fileWritePositional

fn fileWritePositional(
    userdata: ?*anyopaque,
    file: File,
    header: []const u8,
    data: []const []const u8,
    splat: usize,
    offset: u64,
) File.WritePositionalError!usize {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

    var iovecs: [max_iovecs_len]iovec_const = undefined;
    var iovlen: iovlen_t = 0;
    addBuf(&iovecs, &iovlen, header);
    for (data[0 .. data.len - 1]) |bytes| addBuf(&iovecs, &iovlen, bytes);
    const pattern = data[data.len - 1];
    var backup_buffer: [splat_buffer_size]u8 = undefined;
    if (iovecs.len - iovlen != 0) switch (splat) {
        0 => {},
        1 => addBuf(&iovecs, &iovlen, pattern),
        else => switch (pattern.len) {
            0 => {},
            1 => {
                const splat_buffer = &backup_buffer;
                const memset_len = @min(splat_buffer.len, splat);
                const buf = splat_buffer[0..memset_len];
                @memset(buf, pattern[0]);
                addBuf(&iovecs, &iovlen, buf);
                var remaining_splat = splat - buf.len;
                while (remaining_splat > splat_buffer.len and iovecs.len - iovlen != 0) {
                    assert(buf.len == splat_buffer.len);
                    addBuf(&iovecs, &iovlen, splat_buffer);
                    remaining_splat -= splat_buffer.len;
                }
                addBuf(&iovecs, &iovlen, splat_buffer[0..@min(remaining_splat, splat_buffer.len)]);
            },
            else => for (0..@min(splat, iovecs.len - iovlen)) |_| {
                addBuf(&iovecs, &iovlen, pattern);
            },
        },
    };

    var cancel_region: CancelRegion = .init();
    defer cancel_region.deinit();
    return ev.pwritev(&cancel_region, file.handle, iovecs[0..iovlen], offset);
}