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

[Matthew Booth]

On 17/11/09 17:07, Richard W.M. Jones wrote:
>> From 3c5d63be17d281634f036f53aa32103ad4ab1c41 Mon Sep 17 00:00:00 2001
> From: Richard Jones<rjones at redhat.com>
> Date: Tue, 17 Nov 2009 17:02:45 +0000
> 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 ();
>
>     /* This should never happen, but at least it stops us overflowing
> -- 1.6.5.2

I've stared at this til my head hurts, and I don't get it. Why is 1 not 
a valid returned size from guestfs_pread? Why is NULL not an indicator 
of failure?

The documentation says:

        This function returns a buffer, or NULL on error.  The size of the
        returned buffer is written to *size_r.  The caller must free the
        returned buffer after use.

If NULL isn't sufficient to indicate error, guestfs_pread needs to be 
fixed, not this funtion.

Matt
-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team

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