Golang crypto/rand internals: How does it generate a secure random?

I started to geek out and I wanted to see how https://golang.org/src/crypto/rand/rand_unix.go works on the inside.

I wanted to see when it generates rands from dev/random (more secure) and when it generates from dev/urandom(less security)

It looks like if rand_batched.go is initialized (this initializes altGetRandom) and GOOS is not plan9 (then r.name = urandomDevice it will return the length of the random array and not the content (which is surprising, why the length?)

see line 57:

if altGetRandom != nil && r.name == urandomDevice && altGetRandom(b) {
        return len(b), nil
    }

else it will simply return the content of the array which will be based on dev/random only if GOOS=plan9.

So why should it ever return len(b)? Also it looks to me that most of the time it will use dev/urandom which is suboptimal... am I wrong (guess so because of docs, but help me understand)?