The X Window System had a solution that allowed an asynchronous batch of "create thing, do stuff with thing": X allowed the client to allocate XIDs, and just had an operation for "give me an additional batch of XIDs I can allocate". Could we do the same thing in io_uring, as a variant of the "fixed file" concept? Suppose that we add a call to preallocate a large contiguous range of file descriptors, and ensure that the kernel never allocates those file descriptors for an application. We could then allow userspace to *tell* the kernel to use a specific preallocated file descriptor in that range when it does an OPENAT or OPENAT2 operation, and then go on to use that file descriptor in a subsequent read/write/close/etc operation. (If the open operation returns an error, it won't have allocated that file descriptor, so subsequent operations will just fail quickly with EBADF, which seems fine.) This seems simple for userspace to deal with, and would work for many use cases. Thoughts? - Josh Triplett
