cmd/go: malformed module path "xxxx/xxxx/uuid" missing dot in first path element when migrating from GOPATH based dep to go mod

[AyushG3112]

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

$ go version
1.13.3

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="/home/ayush/projects/gojects/bin"
GOCACHE="/home/ayush/.cache/go-build"
GOENV="/home/ayush/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/ayush/projects/gojects"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/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-build570102704=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I had a folder structure as follows:

GOPATH
+---src
     +--- my-api-server
           +--- my-auth-server
                 +--- main.go
           +--- my-utils
                 +--- uuid
                       +--- uuid.go

my-auth-server uses my-api-server/my-utils/uuid as a depenency

Now, when I used the GOPATH based module system, this worked fine. But when using go modules, when I run go run main.go in my-auth-server it returned error:

build command-line-arguments: cannot load my-api-server/my-utils/uuid: malformed module path "my-api-server/my-utils/uuid": missing dot in first path element

What did you expect to see?

My project should have been started successfully

What did you see instead?

build command-line-arguments: cannot load my-api-server/my-utils/uuid: malformed module path "my-api-server/my-utils/uuid": missing dot in first path element

[AyushG3112]

Also, I tried moving my-utils inside my-auth-server, but as a library, my-utils will be used in multiple places.

Now, my-utils also has a go.mod, but that contains a module declaration.
If I place it in my-auth-server, the module path becomes my-api-server/my-auth-server/my-utils

If I have 2 servers,

• my-auth-server
• my-session-server

I cannot place my-utils inside both because there can only be one module declaration per go.mod.

Any help in solving this would also be appreciated.

[dmitshur]

There is currently a requirement that the module path must have a dot in the first path element.

Any help in solving this would also be appreciated.

The only change you have to make is add a dot to the first element of the module path. If you don't intend to make the module available on the internet, you can use a reserved TLD like .localhost that signifies that. For example, the following module path would work:

my-api-server.localhost/my-utils/uuid

You may also find the https://blog.golang.org/migrating-to-go-modules blog post helpful.

/cc @jayconrod @bcmills

[bcmills]

The only change you have to make is add a dot to the first element of the module path.

Or, as an alternative, if both components are in the same version-control repo, you can place a single go.mod file at GOPATH/src/my-api-server/go.mod instead of splitting up the repo into smaller modules.

Or, you can use a replace directive to retarget the locations of the modules and avoid a remote lookup.

Regardless, the best approach is to choose a domain name or prefix under your control for the import path, as @dmitshur suggests. (See #32819.)

[bcmills]

Duplicate of #32819