I'm looking into using FUSE for a quick project but I have come across, what I think is, a major issue.

With the FUSE module, you can provide an 'init' callback where you can return a scalar to be put into fuse_get_context() as 'private'. I want to use this to store the scripts "main" object so I can access the information needed to respond to the various calls (getattr, read, write etc.).

However, as soon as I try to use get_fuse_context with this, Perl gives me a lot of various errors like "Attempt to copy freed scalar" and "Attempt to free unreferenced scalar" and eventually panic-exits. Printing out fuse_get_context with Data::Dumper, shows that the private key is a "dangling pointer" (sorry, I'm not sure if this is the actual term).

It looks as though when fuse_get_context is called, it copies the structure in such a way that the refcount is not increased, so when it exits scope, it is being freed despite still being in use (it works okay the first time, as shown below).

I think this is the minimal code to reproduce the problem:

#!/usr/bin/env perl

use strict;
use warnings;

use Fuse;

my $priv_data = 'PRIVDAT';

Fuse::main (
    debug       => 1,
    init        => sub { return $priv_data },
    mountpoint  => 'mnt',
    getattr     => sub {
        Fuse::fuse_get_context();
        return qw[0 0 0040700 1 0 0 0 0 0 0 0 4096 0]
    },
);

[download]

(making $priv_data a global was just to make sure it was always in a scope)

On the fuller code where more callbacks are implemented, it results in values like this:

$ fuse_init_unref.pl | grep private
          'private' => 'PRIVDAT'
          'private' => 2,
          'private' => 0
          'private' => ' => ',
          'private' => 0,
          'private' => [
          'private' => undef,
          'private' => undef,
          'private' => undef,
          'private' => '',

[download]

Any ideas of whether it is FUSE or myself at fault? The only workarounds I can think of would involve wrapping all the callbacks so I can inject the object or having singletons..

In reply to FUSE, fuse_get_context and private = dangling pointer woes by CandyAngel

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