Why does this simple monitoring app result in a permission denied error on file event?

[bbarker]

Not sure if this is more of a question or a bug, so I'll fill out the below.

System details

• OS/Platform name and version:
  NixOS:

  • system: "x86_64-linux"
  • host os: Linux 5.4.50, NixOS, 20.09.1889.58f9c4c7d3a (Nightingale)
  • multi-user?: yes
  • sandbox: yes
  • version: nix-env (Nix) 2.3.7
  • channels(nixos): ""
  • channels(root): "nixos-20.09.1889.58f9c4c7d3a"
  • channels(brandon): ""
  • nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

• Rust version (if building from source): rustc --version:

  • rustc 1.52.0-nightly (f98721f88 2021-03-10)

• Notify version (or commit hash if building from git):

  • 5.0.0-pre.6

• If you're coming from a project that makes use of Notify, what it is, and a link to the downstream issue if there is one:

  • https://github.com/bbarker/cont-append

• Filesystem type and options:

  • ext4

• If you're running as a privileged user (root, System):

  • No, running as a regular user

• If you're running in a VM, details on the hypervisor:

  • I believe this is running in KVM (in OpenStack)

What you did (as detailed as you can)

I wrote a small program (just starting it) to monitor files in a given directory. The relevant snippet:

pub fn cappend(src: &str, dst: &str) -> Result<()> {
    println!("Hello from cappend");
    let mut watcher: RecommendedWatcher = Watcher::new_immediate(|res| {
        println!("Debug: received an event");
        match res {
            Ok(event) => println!("event: {:?}", event),
            Err(e) => println!("watch error: {:?}", e),
        }
    })?;
    watcher
        .configure(Config::PreciseEvents(true))
        .context("Error configuring watcher for PreciseEvents")?;

    let src = PathDir::create(src)?;
    watch_dir(&mut watcher, &src)?;
    Ok(())
}

fn watch_dir(watcher: &mut RecommendedWatcher, dir: &PathDir) -> notify::Result<()> {
    watcher.watch(dir.as_path(), RecursiveMode::Recursive)?;
    Ok(())
}

What you expected

I expected the program to keep running - I don't know why there would be a permission denied error in this program, especially since there are no writes, and it is just looking at a folder in my home directory with these permissions:

$ ls -last ~/workspace
total 716
  0 -rw-r--r--  1 brandon users       0 Mar 28 13:50 foo.txt
  4 drwxr-xr-x 60 brandon users    4096 Mar 28 13:50 .
  4 drwx------ 29 brandon users    4096 Mar 26 01:12 ..

What happened

After running the program and initiating a filesystem action in another terminal (touch ~/workspace/foo.txt), I received an error and the program exited:

$ ./target/debug/contappend /home/brandon/workspace/ /tmp                                                                                                                                                                                                                                                                                              Hello from cappend
Debug: received an event
Error in cappend: Permission denied (os error 13)

[bbarker]

Running this on a different system (Ubuntu 20.04, bare metal, no-VM), seems to indicate a different OS error ("No space left on device"), which is odd - every non-snap volume listed by df indicates ample free space.

$ ./target/debug/contappend ~/workspace/ /tmp/
Hello from cappend
Debug: received an event
Error in cappendevent: Event { kind: Create(File), paths: ["/home/brandon/workspace/foo.txt"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }
: Debug: received an event
event: Event { kind: Modify(Metadata(Any)), paths: ["/home/brandon/workspace/foo.txt"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }
No space left on device (os error 28)Debug: received an event

event: Event { kind: Access(Close(Write)), paths: ["/home/brandon/workspace/foo.txt"], attr:tracker: None, attr:flag: None, attr:info: None, attr:source: None }

[0xpr03]

No space left on device

See also #266 - are you sure that you aren't at the inotify cap ?

[0xpr03]

Depending on the settings and applications you're running, as well as how big the folder is you want to watch, you may already reach a limit.

[bbarker]

Ah, I see - makes sense. I think you are right about that. It is a very large folder.

[0xpr03]

Note that you can increase your limits (system and user wide). Look for fs.inotify.max_user_watches on linux.

[0xpr03]

Regarding your readme: You can use https://docs.rs/notify/5.0.0-pre.6/notify/poll/struct.PollWatcher.html instead to use a polling based watcher.

[bbarker]

Thanks! I added some notes for that.