[RFC,1/5] nbd: Correct name comparison for export_set_name()

exp->name == name is certainly true if both strings are equal and will
work for both of them being NULL (which is important to check here);
however, the strings may also be equal without having the same address,
in which case there is no need to replace the export's name either.
Therefore, add a check for this case.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 nbd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Sat, May 31, 2014 at 08:43:08PM +0200, Max Reitz wrote:
> exp->name == name is certainly true if both strings are equal and will
> work for both of them being NULL (which is important to check here);
> however, the strings may also be equal without having the same address,
> in which case there is no need to replace the export's name either.
> Therefore, add a check for this case.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  nbd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/nbd.c b/nbd.c
> index e5084b6..0787cba 100644
> --- a/nbd.c
> +++ b/nbd.c
> @@ -832,7 +832,7 @@ NBDExport *nbd_export_find(const char *name)
>  
>  void nbd_export_set_name(NBDExport *exp, const char *name)
>  {
> -    if (exp->name == name) {
> +    if (exp->name == name || (exp->name && name && !strcmp(exp->name, name))) {
>          return;
>      }

It's not clear to me why we even bother.  The function is idempotent and
there are only 2 call sites in QEMU.  This is not a performance-critical
function where it helps to bail early.

Can we just drop the if statement completely?

void nbd_export_set_name(NBDExport *exp, const char *name)
{
    if (exp->name == name) {
        return;
    }

    nbd_export_get(exp);
    if (exp->name != NULL) {
        g_free(exp->name);
        exp->name = NULL;
        QTAILQ_REMOVE(&exports, exp, next);
        nbd_export_put(exp);
    }
    if (name != NULL) {
        nbd_export_get(exp);
        exp->name = g_strdup(name);
        QTAILQ_INSERT_TAIL(&exports, exp, next);
    }
    nbd_export_put(exp);
}

On 04.06.2014 13:52, Stefan Hajnoczi wrote:
> On Sat, May 31, 2014 at 08:43:08PM +0200, Max Reitz wrote:
>> exp->name == name is certainly true if both strings are equal and will
>> work for both of them being NULL (which is important to check here);
>> however, the strings may also be equal without having the same address,
>> in which case there is no need to replace the export's name either.
>> Therefore, add a check for this case.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   nbd.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/nbd.c b/nbd.c
>> index e5084b6..0787cba 100644
>> --- a/nbd.c
>> +++ b/nbd.c
>> @@ -832,7 +832,7 @@ NBDExport *nbd_export_find(const char *name)
>>   
>>   void nbd_export_set_name(NBDExport *exp, const char *name)
>>   {
>> -    if (exp->name == name) {
>> +    if (exp->name == name || (exp->name && name && !strcmp(exp->name, name))) {
>>           return;
>>       }
> It's not clear to me why we even bother.  The function is idempotent and
> there are only 2 call sites in QEMU.  This is not a performance-critical
> function where it helps to bail early.

You're probably right. I just happened to stumble over this code when 
looking into NBD.

> Can we just drop the if statement completely?

Probably, yes, but then again, I think it's not worth bothering with 
dropping it, either. ;-)

Max

> void nbd_export_set_name(NBDExport *exp, const char *name)
> {
>      if (exp->name == name) {
>          return;
>      }
>
>      nbd_export_get(exp);
>      if (exp->name != NULL) {
>          g_free(exp->name);
>          exp->name = NULL;
>          QTAILQ_REMOVE(&exports, exp, next);
>          nbd_export_put(exp);
>      }
>      if (name != NULL) {
>          nbd_export_get(exp);
>          exp->name = g_strdup(name);
>          QTAILQ_INSERT_TAIL(&exports, exp, next);
>      }
>      nbd_export_put(exp);
> }