A Tour of Versioned Go (vgo)
Go & Versioning, Part 2
Russ Cox
February 20, 2018
research.swtch.com/vgo-tour
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 throughmany 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. Yourun 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 welearn 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.

ExamplesThis 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 theprefix “x/vgo:” to the title. Thanks.
Hello, world
Let’s write an interesting “hello, world” program. Create a directory outside yourGOPATH/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A T  V G ()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.
$Notice that there is no explicit vgo get required. Plain vgo build will, upon encountering an unknown import, look up the module that contains it and add thelatest version of that module as a requirement to the current module.
A side effect of running any vgo command is to update go.mod if necessary.
In this case, the vgo build wrote a new go.mod:$ cat go.mod
module github.com/you/hellorequire rsc.io/quote v1.5.2$Because the go.mod was written, the next vgo build will not resolve the importagain or print nearly as much:$ 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 specificversions 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
MODULEgithub.com/you/hellogolang.org/x/text
rsc.io/quote
rsc.io/sampler$VERSION
v0.0.0-20170915032832-14c0d48ead0c
v1.5.2v1.3.0A T  V G ()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.
$Upgrading
We’ve seen that when a new module must be added to a build to resolve a newimport, vgo takes the latest one. Earlier, it needed rsc.io/quote and found thatv1.5.2 was the latest. But except when resolving new imports, vgo uses only versions listed in go.mod files. In our example, rsc.io/quote depended indirectly
on specific versions of golang.org/x/text and rsc.io/sampler. It turns outthat both of those packages have newer releases, as we can see by adding -u(check for updated packages) to the vgo list command:$ vgo list -m -u
MODULEgithub.com/you/hellogolang.org/x/text
rsc.io/quote
rsc.io/sampler$VERSION
v0.0.0-20170915032832-14c0d48ead0c
v1.5.2 (2018-02-14 10:44)
v1.3.0 (2018-02-13 14:05)

LATEST
v0.3.0 (2017-12-14 08:08)
v1.99.99 (2018-02-13 17:20)

Both of those packages have newer releases, so we might want to upgrade themin 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/hellorequire (golang.org/x/text v0.3.0
rsc.io/quote v1.5.2)$The vgo get command looks up the latest version of the given modules andadds that version as a requirement for the current module, by updating go.mod.
Now future builds will use the newer text module:$ vgo list -m
MODULEgithub.com/you/hellogolang.org/x/text
rsc.io/quote
rsc.io/sampler$VERSION
v0.3.0v1.5.2v1.3.0A T  V G ()Of course, after an upgrade, it’s a good idea to test that everything still works.
Our dependencies rsc.io/quote and rsc.io/sampler have not been tested
with the newer text module. We can run their tests in the configuration we’vecreated:$ vgo test all?
github.com/you/hello[no test files]?
golang.org/x/text/internal/gen [no test files]okgolang.org/x/text/internal/tag 0.020s?
golang.org/x/text/internal/testtext [no test files]okgolang.org/x/text/internal/ucd 0.020s
okgolang.org/x/text/language 0.068sokgolang.org/x/text/unicode/cldr 0.063s
ok
rsc.io/quote
0.015sok
rsc.io/sampler 0.016s$
In the original go command, the package pattern all meant all packages foundin 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!
FAILFAIL
rsc.io/quote/buggy 0.014s(exit status 1)$Until something in our module imports buggy, however, it’s irrelevant to us, so
it’s not included in all. In any event, the upgraded x/text seems to work. At thispoint we’d probably commit go.mod.
Another option is to upgrade all modules needed by the build, using vgo get-u:$ vgo get -uvgo: 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/hellorequire (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 upgradedrsc.io/sampler to its latest version, v1.99.99.

A T  V G ()Let’s run our tests:$ vgo test all?
github.com/you/hello[no test files]?
golang.org/x/text/internal/gen [no test files]okgolang.org/x/text/internal/tag (cached)?
golang.org/x/text/internal/testtext [no test files]okgolang.org/x/text/internal/ucd (cached)okgolang.org/x/text/language 0.070s
okgolang.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 "...FAILFAIL
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, .....
hello_teFAIL
rsc.io/sampler 0.014s(exit status 1)$It appears that something is wrong with rsc.io/sampler v1.99.99. Sureenough:$ vgo build$ ./hello
99 bottles of beer on the wall, 99 bottles of beer, ...
$The vgo get -u behavior of taking the latest of every dependency is exactly
what go get does when packages being downloaded aren’t in GOPATH. On asystem with nothing in GOPATH:$ 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
v1.0.0v1.2.0v1.2.1v1.3.0v1.3.1v1.99.99$Then use vgo get to ask for a specific version, like maybe v1.3.1:$ cat go.mod
module github.com/you/hellorequire (golang.org/x/text v0.3.0
A T  V G ()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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quotev1.5.2
rsc.io/sampler
v1.3.1$ cat go.mod
module github.com/you/hellorequire (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]okgolang.org/x/text/internal/tag (cached)?
golang.org/x/text/internal/testtext [no test files]okgolang.org/x/text/internal/ucd (cached)okgolang.org/x/text/language (cached)okgolang.org/x/text/unicode/cldr (cached)ok
rsc.io/quote0.016s
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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quotev1.4.0
rsc.io/samplerv1.2.0$ cat go.mod
module github.com/you/hellorequire (golang.org/x/text v0.3.0
rsc.io/quote v1.4.0
rsc.io/sampler v1.2.0)$A T  V G ()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 selectedrsc.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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quote
v1.3.0$ cat go.mod
module github.com/you/hellorequire (golang.org/x/text v0.3.0
rsc.io/quote v1.3.0)$ vgo test allvgo: 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 -uvgo: 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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quotev1.5.2
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 byadding an exclude directive to go.mod:
exclude rsc.io/sampler v1.99.99

A T  V G ()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
v1.0.0v1.2.0v1.2.1v1.3.0v1.3.1v1.99.99 # excluded$ vgo get -uvgo: 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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quotev1.5.2
rsc.io/sampler
v1.3.1$ cat go.mod
module github.com/you/hellorequire (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]okgolang.org/x/text/internal/tag (cached)?
golang.org/x/text/internal/testtext [no test files]okgolang.org/x/text/internal/ucd (cached)okgolang.org/x/text/language (cached)okgolang.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.

A T  V G ()ReplacingIf you do identify a problem in a dependency, you need a way to replace itwith a fixed copy temporarily. Suppose we want to change something about thebehavior 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 tocheck 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
MODULEVERSIONgithub.com/you/hello golang.org/x/text
v0.3.0rsc.io/quotev1.5.2=> ../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 canfork 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$A T  V G ()Then you can use that as the replacement:$ 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
MODULEVERSIONgithub.com/you/hellogolang.org/x/text
v0.3.0rsc.io/quotev1.5.2=> 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.
$Backwards Compatibility
Even if you want to use vgo for your project, you probably don’t want to require all your users to have vgo. Instead, you can create a vendor directory thatallows go command users to produce nearly the same builds (building inside
GOPATH, of course):$ 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.
$I said the builds are “nearly the same,” because the import paths seen by thetoolchain and recorded in the final binary are different. The vendored builds see
vendor directories:$ 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 orderto 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 ingo.mod files. File issues at golang.org/issue, and please include “x/vgo:” at thestart of the title. More posts tomorrow. Thanks, and have fun!

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