batchSourceCancel

fn batchSourceCancel(context: ?*anyopaque) callconv(.c) void {
    const storage: *Io.Operation.Storage = @ptrCast(@alignCast(context));
    const pending = &storage.pending;
    const operation_userdata: *BatchOperationUserdata = .fromErased(&pending.userdata);
    const batch = operation_userdata.batch;
    const source = operation_userdata.source;
    const index: Io.Operation.OptionalIndex = .fromIndex(storage - batch.storage.ptr);

    switch (pending.node.prev) {
        .none => batch.pending.head = pending.node.next,
        else => |prev_index| batch.storage[prev_index.toIndex()].pending.node.next = pending.node.next,
    }
    switch (pending.node.next) {
        .none => batch.pending.tail = pending.node.prev,
        else => |next_index| batch.storage[next_index.toIndex()].pending.node.prev = pending.node.prev,
    }

    const tail_index = batch.unused.tail;
    switch (tail_index) {
        .none => batch.unused.head = index,
        else => batch.storage[tail_index.toIndex()].unused.next = index,
    }
    storage.* = .{ .unused = .{ .prev = tail_index, .next = .none } };
    batch.unused.tail = index;

    source.as_object().release();
    if (batch.pending.head != .none) return;
    const queue: c.dispatch.queue_t = @ptrCast(batch.userdata);
    const waiter: *BatchWaiter = @ptrCast(@alignCast(queue.as_object().get_context()));
    queue.as_object().release();
    waiter.wake();
}