[Libguestfs] [PATCH 4/5] fuse: Fix read for empty files.

[Jim Meyering]

Richard W.M. Jones wrote:
> Subject: [PATCH 4/5] fuse: Fix read for empty files.
>
> Error handling for the guestfs_pread call was incorrect, which
> meant that empty files could produce spurious error messages.
> ---
>  fuse/guestmount.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
>
> diff --git a/fuse/guestmount.c b/fuse/guestmount.c
> index 9d16cef..669bf80 100644
> --- a/fuse/guestmount.c
> +++ b/fuse/guestmount.c
> @@ -621,8 +621,14 @@ fg_read (const char *path, char *buf, size_t size, off_t offset,
>    if (size > limit)
>      size = limit;
>
> +  /* Note the correct error handling here is tricky, because in the
> +   * case where the call returns a zero-length buffer, it might return
> +   * NULL.  However it won't adjust rsize along the error path, so we
> +   * can set rsize to something beforehand and use that as a flag.
> +   */
> +  rsize = 1;
>    r = guestfs_pread (g, path, size, offset, &rsize);
> -  if (r == NULL)
> +  if (rsize == 1 && r == NULL)
>      return error ();

That change looks fine.

However, that you (and every guestfs_pread caller) would have to
endure such contortions suggests that this API is too hard to use.
Or at least too easy to misuse.

Maybe it will be used so rarely that it doesn't matter...
But it's still early in the life of libguestfs, so if you
think you might want to change it, now is better/easier than later.