gstreamer/

memory_wrapped.rs

// Take a look at the license at the top of the repository in the LICENSE file.

use glib::{prelude::*, translate::*};

use std::{alloc, any::TypeId, mem, ptr};

use crate::{ffi, Memory};

// rustdoc-stripper-ignore-next
/// Error type for `try_into_inner` failures on Memory and Buffer.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum MemoryIntoInnerError {
    #[error("Memory does not use the Rust allocator (uses {actual_allocator:?})")]
    WrongAllocator { actual_allocator: Option<String> },
    #[error("Memory is not writable")]
    NotWritable,
    #[error("Cannot extract wrapped value from sub-memory (shared memory)")]
    SubMemory,
    #[error("Memory does not wrap the requested type (expected {expected:?}, found {actual:?})")]
    TypeMismatch { expected: TypeId, actual: TypeId },
    #[error("Buffer must contain exactly one memory block")]
    MultipleMemoryBlocks,
}

#[repr(C)]
struct WrappedMemory<T> {
    mem: ffi::GstMemory,

    // AsRef / AsMut values
    data: *mut u8,

    // Layout used for allocating this struct, literally `Layout::new<Self>`
    layout: alloc::Layout,

    // Offset from the beginning of the struct until `wrap`
    wrap_offset: usize,
    // `ptr::drop_in_place()` for `T`
    wrap_drop_in_place: Option<unsafe fn(*mut ())>,
    // TypeId of the wrapped type for runtime type checking
    wrap_type_id: TypeId,
    wrap: T,
}

unsafe extern "C" fn alloc(
    allocator: *mut ffi::GstAllocator,
    size: usize,
    params: *mut ffi::GstAllocationParams,
) -> *mut ffi::GstMemory {
    let params = &*params;

    let Some(maxsize) = size
        .checked_add(params.prefix)
        .and_then(|s| s.checked_add(params.padding))
    else {
        return ptr::null_mut();
    };

    let align = params.align | crate::Memory::default_alignment();

    let layout_base = alloc::Layout::new::<WrappedMemory<()>>();

    let layout_data = match alloc::Layout::from_size_align(maxsize, align + 1) {
        Ok(res) => res,
        Err(err) => {
            crate::warning!(
                crate::CAT_RUST,
                "Invalid size {maxsize} or alignment {align}: {err}"
            );
            return ptr::null_mut();
        }
    };
    let (layout, data_offset) = match layout_base.extend(layout_data) {
        Ok(res) => res,
        Err(err) => {
            crate::warning!(
                crate::CAT_RUST,
                "Can't extend base memory layout to {maxsize} or alignment {align}: {err}"
            );
            return ptr::null_mut();
        }
    };
    let layout = layout.pad_to_align();

    let mem = alloc::alloc(layout);
    let data = mem.add(data_offset);

    if params.prefix > 0 && (params.flags & ffi::GST_MEMORY_FLAG_ZERO_PREFIXED) != 0 {
        ptr::write_bytes(data, 0, params.prefix);
    }

    if (params.flags & ffi::GST_MEMORY_FLAG_ZERO_PADDED) != 0 {
        ptr::write_bytes(data.add(params.prefix).add(size), 0, params.padding);
    }

    let mem = mem as *mut WrappedMemory<()>;
    ffi::gst_memory_init(
        ptr::addr_of_mut!((*mem).mem),
        params.flags,
        allocator,
        ptr::null_mut(),
        maxsize,
        params.align,
        params.prefix,
        size,
    );
    ptr::write(ptr::addr_of_mut!((*mem).data), data);
    ptr::write(ptr::addr_of_mut!((*mem).layout), layout);
    ptr::write(ptr::addr_of_mut!((*mem).wrap_type_id), TypeId::of::<()>());
    ptr::write(ptr::addr_of_mut!((*mem).wrap_offset), 0);
    ptr::write(ptr::addr_of_mut!((*mem).wrap_drop_in_place), None);

    mem as *mut ffi::GstMemory
}

unsafe extern "C" fn free(_allocator: *mut ffi::GstAllocator, mem: *mut ffi::GstMemory) {
    debug_assert_eq!((*mem).mini_object.refcount, 0);

    let mem = mem as *mut WrappedMemory<()>;

    if let Some(wrap_drop_in_place) = (*mem).wrap_drop_in_place {
        let wrap = (mem as *mut u8).add((*mem).wrap_offset) as *mut ();
        wrap_drop_in_place(wrap);
    }

    alloc::dealloc(mem as *mut u8, (*mem).layout);
}

unsafe extern "C" fn mem_map(
    mem: *mut ffi::GstMemory,
    _maxsize: usize,
    _flags: ffi::GstMapFlags,
) -> glib::ffi::gpointer {
    let mem = mem as *mut WrappedMemory<()>;

    // `(*mem).offset` is added to the pointer by `gst_memory_map()`
    (*mem).data as glib::ffi::gpointer
}

unsafe extern "C" fn mem_unmap(_mem: *mut ffi::GstMemory) {}

unsafe extern "C" fn mem_share(
    mem: *mut ffi::GstMemory,
    offset: isize,
    size: isize,
) -> *mut ffi::GstMemory {
    let mem = mem as *mut WrappedMemory<()>;

    // Basically a re-implementation of _sysmem_share()

    let parent = if (*mem).mem.parent.is_null() {
        mem
    } else {
        (*mem).mem.parent as *mut WrappedMemory<()>
    };

    // Offset and size are actually usizes and the API assumes that negative values simply wrap
    // around, so let's cast to usizes here and do wrapping arithmetic.
    let offset = offset as usize;
    let mut size = size as usize;

    let new_offset = (*mem).mem.offset.wrapping_add(offset);
    debug_assert!(new_offset < (*mem).mem.maxsize);

    if size == usize::MAX {
        size = (*mem).mem.size.wrapping_sub(offset);
    }
    debug_assert!(new_offset <= usize::MAX - size);
    debug_assert!(new_offset + size <= (*mem).mem.maxsize);

    let layout = alloc::Layout::new::<WrappedMemory<()>>();
    let sub = alloc::alloc(layout) as *mut WrappedMemory<()>;

    ffi::gst_memory_init(
        sub as *mut ffi::GstMemory,
        (*mem).mem.mini_object.flags | ffi::GST_MINI_OBJECT_FLAG_LOCK_READONLY,
        (*mem).mem.allocator,
        parent as *mut ffi::GstMemory,
        (*mem).mem.maxsize,
        (*mem).mem.align,
        new_offset,
        size,
    );
    ptr::write(ptr::addr_of_mut!((*sub).data), (*mem).data);
    ptr::write(ptr::addr_of_mut!((*sub).layout), layout);
    ptr::write(ptr::addr_of_mut!((*sub).wrap_offset), 0);
    ptr::write(ptr::addr_of_mut!((*sub).wrap_drop_in_place), None);

    sub as *mut ffi::GstMemory
}

unsafe extern "C" fn mem_is_span(
    mem1: *mut ffi::GstMemory,
    mem2: *mut ffi::GstMemory,
    offset: *mut usize,
) -> glib::ffi::gboolean {
    let mem1 = mem1 as *mut WrappedMemory<()>;
    let mem2 = mem2 as *mut WrappedMemory<()>;

    // Same parent is checked by `gst_memory_is_span()` already
    let parent1 = (*mem1).mem.parent as *mut WrappedMemory<()>;
    let parent2 = (*mem2).mem.parent as *mut WrappedMemory<()>;
    debug_assert_eq!(parent1, parent2);

    // Basically a re-implementation of _sysmem_is_span()
    if !offset.is_null() {
        // Offset that can be used on the parent memory to create a
        // shared memory that starts with `mem1`.
        //
        // This needs to use wrapping arithmetic too as in `mem_share()`.
        *offset = (*mem1).mem.offset.wrapping_sub((*parent1).mem.offset);
    }

    // Check if both memories are contiguous.
    let is_span = ((*mem1).mem.offset + ((*mem1).mem.size)) == (*mem2).mem.offset;

    is_span.into_glib()
}

unsafe extern "C" fn class_init(class: glib::ffi::gpointer, _class_data: glib::ffi::gpointer) {
    let class = class as *mut ffi::GstAllocatorClass;

    (*class).alloc = Some(alloc);
    (*class).free = Some(free);
}

unsafe extern "C" fn instance_init(
    obj: *mut glib::gobject_ffi::GTypeInstance,
    _class: glib::ffi::gpointer,
) {
    static ALLOCATOR_TYPE: &[u8] = b"RustGlobalAllocatorMemory\0";

    let allocator = obj as *mut ffi::GstAllocator;

    (*allocator).mem_type = ALLOCATOR_TYPE.as_ptr() as *const _;
    (*allocator).mem_map = Some(mem_map);
    (*allocator).mem_unmap = Some(mem_unmap);
    // mem_copy not set because the fallback already does the right thing
    (*allocator).mem_share = Some(mem_share);
    (*allocator).mem_is_span = Some(mem_is_span);

    (*allocator).object.flags |= ffi::GST_OBJECT_FLAG_MAY_BE_LEAKED;
}

pub fn rust_allocator() -> &'static crate::Allocator {
    assert_initialized_main_thread!();

    rust_allocator_internal()
}

fn rust_allocator_internal() -> &'static crate::Allocator {
    static RUST_ALLOCATOR: std::sync::OnceLock<crate::Allocator> = std::sync::OnceLock::new();

    RUST_ALLOCATOR.get_or_init(|| unsafe {
        struct TypeInfoWrap(glib::gobject_ffi::GTypeInfo);
        unsafe impl Send for TypeInfoWrap {}
        unsafe impl Sync for TypeInfoWrap {}

        static TYPE_INFO: TypeInfoWrap = TypeInfoWrap(glib::gobject_ffi::GTypeInfo {
            class_size: mem::size_of::<ffi::GstAllocatorClass>() as u16,
            base_init: None,
            base_finalize: None,
            class_init: Some(class_init),
            class_finalize: None,
            class_data: ptr::null_mut(),
            instance_size: mem::size_of::<ffi::GstAllocator>() as u16,
            n_preallocs: 0,
            instance_init: Some(instance_init),
            value_table: ptr::null(),
        });

        let type_name = {
            let mut idx = 0;

            loop {
                let type_name = glib::gformat!("GstRsAllocator-{}", idx);
                if glib::gobject_ffi::g_type_from_name(type_name.as_ptr())
                    == glib::gobject_ffi::G_TYPE_INVALID
                {
                    break type_name;
                }
                idx += 1;
            }
        };

        let t = glib::gobject_ffi::g_type_register_static(
            crate::Allocator::static_type().into_glib(),
            type_name.as_ptr(),
            &TYPE_INFO.0,
            0,
        );

        assert!(t != glib::gobject_ffi::G_TYPE_INVALID);

        from_glib_none(
            glib::gobject_ffi::g_object_newv(t, 0, ptr::null_mut()) as *mut ffi::GstAllocator
        )
    })
}

// Attempts to extract the underlying wrapped value of type `T` from a raw memory pointer.
// NOTE: The caller must ensure that the memory is freed on success
#[inline]
pub(crate) unsafe fn try_into_from_memory_ptr<T: 'static>(
    mem_ptr: *mut ffi::GstMemory,
) -> Result<T, MemoryIntoInnerError> {
    skip_assert_initialized!();

    // Check if this memory uses our rust allocator
    if (*mem_ptr).allocator.is_null() || (*mem_ptr).allocator != rust_allocator_internal().as_ptr()
    {
        let actual_allocator = if (*mem_ptr).allocator.is_null() {
            None
        } else {
            Some(
                std::ffi::CStr::from_ptr(glib::gobject_ffi::g_type_name_from_instance(
                    (*mem_ptr).allocator as *mut glib::gobject_ffi::GTypeInstance,
                ))
                .to_string_lossy()
                .to_string(),
            )
        };
        return Err(MemoryIntoInnerError::WrongAllocator { actual_allocator });
    }

    if ffi::gst_mini_object_is_writable(mem_ptr as *mut ffi::GstMiniObject) == glib::ffi::GFALSE {
        return Err(MemoryIntoInnerError::NotWritable);
    }

    // Check that this is not a sub-memory
    if !(*mem_ptr).parent.is_null() {
        return Err(MemoryIntoInnerError::SubMemory);
    }

    // Cast to WrappedMemory<T> and create a reference.
    // SAFETY: This cast is safe for reading the wrap_offset and wrap_type_id fields
    // because these fields are in the same position regardless of the generic type T.
    // We verify the actual type using wrap_type_id before accessing the `wrap` field.
    let mem_wrapper = &*(mem_ptr as *mut WrappedMemory<T>);

    // Check that the wrapped type is actually T
    // Only after this check passes is it safe to access mem_wrapper.wrap
    if mem_wrapper.wrap_type_id != TypeId::of::<T>() {
        return Err(MemoryIntoInnerError::TypeMismatch {
            expected: std::any::TypeId::of::<T>(),
            actual: mem_wrapper.wrap_type_id,
        });
    }

    // Extract the wrapped value
    let mem_wrapper_mut = &mut *(mem_ptr as *mut WrappedMemory<T>);
    let value = ptr::read(&mem_wrapper_mut.wrap);

    // Unset drop function to prevent drop from running on the wrapped value
    mem_wrapper_mut.wrap_drop_in_place = None;

    Ok(value)
}

impl Memory {
    #[doc(alias = "gst_memory_new_wrapped")]
    #[doc(alias = "gst_memory_new_wrapped_full")]
    #[inline]
    pub fn from_slice<T: AsRef<[u8]> + Send + 'static>(slice: T) -> Self {
        assert_initialized_main_thread!();

        let len = slice.as_ref().len();
        unsafe {
            let layout = alloc::Layout::new::<WrappedMemory<T>>();
            let mem = alloc::alloc(layout) as *mut WrappedMemory<T>;

            ffi::gst_memory_init(
                mem as *mut ffi::GstMemory,
                ffi::GST_MINI_OBJECT_FLAG_LOCK_READONLY,
                rust_allocator_internal().to_glib_none().0,
                ptr::null_mut(),
                len,
                0,
                0,
                len,
            );

            ptr::write(ptr::addr_of_mut!((*mem).wrap), slice);

            assert_eq!(len, (*mem).wrap.as_ref().len());
            let data = (*mem).wrap.as_ref().as_ptr();
            ptr::write(ptr::addr_of_mut!((*mem).data), mut_override(data));

            ptr::write(ptr::addr_of_mut!((*mem).layout), layout);

            let wrap_offset = ptr::addr_of!((*mem).wrap) as usize - mem as usize;
            ptr::write(ptr::addr_of_mut!((*mem).wrap_offset), wrap_offset);

            ptr::write(
                ptr::addr_of_mut!((*mem).wrap_drop_in_place),
                if mem::needs_drop::<T>() {
                    Some(|ptr| ptr::drop_in_place::<T>(ptr as *mut T))
                } else {
                    None
                },
            );

            ptr::write(ptr::addr_of_mut!((*mem).wrap_type_id), TypeId::of::<T>());

            from_glib_full(mem as *mut ffi::GstMemory)
        }
    }

    #[doc(alias = "gst_memory_new_wrapped")]
    #[doc(alias = "gst_memory_new_wrapped_full")]
    #[inline]
    pub fn from_mut_slice<T: AsMut<[u8]> + Send + 'static>(mut slice: T) -> Self {
        assert_initialized_main_thread!();

        let len = slice.as_mut().len();
        unsafe {
            let layout = alloc::Layout::new::<WrappedMemory<T>>();
            let mem = alloc::alloc(layout) as *mut WrappedMemory<T>;

            ffi::gst_memory_init(
                mem as *mut ffi::GstMemory,
                0,
                rust_allocator_internal().to_glib_none().0,
                ptr::null_mut(),
                len,
                0,
                0,
                len,
            );

            ptr::write(ptr::addr_of_mut!((*mem).wrap), slice);

            assert_eq!(len, (*mem).wrap.as_mut().len());
            let data = (*mem).wrap.as_mut().as_mut_ptr();
            ptr::write(ptr::addr_of_mut!((*mem).data), data);

            ptr::write(ptr::addr_of_mut!((*mem).layout), layout);

            let wrap_offset = ptr::addr_of!((*mem).wrap) as usize - mem as usize;
            ptr::write(ptr::addr_of_mut!((*mem).wrap_offset), wrap_offset);

            ptr::write(
                ptr::addr_of_mut!((*mem).wrap_drop_in_place),
                if mem::needs_drop::<T>() {
                    Some(|ptr| ptr::drop_in_place::<T>(ptr as *mut T))
                } else {
                    None
                },
            );

            ptr::write(ptr::addr_of_mut!((*mem).wrap_type_id), TypeId::of::<T>());

            from_glib_full(mem as *mut ffi::GstMemory)
        }
    }

    // rustdoc-stripper-ignore-next
    /// Attempts to extract the underlying wrapped value of type `T` from this `Memory`.
    ///
    /// This will only succeed if:
    /// - The memory was created with `from_slice()` or `from_mut_slice()` wrapping type `T`
    /// - The memory is not a sub-memory (created via sharing)
    /// - This is the only reference to the memory (memory is writable)
    ///
    /// On success, the `Memory` is consumed and the original wrapped value is returned.
    /// On failure, the original `Memory` and an error are returned in the `Err` variant.
    ///
    /// # Examples
    ///
    /// ```
    /// use gstreamer::Memory;
    ///
    /// gstreamer::init().unwrap();
    ///
    /// let vec = vec![1u8, 2, 3, 4, 5];
    /// let expected = vec.clone();
    /// let mem = Memory::from_slice(vec);
    ///
    /// let extracted: Vec<u8> = mem.try_into_inner().unwrap();
    /// assert_eq!(extracted, expected);
    /// ```
    #[inline]
    pub fn try_into_inner<T: 'static>(self) -> Result<T, (Self, MemoryIntoInnerError)> {
        // Check that this is the only reference
        unsafe {
            let mem_ptr = self.as_mut_ptr();

            // Try to extract the wrapped value using the helper function
            match try_into_from_memory_ptr(mem_ptr) {
                Ok(value) => {
                    // Drop the Memory (which will deallocate the wrapper but
                    // not call the wrapped value's drop)
                    drop(self);
                    Ok(value)
                }
                Err(err) => {
                    // Failed validation
                    Err((self, err))
                }
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_alloc() {
        use crate::prelude::AllocatorExt;

        crate::init().unwrap();

        let data = [0, 1, 2, 3, 4, 5, 6, 7];

        let mut mem = rust_allocator().alloc(data.len(), None).unwrap();
        assert_eq!(mem.size(), data.len());
        assert_eq!(mem.allocator().unwrap(), rust_allocator());

        {
            let mem = mem.get_mut().unwrap();
            let mut map = mem.map_writable().unwrap();
            assert_eq!(
                map.as_ptr() as usize & crate::Memory::default_alignment(),
                0
            );
            map.copy_from_slice(&data);
        }

        let copy = mem.copy();
        assert_eq!(copy.size(), data.len());
        assert_eq!(copy.allocator().unwrap(), rust_allocator());

        {
            let map = copy.map_readable().unwrap();
            assert_eq!(
                copy.as_ptr() as usize & crate::Memory::default_alignment(),
                0
            );
            assert_eq!(map.as_slice(), &data);
        }

        let mut mem = rust_allocator()
            .alloc(
                data.len(),
                Some(&crate::AllocationParams::new(
                    crate::MemoryFlags::empty(),
                    1023,
                    0,
                    0,
                )),
            )
            .unwrap();
        assert_eq!(mem.size(), data.len());
        assert_eq!(mem.maxsize(), data.len());
        assert_eq!(mem.allocator().unwrap(), rust_allocator());

        {
            let mem = mem.get_mut().unwrap();
            let mut map = mem.map_writable().unwrap();
            assert_eq!(map.as_ptr() as usize & 1023, 0);
            map.copy_from_slice(&data);
        }

        let copy = mem.copy();
        assert_eq!(copy.size(), data.len());
        assert_eq!(copy.allocator().unwrap(), rust_allocator());

        {
            let map = copy.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data);
        }

        let share = mem.share(2..4);
        assert_eq!(share.size(), 2);
        assert_eq!(share.allocator().unwrap(), rust_allocator());
        {
            let map = share.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data[2..4]);
        }

        let mut mem = rust_allocator()
            .alloc(
                data.len(),
                Some(&crate::AllocationParams::new(
                    crate::MemoryFlags::ZERO_PADDED | crate::MemoryFlags::ZERO_PREFIXED,
                    1023,
                    32,
                    32,
                )),
            )
            .unwrap();
        assert_eq!(mem.size(), data.len());
        assert_eq!(mem.maxsize(), data.len() + 32 + 32);
        assert_eq!(mem.allocator().unwrap(), rust_allocator());

        {
            let mem = mem.get_mut().unwrap();
            let mut map = mem.map_writable().unwrap();
            assert_eq!((map.as_ptr() as usize - 32) & 1023, 0);
            map.copy_from_slice(&data);
        }

        let copy = mem.copy();
        assert_eq!(copy.size(), data.len());
        assert_eq!(copy.allocator().unwrap(), rust_allocator());

        {
            let map = copy.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data);
        }

        let share = mem.share(2..4);
        assert_eq!(share.size(), 2);
        assert_eq!(share.allocator().unwrap(), rust_allocator());
        {
            let map = share.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data[2..4]);
        }

        let share = mem.share_maxsize(0..(data.len() + 32 + 32));
        assert_eq!(share.size(), data.len() + 32 + 32);
        assert_eq!(share.allocator().unwrap(), rust_allocator());
        {
            let map = share.map_readable().unwrap();
            let padding = [0; 32];
            assert_eq!(&map.as_slice()[..32], &padding);
            assert_eq!(&map.as_slice()[32..][..data.len()], &data);
            assert_eq!(&map.as_slice()[(32 + data.len())..], &padding);
        }
    }

    #[test]
    fn test_wrap_vec_u8() {
        crate::init().unwrap();

        let data = vec![1u8, 2, 3, 4, 5];
        let expected = data.clone();

        let mem = Memory::from_slice(data);
        assert_eq!(mem.size(), 5);
        {
            let map = mem.map_readable().unwrap();
            assert_eq!(&expected, map.as_slice());
        }
    }

    #[test]
    fn test_wrap_array_u8() {
        crate::init().unwrap();

        let data: [u8; 5] = [1u8, 2, 3, 4, 5];
        let expected = data;

        let mem = Memory::from_slice(data);
        assert_eq!(mem.size(), 5);
        assert_eq!(mem.size(), 5);
        {
            let map = mem.map_readable().unwrap();
            assert_eq!(&expected, map.as_slice());
        }
    }

    #[test]
    fn test_wrap_vec_u8_and_back() {
        crate::init().unwrap();

        let data = vec![1u8, 2, 3, 4, 5];
        let expected = data.clone();

        let mem = Memory::from_slice(data);
        assert_eq!(mem.size(), 5);
        {
            let map = mem.map_readable().unwrap();
            assert_eq!(&expected, map.as_slice());
        }

        let extracted: Vec<u8> = mem.try_into_inner().unwrap();
        assert_eq!(extracted, expected);
    }

    #[test]
    fn test_wrap_array_u8_and_back() {
        crate::init().unwrap();

        let data: [u8; 5] = [1u8, 2, 3, 4, 5];
        let expected = data;

        let mem = Memory::from_slice(data);
        assert_eq!(mem.size(), 5);
        assert_eq!(mem.size(), 5);
        {
            let map = mem.map_readable().unwrap();
            assert_eq!(&expected, map.as_slice());
        }

        let extracted: [u8; 5] = mem.try_into_inner().unwrap();
        assert_eq!(extracted, expected);
    }

    #[test]
    fn test_wrap_array_u8_mem_ops() {
        crate::init().unwrap();

        let data = [0, 1, 2, 3, 4, 5, 6, 7];

        let memory = Memory::from_slice(data);
        assert_eq!(memory.size(), data.len());

        {
            let map = memory.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data);
        }

        let copy = memory.copy();
        assert!(copy.parent().is_none());

        {
            let map1 = memory.map_readable().unwrap();
            let map2 = copy.map_readable().unwrap();
            assert_eq!(map1.as_slice(), map2.as_slice());
        }

        let share = memory.share(..);
        assert_eq!(share.parent().unwrap().as_ptr(), memory.as_ptr());

        {
            let map1 = memory.map_readable().unwrap();
            let map2 = share.map_readable().unwrap();
            assert_eq!(map1.as_slice(), map2.as_slice());
        }

        let sub1 = memory.share(..2);
        assert_eq!(sub1.size(), 2);
        assert_eq!(sub1.parent().unwrap().as_ptr(), memory.as_ptr());

        {
            let map = sub1.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data[..2]);
        }

        let sub2 = memory.share(2..);
        assert_eq!(sub2.size(), 6);
        assert_eq!(sub2.parent().unwrap().as_ptr(), memory.as_ptr());

        {
            let map = sub2.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data[2..]);
        }

        let offset = sub1.is_span(&sub2).unwrap();
        assert_eq!(offset, 0);

        let sub3 = sub2.share(2..);
        assert_eq!(sub3.size(), 4);
        assert_eq!(sub3.parent().unwrap().as_ptr(), memory.as_ptr());

        {
            let map = sub3.map_readable().unwrap();
            assert_eq!(map.as_slice(), &data[4..]);
        }
    }
}