Retain the arrays returned from Features's accessors

[mmetcalfe]

Currently, calling accessor methods on a Features object causes a crash due to a double free.

A (reasonably) minimal example of this problem is demonstrated in the following program:

// file: af_bug_mwe.rs
extern crate arrayfire as af;
use std::mem;
fn main() {
    af::set_device(0);
    af::info();

    // Load the test image: http://www.smufl.org/wp-content/uploads/M3.jpeg
    let file_name = String::from("../../smufl-november2-test.jpg");
    let img_grey = af::load_image(file_name, false).unwrap();
    {
        let features = af::fast(&img_grey, 20.0, 9, true, 0.05, 10).unwrap();
        let num_features = features.num_features().unwrap() as usize;
        println!("num_features: {:?}", num_features);

        let af_xpos = features.xpos().unwrap();
        println!("num_elements: {:?}", af_xpos.elements().unwrap());

        // unsafe { mem::forget(af_xpos); }

        println!("End features scope.");
    }
    println!("End program.");
}

Using the current devel branch of arrayfire-rust (f22a5fb) the program outputs the following:

ArrayFire v3.2.2 (OpenCL, 64-bit Mac OSX, build 7507b61)
[0] APPLE   : HD Graphics 4000
-1- APPLE   : GeForce GT 650M
num_features: 3827
num_elements: 3827
End features scope.
af_bug_mwe(18660,0x7fff7aa27300) malloc: *** error for object 0x7facabcf43e0: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug
An unknown error occurred

To learn more, run the command again with --verbose.

Un-commenting the unsafe line in the above example prevents the error.

This pull request changes the feat_func_def macro in the vision module to retain the arrays returned from af_get_features_* functions, in the same way that ArrayFire's current c++ implementation does.

With this change applied the output of the above program is as expected:

ArrayFire v3.2.2 (OpenCL, 64-bit Mac OSX, build 7507b61)
[0] APPLE   : HD Graphics 4000
-1- APPLE   : GeForce GT 650M
num_features: 3827
num_elements: 3827
End features scope.
End program.

[9prady9]

good catch 👍 thank you.

[9prady9]

@mmetcalfe
I don't think we need a new member function for this unless there are more use cases to it. Probably doing just Ok(Array::from(temp).clone()) is enough. Can you please check it and change accordingly.

[mmetcalfe]

I just changed the implementation based on your suggestion.
Using mem::forget here feels like a bit of a hack, but doing it this way doesn't require touching other files.