Add go mod init to quick-start

[zealws]

Updated documentation to make it clear that a go.mod file is required to run golangci-lint

closes #1833

[boring-cyborg]

Hey, thank you for opening your first Pull Request !

[CLAassistant]

All committers have signed the CLA.

[bombsimon]

This isn't really true, as you noticed in your issue you can keep avoiding go modules if you set GO111MODULE to a proper value (off if not using go modules). See the changelog.

[zealws]

Ok, instead of this, could we add a question to the FAQ with that information?

[zealws]

Alternatively, a new paragraph on the quick-start page detailing how golangci-lint finds golang files might be helpful.

Something along the lines of:

golangci-lint uses go modules to find golang files. If your project is a go module, it'll just work.
But if your project is not a go module, you'll need to set the environment variable GO111MODULE=off when running golangci-lint to disable using go modules.
Make sure you set this in your CI toolchain appropriately.

[bombsimon]

I think this is totally fine and an OK place to put it, was just missing some more background like why this is needed and/or the mention of GO111MODULE=off. I guess the environment variable would be nice to know as well for those who want to keep staying away from modules but still have golangci-lint work.

Let's wait for some other voices from @golangci/team, maybe I'm the only one who thinks this way and others will approve it.

[zealws]

I'll change the PR to do that instead while we wait for more feedback. Thanks @bombsimon !

[ldez]

golangci-lint uses go modules to find golang files.

This is false or at least not precise: the use of go modules by default is related to the behavior of your local go version.

go1.11

$ go version
go version go1.11.13 linux/amd64
$ env | rg GO111MODULE
$ go env | grep GO111MODULE
$ golangci-lint run

go1.12

$ go version
go version go1.12.17 linux/amd64
$ env | rg GO111MODULE
$ go env | grep GO111MODULE
$ golangci-lint run

go1.13

$ go version
go version go1.13.15 linux/amd64
$ env | rg GO111MODULE
$ go env | grep GO111MODULE
GO111MODULE=""
$ golangci-lint run

go1.14

$ go version
go version go1.14.15 linux/amd64
$ env | rg GO111MODULE
$ go env | grep GO111MODULE
GO111MODULE=""
$ golangci-lint run

go1.15

$ go version
go version go1.15.9 linux/amd64
$ env | rg GO111MODULE
$ go env | grep GO111MODULE
GO111MODULE=""
$ golangci-lint run

go1.16

$ go version
go version go1.16.1 linux/amd64
$ go env | grep GO111MODULE
GO111MODULE=""
$ golangci-lint run

ERRO Running error: context loading failed: no go files to analyze 

In go1.16 the go modules are on by default instead of auto or off

Also note:

We plan to drop support for GOPATH mode in Go 1.17. In other words, Go 1.17 will ignore GO111MODULE.

https://blog.golang.org/go116-module-changes

[zealws]

Should this specifically state this applies to go 1.16 and up?

[ldez]

The relation between the local go version and GO111MODULE exists since go1.11.

If GO111MODULE is on, even with go<1.16, golangci-lint will use Go modules.

go1.11

$ go version
go version go1.11.13 linux/amd64
$ GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: failed to load packages: failed to load with go/packages: err: exit status 1: stderr: go: cannot determine module path for source directory /home/ldez/sources/go/src/github.com/golangci/sandbox (outside GOPATH, no import comments) 

go1.12

$ go version
go version go1.12.17 linux/amd64
$GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: failed to load packages: failed to load with go/packages: err: exit status 1: stderr: go: cannot determine module path for source directory /home/ldez/sources/go/src/github.com/golangci/sandbox (outside GOPATH, no import comments) 

go1.13

$ go version
go version go1.13.15 linux/amd64
$ GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: no go files to analyze 

go1.14

$ go version
go version go1.14.15 linux/amd64
$ GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: no go files to analyze

go1.15

$ go version
go version go1.15.9 linux/amd64
$ GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: no go files to analyze

go1.16

$ go version
go version go1.16.1 linux/amd64
$ GO111MODULE=on golangci-lint run
ERRO Running error: context loading failed: no go files to analyze

[ldez]

To be precise, golangci-lint relies on Go internal behavior, there is no GO111MODULE related code inside golangci-lint code base.

[ldez]

If your project is not a go module, you can still use golangci-lint for your project.

Same thing here.