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A Tour of Versioned Go (vgo)
(Go & Versioning, Part 2)
Posted on Tuesday, February 20, 2018. PDF

For me, design means building, tearing down, and building again, over and over. To write the new versioning proposal, I built an prototype, vgo, to work through many subtle details. This post shows what it's like to use vgo.

You can download and try vgo today by running go get golang.org/x/vgo. Vgo is a drop-in replacement for (and a forked copy of) the go command. You run vgo instead of go, and then it uses the standard compiler and libraries you already have installed in $GOROOT (Go 1.10beta1 or later).

The details of vgo's semantics and command lines are likely to change as we learn more about what works and what does not. However, we intend to avoid backwards-incompatible changes to the go.mod file format, so that a go.mod added to a project today will keep working in the future. As we refine the proposal, we'll update vgo accordingly.

Examples

This section demonstrates what it's like to use vgo. Please follow along and experiment with variations as you do.

Start by installing vgo:

$ go get -u golang.org/x/vgo

You are certain to run into interesting bugs, since vgo is at best only lightly tested right now. To file issues, please use the main Go issue tracker and add the prefix “x/vgo:” to the title. Thanks.

Hello, world

Let's write an interesting “hello, world” program. Create a directory outside your GOPATH/src tree and change into it:

$ cd $HOME
$ mkdir hello
$ cd hello

Then create a file hello.go:

package main // import "github.com/you/hello"

import (
	"fmt"
	"rsc.io/quote"
)

func main() {
	fmt.Println(quote.Hello())
}

Or download it:

$ curl -sS https://swtch.com/hello.go >hello.go

Create an empty go.mod file to mark the root of this module, and then build and run your new program:

$ echo >go.mod
$ vgo build
vgo: resolving import "rsc.io/quote"
vgo: finding rsc.io/quote (latest)
vgo: adding rsc.io/quote v1.5.2
vgo: finding rsc.io/quote v1.5.2
vgo: finding rsc.io/sampler v1.3.0
vgo: finding golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
vgo: downloading rsc.io/quote v1.5.2
vgo: downloading rsc.io/sampler v1.3.0
vgo: downloading golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c
$ ./hello
Hello, world.
$ 

Notice that there is no explicit vgo get required. Plain vgo build will, upon encountering an unknown import, look up the module that contains it and add the latest version of that module as a requirement to the current module.

A side effect of running any vgo command is to update go.mod if necessary. In this case, the vgo build wrote a new go.mod:

$ cat go.mod
module github.com/you/hello

require rsc.io/quote v1.5.2
$ 

Because the go.mod was written, the next vgo build will not resolve the import again or print nearly as much:

$ vgo build
$ ./hello
Hello, world.
$ 

Even if rsc.io/quote v1.5.3 or v1.6.0 is released tomorrow, builds in this directory will keep using v1.5.2 until an explicit upgrade (see below).

The go.mod file lists a minimal set of requirements, omitting those implied by the ones already listed. In this case, rsc.io/quote v1.5.2 requires the specific versions of rsc.io/sampler and golang.org/x/text that were reported, so it would be redundant to repeat those in the go.mod file.

It is still possible to find out the full set of modules required by a build, using vgo list -m:

$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.0.0-20170915032832-14c0d48ead0c
rsc.io/quote          v1.5.2
rsc.io/sampler        v1.3.0
$ 

At this point you might wonder why our simple “hello world” program uses golang.org/x/text. It turns out that rsc.io/quote depends on rsc.io/sampler, which in turn uses golang.org/x/text for language matching.

$ LANG=fr ./hello
Bonjour le monde.
$ 

Upgrading

We've seen that when a new module must be added to a build to resolve a new import, vgo takes the latest one. Earlier, it needed rsc.io/quote and found that v1.5.2 was the latest. But except when resolving new imports, vgo uses only versions listed in go.mod files. In our example, rsc.io/quote depended indirectly on specific versions of golang.org/x/text and rsc.io/sampler. It turns out that both of those packages have newer releases, as we can see by adding -u (check for updated packages) to the vgo list command:

$ vgo list -m -u
MODULE                VERSION                             LATEST
github.com/you/hello  -                                   -
golang.org/x/text     v0.0.0-20170915032832-14c0d48ead0c  v0.3.0 (2017-12-14 08:08)
rsc.io/quote          v1.5.2 (2018-02-14 10:44)           -
rsc.io/sampler        v1.3.0 (2018-02-13 14:05)           v1.99.99 (2018-02-13 17:20)
$

Both of those packages have newer releases, so we might want to upgrade them in our hello program.

Let's upgrade golang.org/x/text first:

$ vgo get golang.org/x/text
vgo: finding golang.org/x/text v0.3.0
vgo: downloading golang.org/x/text v0.3.0
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.5.2
)
$ 

The vgo get command looks up the latest version of the given modules and adds that version as a requirement for the current module, by updating go.mod. Now future builds will use the newer text module:

$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.5.2
rsc.io/sampler        v1.3.0
$

Of course, after an upgrade, it's a good idea to test that everything still works. Our dependencies rsc.io/quote and rsc.io/sampler have not been tested with the newer text module. We can run their tests in the configuration we've created:

$ vgo test all
?   	github.com/you/hello	[no test files]
?   	golang.org/x/text/internal/gen	[no test files]
ok  	golang.org/x/text/internal/tag	0.020s
?   	golang.org/x/text/internal/testtext	[no test files]
ok  	golang.org/x/text/internal/ucd	0.020s
ok  	golang.org/x/text/language	0.068s
ok  	golang.org/x/text/unicode/cldr	0.063s
ok  	rsc.io/quote	0.015s
ok  	rsc.io/sampler	0.016s
$ 

In the original go command, the package pattern all meant all packages found in GOPATH. That's almost always too many to be useful. In vgo, we've narrowed the meaning of all to be “all packages in the current module, and the packages they import, recursively.” Version 1.5.2 of the rsc.io/quote module contains a buggy package:

$ vgo test rsc.io/quote/...
ok  	rsc.io/quote	(cached)
--- FAIL: Test (0.00s)
	buggy_test.go:10: buggy!
FAIL
FAIL	rsc.io/quote/buggy	0.014s
(exit status 1)
$

Until something in our module imports buggy, however, it's irrelevant to us, so it's not included in all. In any event, the upgraded x/text seems to work. At this point we'd probably commit go.mod.

Another option is to upgrade all modules needed by the build, using vgo get -u:

$ vgo get -u
vgo: finding golang.org/x/text latest
vgo: finding rsc.io/quote latest
vgo: finding rsc.io/sampler latest
vgo: finding rsc.io/sampler v1.99.99
vgo: finding golang.org/x/text latest
vgo: downloading rsc.io/sampler v1.99.99
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.5.2
	rsc.io/sampler v1.99.99
)
$ 

Here, vgo get -u has kept the upgraded text module and also upgraded rsc.io/sampler to its latest version, v1.99.99.

Let's run our tests:

$ vgo test all
?   	github.com/you/hello	[no test files]
?   	golang.org/x/text/internal/gen	[no test files]
ok  	golang.org/x/text/internal/tag	(cached)
?   	golang.org/x/text/internal/testtext	[no test files]
ok  	golang.org/x/text/internal/ucd	(cached)
ok  	golang.org/x/text/language	0.070s
ok  	golang.org/x/text/unicode/cldr	(cached)
--- FAIL: TestHello (0.00s)
	quote_test.go:19: Hello() = "99 bottles of beer on the wall, 99 bottles of beer, ...", want "Hello, world."
FAIL
FAIL	rsc.io/quote	0.014s
--- FAIL: TestHello (0.00s)
	hello_test.go:31: Hello([en-US fr]) = "99 bottles of beer on the wall, 99 bottles of beer, ...", want "Hello, world."
	hello_test.go:31: Hello([fr en-US]) = "99 bottles of beer on the wall, 99 bottles of beer, ...", want "Bonjour le monde."
FAIL
FAIL	rsc.io/sampler	0.014s
(exit status 1)
$ 

It appears that something is wrong with rsc.io/sampler v1.99.99. Sure enough:

$ vgo build
$ ./hello
99 bottles of beer on the wall, 99 bottles of beer, ...
$ 

The vgo get -u behavior of taking the latest of every dependency is exactly what go get does when packages being downloaded aren't in GOPATH. On a system with nothing in GOPATH:

$ go get -d rsc.io/hello
$ go build -o badhello rsc.io/hello
$ ./badhello
99 bottles of beer on the wall, 99 bottles of beer, ...
$ 

The important difference is that vgo does not behave this way by default. Also you can undo it by downgrading.

Downgrading

To downgrade a package, use vgo list -t to show the available tagged versions:

$ vgo list -t rsc.io/sampler
rsc.io/sampler
	v1.0.0
	v1.2.0
	v1.2.1
	v1.3.0
	v1.3.1
	v1.99.99
$ 

Then use vgo get to ask for a specific version, like maybe v1.3.1:

$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.5.2
	rsc.io/sampler v1.99.99
)
$ vgo get rsc.io/sampler@v1.3.1
vgo: finding rsc.io/sampler v1.3.1
vgo: downloading rsc.io/sampler v1.3.1
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.5.2
rsc.io/sampler        v1.3.1
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.5.2
	rsc.io/sampler v1.3.1
)
$ vgo test all
?   	github.com/you/hello	[no test files]
?   	golang.org/x/text/internal/gen	[no test files]
ok  	golang.org/x/text/internal/tag	(cached)
?   	golang.org/x/text/internal/testtext	[no test files]
ok  	golang.org/x/text/internal/ucd	(cached)
ok  	golang.org/x/text/language	(cached)
ok  	golang.org/x/text/unicode/cldr	(cached)
ok  	rsc.io/quote	0.016s
ok  	rsc.io/sampler	0.015s
$ 

Downgrading one package may require downgrading others. For example:

$ vgo get rsc.io/sampler@v1.2.0
vgo: finding rsc.io/sampler v1.2.0
vgo: finding rsc.io/quote v1.5.1
vgo: finding rsc.io/quote v1.5.0
vgo: finding rsc.io/quote v1.4.0
vgo: finding rsc.io/sampler v1.0.0
vgo: downloading rsc.io/sampler v1.2.0
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.4.0
rsc.io/sampler        v1.2.0
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.4.0
	rsc.io/sampler v1.2.0
)
$

In this case, rsc.io/quote v1.5.0 was the first to require rsc.io/sampler v1.3.0; earlier versions only needed v1.0.0 (or later). The downgrade selected rsc.io/quote v1.4.0, the last version compatible with v1.2.0.

It is also possible to remove a dependency entirely, an extreme form of downgrade, by specifying none as the version.

$ vgo get rsc.io/sampler@none
vgo: downloading rsc.io/quote v1.4.0
vgo: finding rsc.io/quote v1.3.0
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.3.0
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.3.0
)
$ vgo test all
vgo: downloading rsc.io/quote v1.3.0
?   	github.com/you/hello	[no test files]
ok  	rsc.io/quote	0.014s
$ 

Let's go back to the state where everything is the latest version, including rsc.io/sampler v1.99.99:

$ vgo get -u
vgo: finding golang.org/x/text latest
vgo: finding rsc.io/quote latest
vgo: finding rsc.io/sampler latest
vgo: finding golang.org/x/text latest
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.5.2
rsc.io/sampler        v1.99.99
$

Excluding

Having identified that v1.99.99 isn't okay to use in our hello world program, we may want to record that fact, to avoid future problems. We can do that by adding an exclude directive to go.mod:

exclude rsc.io/sampler v1.99.99

Future operations behave as if that module does not exist:

$ echo 'exclude rsc.io/sampler v1.99.99' >>go.mod
$ vgo list -t rsc.io/sampler
rsc.io/sampler
	v1.0.0
	v1.2.0
	v1.2.1
	v1.3.0
	v1.3.1
	v1.99.99 # excluded
$ vgo get -u
vgo: finding golang.org/x/text latest
vgo: finding rsc.io/quote latest
vgo: finding rsc.io/sampler latest
vgo: finding rsc.io/sampler latest
vgo: finding golang.org/x/text latest
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.5.2
rsc.io/sampler        v1.3.1
$ cat go.mod
module github.com/you/hello

require (
	golang.org/x/text v0.3.0
	rsc.io/quote v1.5.2
	rsc.io/sampler v1.3.1
)

exclude "rsc.io/sampler" v1.99.99
$ vgo test all
?   	github.com/you/hello	[no test files]
?   	golang.org/x/text/internal/gen	[no test files]
ok  	golang.org/x/text/internal/tag	(cached)
?   	golang.org/x/text/internal/testtext	[no test files]
ok  	golang.org/x/text/internal/ucd	(cached)
ok  	golang.org/x/text/language	(cached)
ok  	golang.org/x/text/unicode/cldr	(cached)
ok  	rsc.io/quote	(cached)
ok  	rsc.io/sampler	(cached)
$

Exclusions only apply to builds of the current module. If the current module were required by a larger build, the exclusions would not apply.= For example, an exclusion in rsc.io/quote's go.mod will not apply to our “hello, world” build. This policy balances giving the authors of the current module almost arbitrary control over their own build, without also subjecting them to almost arbitrary control exerted by the modules they depend on.

At this point, the right next step is to contact the author of rsc.io/sampler and report the problem in v1.99.99, so it can be fixed in v1.99.100. Unfortunately, the author has a blog post that depends on not fixing the bug.

Replacing

If you do identify a problem in a dependency, you need a way to replace it with a fixed copy temporarily. Suppose we want to change something about the behavior of rsc.io/quote. Perhaps we want to work around the problem in rsc.io/sampler, or perhaps we want to do something else. The first step is to check out the quote module, using an ordinary git command:

$ git clone https://github.com/rsc/quote ../quote
Cloning into '../quote'...

Then edit ../quote/quote.go to change something about func Hello. For example, I'm going to change its return value from sampler.Hello() to sampler.Glass(), a more interesting greeting.

$ cd ../quote
$ <edit quote.go>
$ 

Having changed the fork, we can make our build use it in place of the real one by adding a replacement directive to go.mod:

replace rsc.io/quote v1.5.2 => ../quote

Then we can build our program using it:

$ cd ../hello
$ echo 'replace rsc.io/quote v1.5.2 => ../quote' >>go.mod
$ vgo list -m
MODULE                VERSION
github.com/you/hello  -
golang.org/x/text     v0.3.0
rsc.io/quote          v1.5.2
 => ../quote
rsc.io/sampler        v1.3.1
$ vgo build
$ ./hello
I can eat glass and it doesn't hurt me.
$

You can also name a different module as a replacement. For example, you can fork github.com/rsc/quote and then push your change to your fork.

$ cd ../quote
$ git commit -a -m 'my fork'
[master 6151719] my fork
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git tag v0.0.0-myfork
$ git push https://github.com/you/quote v0.0.0-myfork
To https://github.com/you/quote
 * [new tag]         v0.0.0-myfork -> v0.0.0-myfork
$ 

Then you can use that as the replacement:

$ cd ../hello
$ echo 'replace rsc.io/quote v1.5.2 => github.com/you/quote v0.0.0-myfork' >>go.mod
$ vgo list -m
vgo: finding github.com/you/quote v0.0.0-myfork
MODULE                    VERSION
github.com/you/hello      -
golang.org/x/text         v0.3.0
rsc.io/quote              v1.5.2
 => github.com/you/quote  v0.0.0-myfork
rsc.io/sampler            v1.3.1
$ vgo build
vgo: downloading github.com/you/quote v0.0.0-myfork
$ LANG=fr ./hello
Je peux manger du verre, ça ne me fait pas mal.
$ 

Backwards Compatibility

Even if you want to use vgo for your project, you probably don't want to require all your users to have vgo. Instead, you can create a vendor directory that allows go command users to produce nearly the same builds (building inside GOPATH, of course):

$ vgo vendor
$ mkdir -p $GOPATH/src/github.com/you
$ cp -a . $GOPATH/src/github.com/you/hello
$ go build -o vhello github.com/you/hello
$ LANG=es ./vhello
Puedo comer vidrio, no me hace daño.
$ 

I said the builds are “nearly the same,” because the import paths seen by the toolchain and recorded in the final binary are different. The vendored builds see vendor directories:

$ go tool nm hello | grep sampler.hello
 1170908 B rsc.io/sampler.hello
$ go tool nm vhello | grep sampler.hello
 11718e8 B github.com/you/hello/vendor/rsc.io/sampler.hello
$ 

Except for this difference, the builds should produce the same binaries. In order to provide for a graceful transition, vgo-based builds ignore vendor directories entirely, as will module-aware go command builds.

What's Next?

Please try vgo. Start tagging versions in your repositories. Create and check in go.mod files. File issues at golang.org/issue, and please include “x/vgo:” at the start of the title. More posts tomorrow. Thanks, and have fun!