[sheepdog] [PATCH 2/2] fix array index error in function add_new_participant

[Hitoshi Mitake]

At Tue, 12 Aug 2014 09:25:49 +0800,
Ruoyu wrote:
> 
> 
> On 2014年08月11日 21:45, Hitoshi Mitake wrote:
> > At Mon, 11 Aug 2014 18:11:44 +0800,
> > Ruoyu wrote:
> >> Nothing wrong with fixing it, alhough I don't know exactly
> >> what will be happened if not patching it.
> >>
> >> Signed-off-by: Ruoyu <liangry at ucweb.com>
> >> ---
> >>   sheep/vdi.c | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/sheep/vdi.c b/sheep/vdi.c
> >> index 05cae7b..77dc253 100644
> >> --- a/sheep/vdi.c
> >> +++ b/sheep/vdi.c
> >> @@ -372,11 +372,12 @@ static bool add_new_participant(struct vdi_state_entry *entry,
> >>   		return true;
> >>   	}
> >>   
> >> -	idx = entry->nr_participants++;
> >> +	idx = entry->nr_participants;
> >>   	memcpy(&entry->participants[idx], owner, sizeof(*owner));
> >>   	entry->participants_state[idx] =
> >>   		is_modified(entry) ?
> >>   		SHARED_LOCK_STATE_INVALIDATED : SHARED_LOCK_STATE_SHARED;
> >> +	entry->nr_participants++;
> > I think this patch doesn't change the function. Does this fixes a bug?
> It fixes a logical error. Otherwise, there should be a hole in the array 
> if new participant is added.
> For example,
> 
> 	// supposed entry->nr_participants is 2;
> 	idx = entry->nr_participants++;
> 	// idx is 3 now;

No, idx is 2 in this case. Specification of C guarantees it. e.g.

#include <stdio.h>

int main(void)
{
	int n = 2, m;

	m = n++;
	printf("n: %d, m: %d\n", n, m);

	return 0;
}

if you execute the above program, you can see an output:
n: 3, m: 2

If the expression of increment is put before the incremented variable,
the assigned variable will be equal to the incremented result.

Thanks,
Hitoshi