make: check that Go major/minor version matches required version

[tklauser]

Currently, when building Cilium with a Go major/minor version other than
the one specified in GO_VERSON, the build currently fails with:

package github.com/cilium/cilium/cilium: build constraints exclude all Go files

It's not obvious from the compiler error message that this is due to
mismatching Go version.

This change adds a check-go-version target which checks the Go
compiler version used for building Cilium (as specified in $(GO)
against the version pinned in the GO_VERSION file, i.e. the version
used to build Cilium in CI. This check is required to pass in the
precheck target which should surface mismatching Go versions in a more
user-friendly way.

Example with matching version:

$ go version
go version go1.18.1 linux/amd64
$ make check-go-version

Example with mismatching version:

$ go1.17.9 version
go version go1.17.9 linux/amd64
$ make GO=go1.17.9 check-go-version
Installed Go version 1.17 does not match requested Go version 1.18
make: *** [Makefile:602: check-go-version] Error 1

[aditighag]

Thank you! 🚀

[aditighag]

@tklauser We don't seem to match the minor version though - https://github.com/cilium/cilium/blob/master/daemon/main.go#L6-L6.

[tklauser]

@tklauser We don't seem to match the minor version though - https://github.com/cilium/cilium/blob/master/daemon/main.go#L6-L6.

I assume you meant the patch version? If yes, that's intentional. The //go:build goX.Y tags only use major/minor number and e.g. //go:build go1.18 will evaluate to true for all Go 1.18.z versions and all major/minor versions newer than Go 1.18. The documentation unfortunately isn't very clear on this: https://pkg.go.dev/cmd/go (Ctrl-F for go1.)

[aditighag]

@tklauser Should we backport this to older versions?

[tklauser]

@tklauser Should we backport this to older versions?

Yes, good point. Let's do that.