Comment 0 for bug 2031411

rust-loopdev in mantic has been failing[1] on arm64. Here is one example[2]:

864s ---- detach_a_backing_file_default stdout ----
864s thread 'detach_a_backing_file_default' panicked at 'assertion failed: `(left == right)`
864s left: `0`,
864s right: `1`: there should be no loopback devices mounted', tests/integration_test.rs:176:5

That test looks pretty simple. It counts how many loop devices are at the start, attached a backing file, checks, detached, and counts the number of attached loop devices at the end. Both counts should be equal.

But, according to the test failure, they are not.

I patched the function the test uses with some println() debug statements, and it looks like this:

fn detach_a_backing_file(offset: u64, sizelimit: u64, file_size: i64) {
    let num_devices_at_start = list_device(None).len();
    println!("XXX num_devices_at_start={}",num_devices_at_start);
    println!("XXX list_device(None).len()={}", list_device(None).len());
    let _lock = setup();

    println!("XXX start list_device(None): {:?}, len={}, var={}", list_device(None), list_device(None).len(), num_devices_at_start);
    {
        let file = create_backing_file(file_size);
        attach_file(
            "/dev/loop3",
            file.to_path_buf().to_str().unwrap(),
            offset,
            sizelimit,
        );

        let ld0 = LoopDevice::open("/dev/loop3")
            .expect("should be able to open the created loopback device");

        ld0.detach()
            .expect("should not error detaching the backing file from the loopdev");

        file.close().expect("should delete the temp backing file");
    };

    std::thread::sleep(std::time::Duration::from_millis(500));

    println!("XXX end list_device(None): {:?}, num_devices_at_start={}", list_device(None), num_devices_at_start);
    assert_eq!(
        list_device(None).len(),
        num_devices_at_start,
        "there should be no loopback devices mounted"
    );
    detach_all();
}

And, when it fails, this is the output:
---- detach_a_backing_file_with_offset stdout ----
XXX num_devices_at_start=4
XXX list_device(None).len()=4
XXX start list_device(None): [LoopDeviceOutput { name: "/dev/loop1", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/lxd_25116.snap") }, LoopDeviceOutput { name: "/dev/loop2", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/snapd_19459.snap") }, LoopDeviceOutput { name: "/dev/loop0", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/core22_821.snap") }], len=3, var=4
XXX end list_device(None): [LoopDeviceOutput { name: "/dev/loop1", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/lxd_25116.snap") }, LoopDeviceOutput { name: "/dev/loop2", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/snapd_19459.snap") }, LoopDeviceOutput { name: "/dev/loop0", size_limit: Some(0), offset: Some(0), back_file: Some("/var/lib/snapd/snaps/core22_821.snap") }], num_devices_at_start=4
thread 'detach_a_backing_file_with_offset' panicked at 'assertion failed: `(left == right)`
  left: `3`,
 right: `4`: there should be no loopback devices mounted', tests/integration_test.rs:180:5

Something is changing the list_device(None) output under our feet, as the test runs. The test starts with 4, and right after "let _lock = setup();", it's 3. This doesn't happen always.

1. https://autopkgtest.ubuntu.com/packages/r/rust-loopdev/mantic/arm64
2. https://autopkgtest.ubuntu.com/results/autopkgtest-mantic/mantic/arm64/r/rust-loopdev/20230814_100137_214a4@/log.gz