cmd/go: accept tags of the form X.Y.Z (without leading 'v') as semantic versions

[tamalsaha]

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

$ go1.13beta1 version
go version go1.13beta1 linux/amd64
$ go version
go version go1.12.6 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
GOARCH="amd64"

GOBIN=""

GOCACHE="/home/tamal/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="/home/tamal/go"

GOPROXY=""

GORACE=""

GOROOT="/usr/local/go"

GOTMPDIR=""

GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD="/dev/null"

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-build803673677=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ go1.13beta1 get github.com/kubedb/apimachinery@v0.11.0
go: finding github.com/kubedb/apimachinery v0.11.0
go: finding github.com/kubedb/apimachinery v0.11.0
go: finding github.com v0.11.0
go: finding github.com/kubedb v0.11.0
go get github.com/kubedb/apimachinery@v0.11.0: unknown revision v0.11.0

$ go1.13beta1 get github.com/kubedb/apimachinery@0.11.0
go: finding github.com/kubedb/apimachinery 0.11.0

We use the git tag also as the Docker image tag. I like kubedb/operator:1.0.0 (without v) over kubedb/operator:v1.0.0. So, we went with that. Tools like glide, dep will automatically handle the presence or absence of v. But go mod does not do that.

What did you expect to see?

My question is can go mod automatically search for both of these prefixes?

What did you see instead?

[bcmills]

See previously #30146.

[bcmills]

CC @jayconrod

To reiterate from that discussion: the difference is mainly aesthetic and relatively arbitrary. Absent a compelling need otherwise, Go usually resolves aesthetic differences by picking one to use consistently (think of gofmt), not supporting both.

[jayconrod]

+1 to not supporting this. It's unfortunate that different systems have different standards for this, but supporting both formats would lead to ambiguity and confusion. For example, both vX.Y.Z and X.Y.Z might exist and point to different commits.

go release should probably warn against creating such a version accidentally in the future.

[bcmills]

supporting both formats would lead to ambiguity and confusion

Note that there is a similar problem for build metadata, which we resolve (in Go 1.13) by resolving to a unique pseudo-version derived from the tagged version.

[mattharr-is]

i think the ironic part about all of this is this bit in the semver.org FAQ
https://semver.org/#is-v123-a-semantic-version
In my case, we have an existing build system that handles repo versioning and tagging for us, using standard semver format (no v prefix). When one reads the go module docs about how it all works with a "standard semver format", and then seeing that it's not actually standard semver format, is a bit confusing. It also requires retooling some build processes that actually do follow the standard.

[alexellis]

+1 cc @LucasRoesler @Waterdrips @stefanprodan - we were all hit by this today, we use semver but "the valid way" i.e. without a "v" prefix and dep was fine for us - we could have a proper tag in our dependency file, but now we're stuck with lots of v0.0.0 SHA in all our code. Was hoping that moving to go modules was going to be a pleasant, pain-free upgrade.

We don't want to change the tags if we can avoid it because semver itself recommends against no prefix and also, we rely on the numbering for Docker images.