Re: [Qemu-devel] [PATCH v2 09/20] cow: correctly propagate errors

[Kevin Wolf]

Am 17.02.2014 um 14:15 hat Fam Zheng geschrieben:
> On Sat, 02/15 11:01, Markus Armbruster wrote:
> > Jeff Cody <address@hidden> writes:
> > 
> > > On Fri, Feb 14, 2014 at 05:45:40PM +0100, Kevin Wolf wrote:
> > >> Am 11.02.2014 um 18:03 hat Paolo Bonzini geschrieben:
> > >> > Signed-off-by: Paolo Bonzini <address@hidden>
> > >> > ---
> > >> >  block/cow.c | 12 +++---------
> > >> >  1 file changed, 3 insertions(+), 9 deletions(-)
> > >> > 
> > >> > diff --git a/block/cow.c b/block/cow.c
> > >> > index 7fc0b12..43a2150 100644
> > >> > --- a/block/cow.c
> > >> > +++ b/block/cow.c
> > >> > @@ -82,7 +82,7 @@ static int cow_open(BlockDriverState *bs, QDict 
> > >> > *options, int flags,
> > >> >          char version[64];
> > >> >          snprintf(version, sizeof(version),
> > >> >                 "COW version %d", cow_header.version);
> > >> > -        qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
> > >> > +        error_set(errp, QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
> > >> >              bs->device_name, "cow", version);
> > >> >          ret = -ENOTSUP;
> > >> >          goto fail;
> > >> > @@ -330,7 +330,6 @@ static int cow_create(const char *filename, 
> > >> > QEMUOptionParameter *options,
> > >> >      struct stat st;
> > >> >      int64_t image_sectors = 0;
> > >> >      const char *image_filename = NULL;
> > >> > -    Error *local_err = NULL;
> > >> >      int ret;
> > >> >      BlockDriverState *cow_bs;
> > >> >  
> > >> > @@ -344,18 +343,13 @@ static int cow_create(const char *filename, 
> > >> > QEMUOptionParameter *options,
> > >> >          options++;
> > >> >      }
> > >> >  
> > >> > -    ret = bdrv_create_file(filename, options, &local_err);
> > >> > +    ret = bdrv_create_file(filename, options, errp);
> > >> >      if (ret < 0) {
> > >> > -        qerror_report_err(local_err);
> > >> > -        error_free(local_err);
> > >> >          return ret;
> > >> >      }
> > >> >  
> > >> > -    ret = bdrv_file_open(&cow_bs, filename, NULL, NULL, BDRV_O_RDWR,
> > >> > -                         &local_err);
> > >> > +    ret = bdrv_file_open(&cow_bs, filename, NULL, NULL, BDRV_O_RDWR, 
> > >> > errp);
> > >> >      if (ret < 0) {
> > >> > -        qerror_report_err(local_err);
> > >> > -        error_free(local_err);
> > >> >          return ret;
> > >> >      }
> > >> 
> > >> This is technically correct, but I think general policy is that using
> > >> the local_err pattern is preferred anyway.
> > >>
> > >
> > > If I recall correct, I think there are several places that pass errp
> > > along.  How about this for a rule of thumb policy: use the local_err
> > > method if the function does not indicate error outside of the passed
> > > Error pointer.
> > 
> > Use &local_err when you need to examine the error object.  Passing errp
> > directly is no good then, because it may be null.
> > 
> > When you're forwarding errors without examining them, then passing errp
> > directly is just fine.
> > 
> 
> Does this mean that error_is_set() is always used by programmer to check a
> non-NULL error pointer? Is there any case to call error_is_set(errp) without
> knowing if errp is NULL or not? If no, should we enforce the rule and add
> assert(errp) in error_is_set()?

Sounds like a good idea to me, it would catch bugs where you forget to
use a local_err. Of course, it requires that error_is_set() is used
instead of just using errp as a boolean, but such an assertion could
actually be a reason to make this the policy.

Kevin