librustuv: avoid using the magic number -1 for offset

[nodakai]

The signature of uv_fs_read() is

UV_EXTERN int uv_fs_read(uv_loop_t* loop, uv_fs_t* req, uv_file file,
    void* buf, size_t length, int64_t offset, uv_fs_cb cb);

and when we pass -1 as offset, libuv discards it in favor of the intrinsic file offset.

• https://github.com/rust-lang/libuv/blob/master/src/win/fs.c#L527

  if (offset != -1) {
    memset(&overlapped, 0, sizeof overlapped);

    offset_.QuadPart = offset;
    overlapped.Offset = offset_.LowPart;
    overlapped.OffsetHigh = offset_.HighPart;

    overlapped_ptr = &overlapped;
  } else {
    overlapped_ptr = NULL;
  }

• https://github.com/rust-lang/libuv/blob/master/src/unix/fs.c#L191

static ssize_t uv__fs_read(uv_fs_t* req) {
  if (req->off < 0)
    return read(req->file, req->buf, req->len);
  else
    return pread(req->file, req->buf, req->len, req->off);
}

(The same goes for write operations.)

Such an interface design is fine for libuv itself, but librustuv should present more meaningful interface such as offset: Option<u64>. Note that the role of the magic number -1 is documented nowhere in libuv (sample codes in the uvbook use it without any explanations.) That pretty much means the users of libuv are expected to go through its implementation code in C. But all the developers of Rust don't need to do so.

[thestinger]

libuv was removed as part of #17325 so this is no longer an issue for this repository