

» go mod tidy
go: errors parsing go.mod:
/home/arccy/testrepo-291/go.mod:5: malformed module path "": empty string

[komuw]

I had reported a slight variant of this in gophers slack, where I was trying to retract a version 2 while the module is still in version 0.

repro:

mkdir -p /tmp/example; cd /tmp/example; go mod init example
rm -rf main.go
tee -a main.go <<EOF
package main
func main(){println("hello")}
EOF
go mod edit -retract=v2.0.2
go mod tidy

The error message I got is;

go: errors parsing go.mod:
/tmp/example/go.mod:5: malformed module path "": empty string

Seankhliao pointed out it should not be possible to retract a v2 while still in v0.
So, I'm just posting this here so that the error message can be improved; malformed module path would lead someone to believe that the thing they need to fix is their module name/path

[bcmills]

CC @jayconrod @matloob

[bcmills]

@gopherbot, please backport to Go 1.16.

[gopherbot]

Backport issue(s) opened: #44496 (for 1.16).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

[bcmills]

Just FYI: this is not fixed by CL 295150, so #44497 appears to be unrelated.

[gopherbot]

Change https://golang.org/cl/296130 mentions this issue: modfile: defer fixing versions in retract directives

[gopherbot]

Change https://golang.org/cl/296131 mentions this issue: cmd: upgrade golang.org/x/mod to fix go.mod parser