util: conditionally build CGO functions

[heyitsanthony]

Some projects don't need the CGO functions in the util package;
split out the functions into a separate file so CGO_ENABLED=0
works if those functions are never used.

Fixes #171

[lucab]

This sounds good to me.

I'm just wondering if the non-cgo stub should still implement the functions that are being moved out here, just by erroring out. This would keep the same public interface, regardless of tags.

I'm not sure what is the idiomatic Go way here, @jonboulle?

[gyuho]

@lucab @jonboulle Can we get this merged?

go-systemd build in etcd is broken.

And I believe this is idiomatic, based on Go source tree.

$ find . -name '*_cgo_*'

./src/crypto/x509/root_cgo_darwin.go
./src/runtime/crash_cgo_test.go

And more

$ find . -name '*cgo_*'

./src/net/cgo_unix_test.go
./src/net/cgo_bsd.go
...

Thanks!

[lucab]

Function declaration and docstring should probably be retained, otherwise this part will be missing from godoc and we would get different public symbols depending on build tags. It can be simply implemented as a stub that always errors out. Same for the other functions moved out from this file.

[lucab]

I had another look at this, and I think that the current approach may be non-optimal: some public symbols will depend on build tags and will disappear from godoc (I guess, not sure).

Thus, I'd suggest a different way this could be done, by splitting current util.go into three parts:

• util.go without any tag, only with the pure-Go functions (GetMachineID and IsRunningSystemd)
• util_cgo.go gated behind a cgo tag, as you already did
• util_stub.go gated behind a not-cgo tag, with stubs and docstring to match the cgo counterparts

[heyitsanthony]

@lucab all fixed up; I made the public functions call into private build-specific functions. It seemed weird to have the full godoc in _stub.go when the code there does nothing. Thanks!

[lucab]

This should be moved to util.go in order to let consumers error-match on this in all situations.

[lucab]

I made the public functions call into private build-specific functions. It seemed weird to have the full godoc in _stub.go when the code there does nothing.

I agree this is even better than my half-botched proposal, thanks! Just one final remark inline, other than that it should be ok.

[heyitsanthony]

@lucab ok, ErrNoCGO exported

[lucab]

Great, LGTM!