Give a helpful error if an unsupported Go version is used

[mvdan]

As suggested by @pagran in https://github.com/mvdan/garble/pull/116#issuecomment-687664313. We could call go version and check that it's newer.

Extra care should be taken with master builds, since they won't follow a semver-like version number. For example, my current build from yesterday, which is signifiantly newer than 1.15:

$ go version
go version devel +bf833ead62 Sat Sep 5 14:11:21 2020 +0000 linux/amd64

For these dev builds, it should be enough to check that their timestamp is newer than the time of the Go 1.15 release.

[lu4p]

I don't know if we would overstep some boundaries with this (prompt the user before installing), but we could also just use go to install a currently supported version of go, and then use that.

https://golang.org/doc/manage-install

[mvdan]

I think that's too far. If someone is using garble, they need to know how to install a recent Go version. If they can't manage that, they should probably not be obfuscating their Go builds.

[mvdan]

Also, I think we could print a warning if the version is too new and we aren't testing against it. Because right now, using garble with 1.16 (master) also errors with confusing output, but we could prefix the output with a helpful warning.

[pagran]

I suggest adding 3 messages.

If the output of go version has an unexpected format - "You are using an unsupported version of go (devel) "

If go version is too old - "Your goXXX version is outdated, upgrade to goNNN"

If go version is too new - "Your goXXX version is not supported yet, please update garble"

[mvdan]

I'm surprised that this was approved and closed without any tests. It's easy to place a mock go in $PATH, like a tiny shell script that just echoes a specific version.

I'm reopening this to track adding some tests. Otherwise this feature will likely break the next time we touch the code, which is going to be every six months.

[pagran]

Got it, my mistake. I will add tests soon