Do not suppress Chmod on non-existent file

[kolyshkin]

Currently fsnorify suppresses a Chmod event if the file does not exist
when the event is received.

This makes it impossible to use fsnotify to detect when an opened
file is removed. In such case the Linux kernel sends IN_ATTRIB event,
as described in inotify(7) man page:

IN_ATTRIB (*)
Metadata changed—for example, permissions (e.g., chmod(2)),
timestamps (e.g., utimensat(2)), extended attributes (setx‐
attr(2)), link count (since Linux 2.6.25; e.g., for the tar‐
get of link(2) and for unlink(2)), and user/group ID (e.g.,
chown(2)).

(in this very case it's link count that changes).

To fix:

• Modify the code to only suppress MODIFY and CREATE events.
• Add a test case to verify Chmod event is delivered.

While at it, fix the comment in ignoreLinux() to use the up-to-date
terminology (event ops).

This is heavily based on #205; kudos to @vladlosev for his work.

Fixes #194.

Update

Please indicate that you have signed the CLA in your pull request.

Yes I did

[kolyshkin]

@tsachiherman @zellyn @nathany @nhooyr @markbates PTAL

[kolyshkin]

@nathany @markbates @ppknap PTAL

[kolyshkin]

Rebased; the PR description and the commit message slightly updated for clarity.

[kolyshkin]

It's a pity that no one took a look at this :( the issue is still valid

[nathany]

Looks good.

Let's just ensure that tests are passing. May need a rebase.

[vladlosev]

Some cleanup note, otherwise LGTM.

[kolyshkin]

Rebased; addressed @vladlosev review comment.

@nathany PTAL

[kolyshkin]

CI failures look legit but unrelated

[kolyshkin]

@nathany PTAL

[amitsingh21]

@kolyshkin could you please rebase with master, above CI failure should be fixed with this 7f4cf4d
waiting to get this merged to master, as we need this fix

[alisson276]

I confirm that small change is working. I'm having this problem using lumberjack, because it keeps the file opened. After this change the following code starts to work:

package main

import (
        "os"
	"log"

	"github.com/fsnotify/fsnotify"
        "gopkg.in/natefinch/lumberjack.v2"
)

func main() {
	myFile := "/var/log/x.log"
	file := &lumberjack.Logger{
		Filename: myFile,
	}
        file.Write([]byte{})
        file.Close()
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		log.Println(err)
	}
	defer watcher.Close()

	done := make(chan bool)
	go func() {
		for {
			select {
			case event, ok := <-watcher.Events:
                                log.Println("ok:", ok)
				if !ok {
					return
				}
				log.Println("event:", event)
                                if event.Op&fsnotify.Chmod == fsnotify.Chmod {
					file.Close()
				} else if event.Op&fsnotify.Rename == fsnotify.Rename || event.Op&fsnotify.Remove == fsnotify.Remove {
					file.Write([]byte{})
					log.Println("modified file:", event.Name)
					if err := watcher.Add(event.Name); err != nil {
						log.Println("Unable to watch file: ", event.Name, " ", err)
						os.Exit(1)
					}
					log.Println("started watching file: ", event.Name)
				}
			case err, ok := <-watcher.Errors:
                                log.Println("err ok:", ok)
				if !ok {
					return
				}
				log.Println("error:", err)
			}
		}
	}()

	err = watcher.Add(myFile)
	if err != nil {
		log.Println(err)
	}
	<-done
}

Before this change, after doing a rm /var/log/x.log didn't recreate the file.

[kolyshkin]

Rebased

[nxadm]

I validated this PR for https://github.com/nxadm/tail (a tail Go library, using fsnotify). It looks good to me and it fixes bugs related to reopening deleted files. Thanks for your work, @kolyshkin!

It would be great if this PR could be reviewed and merged.

[ShansOwn]

Looks like the CI problem was resolved in #378 and #381.
Can this one be moved forward?

[arp242]

Rebased on main, and seems good.

One possible concern is that CHMOD events may arrive after the REMOVE, which is what this check addresses in the first place. In this case, the cure seems worse than the disease, but should probably make a note of this in the changelog for the next version.