Add boolean value to type assertion, for skipping assertion panic

[ridewindx]

License: MIT
Signed-off-by: Jason Chang ridewindx@163.com

[Stebalien]

Where did you run across this issue? The parent should always be a pointer to a directory, as far as I know.

[ridewindx]

@Stebalien the parent field of Directory may be the *Root:
rval, err := NewDirectory(parent, node.String(), node, root, ds)
https://github.com/ipfs/go-ipfs/blob/5622cc5bb34ddccd2eee7116700bbaf81940d8d4/mfs/system.go#L93
But *Root is obviously not a *Directory. Uh, is that right?

[Stebalien]

Oh. I see. Um, yeah... that can't currently do anything but panic (it's currently dead code). Thanks for catching this!

I'd rather use a type switch and panic if the parent is neither a directory nor root:

switch parent := cur.parent.(type) {
case *Directory:
	cur = parent
	// ...
case *Root:
	return path
default:
	panic("directory parent neither a directory nor a root")
}

[Kubuxu]

Could you also add simple unit test for it?