cannot watch non-existent files

[tve]

Maybe I'm mis-understanding the semantics of fsnotify, but I'm surprised by the fact that I can't watch non-existent files. E.g., I can't watch a filename and then get a CREATE event when it's created. Or, if I watch a file and it's replaced via a REMOVE then I can no longer track it. This means that to reliably track changes to a file I must watch the file's directory!?

Example:

package main

import (
        "time"

        "gopkg.in/fsnotify.v1"
)

func main() {
        w, _ := fsnotify.NewWatcher()
        w.Add("foo")
        go func() {
                for ev := range w.Events {
                        println("Event: ", ev.String())
                }
        }()
        go func() {
                for err := range w.Errors {
                        println("Error: ", err)
                }
        }()
        time.Sleep(60 * time.Second)

}

Test:

$ go version
go version go1.3 linux/amd64
$ rm foo
$ go run notify.go &
[1] 6990
$ echo hello >foo
# I was expecting a CREATE event at this point...
$ kill %1
[1]   Exit 2                  go run notify.go

$ go run notify.go &
[2] 7004
$ echo hello >foo
Event:  "foo": WRITE
Event:  "foo": WRITE
# This time I got the event because foo already existed, but why do I get two write events?
$ rm foo
$ Event:  "foo": REMOVE
$ echo hello >foo
# I was expecting a CREATE event at this point...
$ kill %2

[nathany]

It depends a lot on which OS you're using, but you probably want to watch the directory that you will be creating a file in.

[freaker2k7]

-1

No I'm not, because the folder doesn't exist as well.

[sorenisanerd]

@freaker2k7 Then keep going up the directory tree until something exists.

[freaker2k7]

That misses the whole point.

The whole beauty is to be able to watch the folder once it's created and on.

[sorenisanerd]

Ok. I'm just telling you what will work.

[sorenisanerd]

What I've done in a few situations is to watch a directory and whenever it changes, I walk the whole tree and add any missing directories to my watch list. Easy peasy.

[freaker2k7]

That's an awful solution when you have a big folder like /etc, /dev or any other directory with many files.
Your solution just abuses the IO.

[sorenisanerd]

Well, that's how inotify works.

Whether you do this from your application or if this functionality is added to fsnotify, it's going to "abuse the IO" just the same.

[arp242]

Pretty much all file notification backends work by getting a file descriptor or handle to the path you want to watch; if this path doesn't exist, then we can't get a file descriptor.

The general strategy for this will be to wait until the path exists:

for {
    if fileExists(path) {
        watcher.Add(path)
        break
    }
    time.Sleep(100*time.Millisecond)
}

I'm not convinced that this is something that needs to be added to this library; it's quite a bit of plumbing to do this well which makes this more complex, and this is the only issue raised about this that I can find in ten years, so there doesn't seem to be that much demand for it.

Should probably document though.