Ok, in my case it seems its not coming from go core, but from https://github.com/kardianos/osext/blob/master/osext_procfs.go

Which also had nearly the same code as in go core:

        case "openbsd", "dragonfly":
                return os.Readlink("/proc/curproc/file")

But this was removed with this commit:
kardianos/osext@b4814f4

Maybe go core should do the same thing?

And with this commit it uses the core executable(). So an update of osext won't fix this if you want to use go 1.8:
kardianos/osext@9b883c5

/cc @mdempsky

The report is correct: there's no procfs on OpenBSD. But unfortunately, there's also no alternative kernel API for implementing os.Executable. (This has a long contentious history within OpenBSD.)

So at best we can emit a better "not implemented" error instead of complaining that /proc doesn't exist.

Or we can try looking for os.Argv[0] in $PATH, and hope it's right. (I believe this is what apps ported to OpenBSD usually do when trying to port this functionality.)

CC @minux

As mentioned kardianos/osext has an workaround for this problem from an OpenBSD Developer: kardianos/osext@b4814f4

Maybe this can be included in go core?
(I have no opinion if this is a bad or good workaround)

CC @ajacoutot

What I implemented in osext is not a workaround per se; as mentioned it's a "best effort" :-)
As @mdempsky said, the history behind implementing such a functionality in OpenBSD has been complex... and I wouldn't expect it to change.
The result is that apps implement it themselves which is why I think it'd make sense to have something in go core if possible.

@ianlancetaylor will copy the gccgo AIX code for argv[0] into std for openbsd.

And then enable the test:

func TestExecutable(t *testing.T) {
        testenv.MustHaveExec(t) // will also execlude nacl, which doesn't support Executable anyway                                                        
        ep, err := os.Executable()
        if err != nil {
                switch goos := runtime.GOOS; goos {
                case "openbsd": // procfs is not mounted by default                                                                                        
                        t.Skipf("Executable failed on %s: %v, expected", goos, err)
                }

We have OpenBSD trybots.

CL https://golang.org/cl/46004 mentions this issue.