Misaligned pointer dereference

[aliddell]

One issue preventing using the Runtime as a module-level fixture in tests (or using it in loops in a script) is that, in Rust, raw pointers want to be aligned when dereferenced (see).

To reproduce this, it's enough to acquire with a frame size that's not divisible by 8 and iterate over the frames:

import acquire


def acquire_unaligned():
    runtime = acquire.Runtime()
    dm = runtime.device_manager()
    props = runtime.get_configuration()
    props.video[0].camera.identifier = dm.select(
        acquire.DeviceKind.Camera, ".*empty.*")

    # sizeof(VideoFrame) + 33 * 47 is not divisible by 8
    props.video[0].camera.settings.shape = (33, 47)
    props.video[0].storage.identifier = dm.select(
        acquire.DeviceKind.Storage, "trash")

    props.video[0].max_frame_count = 3
    runtime.set_configuration(props)

    nframes = 0
    runtime.start()
    while nframes < props.video[0].max_frame_count:
        with runtime.get_available_data(0) as packet:
            for i in range(packet.get_frame_count()):
                _ = next(packet.frames())
                nframes += 1
    runtime.stop()


if __name__ == "__main__":
    acquire_unaligned()

giving something like the following error:

thread '<unnamed>' panicked at src/runtime.rs:394:54:
misaligned pointer dereference: address must be a multiple of 0x8 but is 0x300000c7e
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread caused non-unwinding panic. aborting.

Process finished with exit code 134 (interrupted by signal 6:SIGABRT)

This is VideoFrameIterator::next():

fn next(&mut self) -> Option<Self::Item> {
        let mut cur = self.cur.lock();
        if (*self.store.lock()).is_some() && *cur < self.end {
            let out = VideoFrame {
                _store: self.store.clone(),
                cur: *cur,
            };

            let c = cur.as_ptr();
            let o = unsafe { (c as *const u8).offset((*c).bytes_of_frame as _) } // OFFENDING DEREFERENCE HERE
                as *mut capi::VideoFrame;
            *cur = unsafe { NonNull::new_unchecked(o) };

            Some(out)
        } else {
            None
        }
    }

We should consider padding video frames and making next aware of this.