` io_uring_prep_link_timeout` may return undocumented error -ENOENT

[buracchi]

When using io_uring_prep_link_timeout the resulting CQE may have the res field set to -ENOENT. This return value is not documented in the io_uring_prep_link_timeout(3) man page and understanding what the error code is trying to convey is non-trivial.

The following example

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#include <liburing.h>

int main() {
    enum { NOP, TIMEOUT };
    struct io_uring ring;
    io_uring_queue_init(2, &ring, 0);

    struct io_uring_sqe *sqe = io_uring_get_sqe(&ring);
    io_uring_prep_nop(sqe);
    io_uring_sqe_set_data(sqe, (void *)NOP);
    io_uring_sqe_set_flags(sqe, IOSQE_IO_LINK);
    sqe = io_uring_get_sqe(&ring);
    io_uring_prep_link_timeout(sqe, &(struct __kernel_timespec) {.tv_nsec = 1}, 0);
    io_uring_sqe_set_data(sqe, (void *)TIMEOUT);
    io_uring_submit(&ring);

    struct io_uring_cqe *cqe;
    for (int i = 0; i < 2; i++) {
        io_uring_wait_cqe(&ring, &cqe);
        printf("Get %s\n", io_uring_cqe_get_data(cqe) == (void *)NOP ? "NOP" : "TIMEOUT");
        if (cqe->res < 0) {
            printf("%s%s\n", strerror(-cqe->res), -cqe->res == ENOENT ? " (ENOENT)" : "");
            return EXIT_FAILURE;
        }
    io_uring_cqe_seen(&ring, cqe);
    }
    io_uring_queue_exit(&ring);
    return EXIT_SUCCESS;
}

Produces the output:

Get NOP Get TIMEOUT No such file or directory (ENOENT)

Running on kernels

• 6.6.36.3-microsoft-standard-WSL2
• 6.5.0-1022-azure

[axboe]

Yep that can trigger, when the previous request has already completed, like in your case. I'll augment the man page.

[buracchi]

When the previous request has already completed shouldn't it trigger a -ECANCELED? I've noticed this behavior happening even if the linked request is still pending. It seems related to "small" timeout values.

[axboe]

-ECANCELED would mean that it canceled the previous request. But yes, if the timeout is small, races are more likely to happen, that's just unavoidable, and then you may see either one.