Ensure nil is accepted as FS config

[evacchi]

In previous releases, passing a nil value as an FS config did not cause any error. However, in 1.0.0-rc.1 this leads
to the creation of an invalid adapter{fs: nil}, which eventually leads to a panic (nil):

(f *FileEntry) Stat(st *platform.Stat_t) =>
    (r *lazyDir) file() =>
        r.fs.OpenFile(".", os.O_RDONLY, 0)

with fs == nil

The backwards-compatible fix is to make Adapt() return UnimplementedFS.

However, we may want to consider returning an error somewhere, because configuring a nil FS may be unintended.

Signed-off-by: Edoardo Vacchi evacchi@users.noreply.github.com

[codefromthecrypt]

I think it is cleaner to handle this at the call site. The adapter is too late imho, and nil there a bug not an intended behavior. otoh keeping the nil guard in adapt is also fine!

so main change I prefer is to check nil inside here:

// WithFS implements ModuleConfig.WithFS
func (c *moduleConfig) WithFS(fs fs.FS) ModuleConfig {
	return c.WithFSConfig(NewFSConfig().WithFSMount(fs, ""))
}

to

// WithFS implements ModuleConfig.WithFS
func (c *moduleConfig) WithFS(fs fs.FS) ModuleConfig {
	var config FSConfig
	if fs != nil {
		config = NewFSConfig().WithFSMount(fs, "")
	}
	return c.WithFSConfig(config)
}

[evacchi]

I originally did it that way and figured that it would do the same if I handled at Adapt; but it's ok to be a tad more defensive. I pushed the following, which avoids invoking withFSConfig() when fs is nil

// WithFS implements ModuleConfig.WithFS
func (c *moduleConfig) WithFS(fs fs.FS) ModuleConfig {
	if fs == nil {
		return c
	}
	return c.WithFSConfig(NewFSConfig().WithFSMount(fs, ""))
}

[codefromthecrypt]

sorry to be a nitpicker, but any WithXXX(nil) should clear previous don't you think?

[evacchi]

oh right I didn't think about that case!

[codefromthecrypt]

Thanks for bearing with me!