Why is GO111MODULE everywhere, and everything about Go Modules

You might have noticed that GO111MODULE=on is flourishing everywhere. Many readmes have that:

GO111MODULE=on go get golang.org/x/tools/gopls@latest

In this short post, I will explain why GO111MODULE exists, its caveats and interesting bits that you need to know when dealing with Go Modules.

Table of content:

• From GOPATH to GO111MODULE
• The GO111MODULE environment variable
  • GO111MODULE with Go 1.11 and 1.12
  • GO111MODULE with Go 1.13
  • GO111MODULE with Go 1.14
  • So, why is GO111MODULE everywhere?!
  • The pitfall of go.mod being silently updated
  • The -u and @version pitfall
• Caveats when using Go Modules
  • Remember that go get also updates your go.mod
  • Where are the sources of the dependencies with Go Modules
  • Set GO111MODULE on a per-folder basis with direnv
  • Private Go Modules and Dockerfile
  • Solution 1: vendoring
  • Solution 2: no vendoring

From GOPATH to GO111MODULE

First off, let's talk about GOPATH. When Go was first introduced in 2009, it was not shipped with a package manager. Instead, go get would fetch all the sources by using their import paths and store them in $GOPATH/src. There was no versioning and the 'master' branch would represent a stable version of the package.

Go Modules (previously called vgo -- versioned Go) were introduced with Go 1.11. Instead of using the GOPATH for storing a single git checkout of every package, Go Modules stores tagged versions with go.mod keeping track of each package's version.

Since then, the interaction between the 'GOPATH behavior' and the 'Go Modules behavior' has become one of the biggest gotchas of Go. One environment variable is responsible for 95% of this pain: GO111MODULE.

The GO111MODULE environment variable

GO111MODULE is an environment variable that can be set when using go for changing how Go imports packages. One of the first pain-points is that depending on the Go version, its semantics change.

GO111MODULE with Go 1.11 and 1.12

• GO111MODULE=on will force using Go modules even if the project is in your GOPATH. Requires go.mod to work.
• GO111MODULE=off forces Go to behave the GOPATH way, even outside of GOPATH.

• GO111MODULE=auto is the default mode. In this mode, Go will behave

  • similarly to GO111MODULE=on when you are outside of GOPATH,
  • similarly to GO111MODULE=off when you are inside the GOPATH even if a go.mod is present.

Whenever you are in your GOPATH and you want to do an operation that requires Go modules (e.g., go get a specific version of a binary), you need to do:

GO111MODULE=on go get github.com/golang/mock/tree/master/mockgen@v1.3.1

GO111MODULE with Go 1.13

Using Go 1.13, GO111MODULE's default (auto) changes:

• behaves like GO111MODULE=on anywhere there is a go.mod OR anywhere outside the GOPATH even if there is no go.mod. So you can keep all your repositories in your GOPATH with Go 1.13.
• behaves like GO111MODULE=off in the GOPATH with no go.mod.

GO111MODULE with Go 1.14

The GO111MODULE variable has the same behavior as with Go 1.13.

Note that some slight changes in behaviors unrelated to GO111MODULE happened:

• The vendor/ is picked up automatically. That has the tendency of breaking Gomock (issue) which were unknowingly not using vendor/ before 1.14.
• You still need to use cd && GO111MODULE=on go get when you don't want to mess up your current project’s go.mod (that's so annoying).

So, why is GO111MODULE everywhere?!

Now that we know that GO111MODULE can be very useful for enabling the Go Modules behavior, here is the answer: that's because GO111MODULE=on allows you to select a version. Without Go Modules, go get fetches the latest commit from master. With Go Modules, you can select a specific version based on git tags.

I use GO111MODULE=on very often when I want to switch between the latest version and the HEAD version of gopls (the Go Language Server):

GO111MODULE=on go get golang.org/x/tools/gopls@latest
GO111MODULE=on go get golang.org/x/tools/gopls@master
GO111MODULE=on go get golang.org/x/tools/gopls@v0.1
GO111MODULE=on go get golang.org/x/tools/gopls@v0.1.8
GO111MODULE="on" go get sigs.k8s.io/kind@v0.7.0

The pitfall of go.mod being silently updated

And to make things even worse, some projects have an even more complicated one-liners:

(cd && GO111MODULE=on go get golang.org/x/tools/gopls@latest)

Note: the @latest suffix will use the latest git tag of gopls. Note that -u (which means 'update') is not needed for @v0.1.8 since this is a 'fixed' version, and updating a fixed version does not really make sense. It is also interesting to note that with @v0.1, go get will fetch the latest patch version for that tag.

That's yet another Go ideocracy: by default (and you can't turn that off), if you are in a folder that has a go.mod, go get will update that go.mod with what you just installed. And in the case of development binaries like gopls or kind, you definitely don't want to have these appearing in the go.mod file!

So the workaround is to give a one-liner that makes sure that you won't be in a go.mod-enabled folder: (cd && go get) does exactly that.

I hope that (sooner or later) we will have a clear separation of concerns between go get that is adding a dependency to your go.mod (like npm install) and go install that is meant to install a binary without messing up your go.mod.

• First caveat: we all use go get to install dev dependencies, so moving to go install would kind of not work (habits...)
• Second caveat: go install doesn’t allow you to give a version (e.g., @latest or @v1.4.5), and go run either by the way. So go install isn't that useful after all... 😞

$ export GO111MODULE=on

$ go get golang.org/x/tools/gopls@v0.1.8              # ✅

$ go install golang.org/x/tools/gopls@v0.1.8          # ❌
can t load package: package golang.org/x/tools/gopls@v0.1.8: cannot use path@version syntax in GOPATH mode

$ go run golang.org/x/tools/gopls@v0.1.8              # ❌
package golang.org/x/tools/gopls@v0.1.8: can only use path@version syntax with 'go get'

The -u and @version pitfall

I have been bitten multiple times by this: when using go get @latest (for a binary, at least), you should avoid using -u so that it uses the dependencies as defined in go.sum. Otherwise, it will update all the dependencies to their latest minor revision... And since a ton of projects choose to have breaking changes between minor versions (e.g. v0.2.0 to v0.3.0), using -u has a large chance of breaking things.

So if you see this:

# Both -u and @latest!
GO111MODULE=on go get -u golang.org/x/tools/gopls@latest

then you will immediately realize that it is wrong: you want to be using the recorded versions given in go.sum when go-getting a binary!

Rebecca Stambler reminds us that we should not use -u in conjunction with a version:

-u should not be used in conjunction with the @latest tag, as it will give you incorrect versions of the dependencies.

But it's kind of hidden in this issue... I guess it is written somewhere in the Go help (btw, what a hideous help compared to git help) but that kind of caveat should be more visible: maybe print a warning when installing a binary with both @version and -u?

Caveats when using Go Modules

Now, let's go through some caveats I encountered when working with Go Modules.

Remember that go get also updates your go.mod

That’s one of the weird things with go get: sometimes, it serves the purpose of installing binaries or downloading packages. But with Go modules, if you are in a repo with a go.mod, it will silently add the package you go get to your go.mod.

That’s one of the catches of Go modules! 😁

Where are the sources of the dependencies with Go Modules

When using Go Modules, the packages that are used during go build are stored in $GOPATH/pkg/mod. When trying to inspect an 'import' in vim or VSCode, you might end up in the GOPATH version of the package instead of the pkg/mod one used during compilation.

A second issue that arises is when you want to hack one of your dependencies, for example for testing purposes.

Solution 1: use go mod vendor + go build -mod=vendor. That will force go to use the vendor/ files instead of using the $GOPATH/pkg/mod ones. This option also solves the problem of vim and VSCode not opening the right version of a package’s file.

Solution 2: add a 'replace' line at the end of your go.mod:

replace github.com/maelvls/beers => ../beers

where ../beers is a local copy I made of the dependency I want to inspect and hack.

Set GO111MODULE on a per-folder basis with direnv

During the migration from GOPATH-based projects (mainly using Dep) to Go Modules, I found myself struggling with two different places: inside and outside GOPATH. All Go Modules had to be kept outside of GOPATH, which meant my projects were in different folders.

To remediate that, I used GO111MODULE extensively. I would keep all my projects in the GOPATH, and for the Go Modules-enabled projects, I would set export GO111MODULE=on.

This is where direnv comes in handy. Direnv is a lightweight command written in Go that will load a file, .envrc, whenever you enter a directory and .envrc is present. For every Go Module-enabled project, I would have this .envrc:

# .envrc
export GO111MODULE=on
export GOPRIVATE=github.com/mycompany/\*
export GOFLAGS=-mod=vendor

The GOPRIVATE disables the Go Proxy (Go 1.13) for certain import paths. I also found useful to set -mod=vendor so that every command uses the vendor folder (go mod vendor).

Private Go Modules and Dockerfile

At my company, we use a lot of private repositories. As explained above, we can use GOPRIVATE in order to tell Go 1.13 to skip the package proxy and fetch our private packages directly from Github.

But what about building Docker images? How can go get fetch our private repositories from a docker build?

Solution 1: vendoring

With go mod vendor, no need to pass Github credentials to the docker build context. We can just put everything in vendor/ and the problem is solved. In the Dockerfile, -mod=vendor will be required, but developers don't even have to bother with -mod=vendor since they have access to the private Github repositories anyway using their local Git config

• Pros: faster build on CI (~10 to 30 seconds less)
• Cons: PRs are bloated with vendor/ changes and the repo's size might be big

Solution 2: no vendoring

If vendor/ is just too big (e.g., for Kubernetes controllers, vendor/ is about 30MB), we can very well do it without vendoring. That would require to pass some form of GITHUB_TOKEN as argument of docker build, and in the Dockerfile, set something like:

git config --global url."https://foo:${GITHUB_TOKEN}@github.com/company".insteadOf "https://github.com/company"
export GOPRIVATE=github.com/company/\*

Illustration by Bailey Beougher, from The Illustrated Children's Guide to Kubernetes.

Update 22 June 2020: it said use replace instead of just replace