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

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> 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()?

If you know ERRP can't be null, then error_is_set(ERRP) is pointless.
Just test *ERRP instead.

If ERRP may be null, then error_is_set(ERRP) makes some sense: it saves
you spelling out ERRP && *ERRP.  Personally, I'd prefer it spelled out,
though.

Note that testing an Error ** that may be null to answer the question
"did this fail" is *wrong*, regardless of whether you use error_is_set()
or spell it out.