[Libguestfs] [PATCH 4/5] fuse: Fix read for empty files.
Jim Meyering
jim at meyering.net
Wed Nov 18 09:11:41 UTC 2009
Matthew Booth wrote:
> 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?
1 is a valid result size, but the only way to get
rsize of 1 (assuming the prior initialization) and
*also* r == NULL is in this failure case.
>> + if (rsize == 1&& r == NULL)
(oops, spacing: should be
if (rsize == 1 && r == NULL)
> 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.
Right. The API is too hard to use properly.
An alternative is to make it so guestfs_pread returns a non-NULL
pointer to one malloc'd (and unused) byte for an empty read,
and also sets *rsize to 0. *THEN* a NULL return value could
suffice to indicate failure.
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