No error when stopping watching a non watched file

[MidasLamb]

What does this pull request do?

Currently, when a non-watched item is requested to be removed from a
watch, an error is returned.
However, the post-condition of removing it is: "This file/dir should not
be watched anymore", which is the case also when the item wasn't being
watched in the first place.
Rather than returning an error. Return nil and skip a part of the work.

Where should the reviewer start?

inotify.go, the Remove function definition.

How should this be manually tested?

We've noticed this because this lib is being used in promtail and it was logging errors because of this.

[mattn]

If this Remove function returns a wrapped error for os.ErrNotExist, will your problem be resolved with using erros.Is?

[MidasLamb]

If this Remove function returns a wrapped error for os.ErrNotExist, will your problem be resolved with using erros.Is?

That could also be a possibility. I'm not that experienced in go so I don't know what would be the most idiomatic way.
I can update the PR if you think it's a better approach

[mattn]

I prefer to add ErrNonExistentWatch error in fsnotify.go

var (
    ErrNonExistentWatch = errors.New("can't remove non-existent watcher")
    ErrEventOverflow    = errors.New("fsnotify queue overflow")
)

and return it in inotify.go, kqueue.go, windows.go

return fmt.Errorf("%w: %s", ErrNonExistentWatch, name)

any thought? @nathany @shogo82148

[nathany]

@mattn a new error sounds good to me

[arp242]

I agree that outright removing the error is not a good solution; I opened #460 to return a specific error that can be tested for, which should address the issue better.