Function probe_file_sparse_support

block/src/lib.rs:362–402

Probe sparse support for a regular file using fallocate().

(fd: libc::c_int)

/// Probe sparse support for a regular file using fallocate().
fn probe_file_sparse_support(fd: libc::c_int) -> bool {
    // SAFETY: FFI call with valid fd
    let file_size = unsafe { libc::lseek(fd, 0, libc::SEEK_END) };
    if file_size < 0 {
        let err = io::Error::last_os_error();
        warn!("Failed to get file size for sparse probe: {err}");
        return false;
    }

    // SAFETY: FFI call with valid fd, probing past EOF is safe with KEEP_SIZE
    let punch_hole =
        unsafe { libc::fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, file_size, 1) }
            == 0;

    if !punch_hole {
        let err = io::Error::last_os_error();
        if err.raw_os_error() == Some(libc::EOPNOTSUPP) {
            debug!("File does not support FALLOC_FL_PUNCH_HOLE: {err}");
        } else {
            debug!("PUNCH_HOLE probe returned unexpected error: {err}");
        }
    }

    // SAFETY: FFI call with valid fd, probing past EOF is safe with KEEP_SIZE
    let zero_range =
        unsafe { libc::fallocate(fd, FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, file_size, 1) }
            == 0;

    if !zero_range {
        let err = io::Error::last_os_error();
        if err.raw_os_error() == Some(libc::EOPNOTSUPP) {
            debug!("File does not support FALLOC_FL_ZERO_RANGE: {err}");
        }
    }

    let supported = punch_hole || zero_range;
    info!(
        "Probed file sparse support: punch_hole={punch_hole}, zero_range={zero_range} => {supported}"
    );
    supported
}

/// Probe sparse support for a block device.
///