Re: [PATCH v6] error: rename errp to errp_in where it is IN-argument

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

29.11.2019 17:35, Markus Armbruster wrote:
> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:
> 
>> 28.11.2019 23:29, Markus Armbruster wrote:
>>> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:
>>>
>>>> 28.11.2019 17:23, Markus Armbruster wrote:
>>>>> Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> writes:
>>>>>
>>>>>> Error **errp is almost always OUT-argument: it's assumed to be NULL, or
>>>>>> pointer to NULL-initialized pointer, or pointer to error_abort or
>>>>>> error_fatal, for callee to report error.
>>>>>>
>>>>>> But very few functions instead get Error **errp as IN-argument:
>>>>>> it's assumed to be set (or, maybe, NULL), and callee should clean it,
>>>>>> or add some information.
>>>>>>
>>>>>> In such cases, rename errp to errp_in.
>>>>>
>>>>> Missing: why is the rename useful?
>>>>
>>>> The main reason is to prepare for coccinelle part.
>>>
>>> It's not a prerequisite for applying the patches Coccinelle produces,
>>> only a prerequisite for running Coccinelle.
>>>
>>>>> It's useful if it helps readers recognize unusual Error ** parameters,
>>>>> and recognizing unusual Error ** parameters is actually a problem.  I'm
>>>>> not sure it is, but my familiarity with the Error interface may blind
>>>>> me.
>>>>>
>>>>> How many functions have unusual Error **parameters?  How are they used?
>>>>> Any calls that could easily be mistaken as the usual case?  See [*]
>>>>> below.
> [...]
>>>>> [*] The information I asked for above is buried in these patches.  I'll
>>>>> try to dig it up as I go reviewing them.
>>>>>
>>>>> Second, it risks some of these "further patches" overtake this one, and
>>>>> then its commit message will be misleading.  Moreover, the other commits
>>>>> will lack context.
> [...]
>>>>>> diff --git a/util/error.c b/util/error.c
>>>>>> index d4532ce318..275586faa8 100644
>>>>>> --- a/util/error.c
>>>>>> +++ b/util/error.c
> [...]
>>>>>> @@ -271,11 +271,11 @@ void error_free(Error *err)
>>>>>>         }
>>>>>>     }
>>>>>>     
>>>>>> -void error_free_or_abort(Error **errp)
>>>>>> +void error_free_or_abort(Error **errp_in)
>>>>>>     {
>>>>>> -    assert(errp && *errp);
>>>>>> -    error_free(*errp);
>>>>>> -    *errp = NULL;
>>>>>> +    assert(errp_in && *errp_in);
>>>>>> +    error_free(*errp_in);
>>>>>> +    *errp_in = NULL;
>>>
>>> This one is actually in/out.
>>>
>>> To make the compiler check errp_in is truly an in-argument, we can
>>> declare it as Error *const *errp_in.
>>>
>>> But we can save ourselves the trouble of renaming it; the const should
>>> suffice to tell both human readers and Coccinelle that this is not your
>>> common out-argument.  I think I like this better than relying on a
>>> naming convention.  What about you?
>>
>> I think it's good idea... But what to do with error_free_or_abort, and other functions
>> which get filled errp, and want to set it to NULL? Patch 21 adds three such functions..
>>
>> Maybe, add assert(errp) at start of such functions, and catch it by coccinelle (not sure
>> that it is possible)?
> 
> I went through the git branch you provided.
> 
> These get rid of unusual Error ** parameters:
> 
>      01e10667d1 hmp: drop Error pointer indirection in hmp_handle_error
>      606bfb7520 vnc: drop Error pointer indirection in vnc_client_io_error
> 
>      Get rid of them by peeling off an indirection.
> 
> These try to make unusual Error ** parameters stand out:
> 
>      51e73b3305 error: rename errp to errp_in where it is IN-argument
> 
>      Four renames.
> 
>      Three functions modify the error, name @errp_in is okay, my const
>      proposal works.
> 
>      error_free_or_abort() clears *errp_in, name @errp_in is misleading,
>      const doesn't work.
> 
>      f7bdfd42da qdev-monitor: well form error hint helpers
> 
>      Two renames.  Both functions modify the error, rename is okay, const
>      works.
> 
>      9d4aac7299 nbd: well form nbd_iter_channel_error errp handler
> 
>      One rename, from @local_err.  nbd_iter_channel_error() clears *errp_in,
>      name @errp_in is misleading, const doesn't work.
> 
>      fb1bd83c40 ppc: well form kvmppc_hint_smt_possible error hint helper
> 
>      One rename.  The function modify the error, rename is okay, const works.
> 
>      e024e89b10 9pfs: well form error hint helpers
> 
>      Two renames.  Both functions modify the error, rename is okay, const
>      works.
> 
> These make usual Error ** parameters look, well, more usual:
> 
>      c01649d999 hw/core/qdev: cleanup Error ** variables
>      a5f6424163 block/snapshot: rename Error ** parameter to more common errp
>      ae200ca21e hw/i386/amd_iommu: rename Error ** parameter to more common errp
>      561f73e681 qga: rename Error ** parameter to more common errp
>      12589a96cd monitor/qmp-cmds: rename Error ** parameter to more common errp
>      f608fc5999 hw/s390x: rename Error ** parameter to more common errp
>      747a90d044 hw/tpm: rename Error ** parameter to more common errp
>      3d19e66610 hw/usb: rename Error ** parameter to more common errp
>      07366648ef include/qom/object.h: rename Error ** parameter to more common errp
> 
> These don't touch Error ** parameter declarations:
> 
>      f6e4fffc16 hw/core/loader-fit: fix freeing errp in fit_load_fdt
>      b5bba880ae net/net: Clean up variable shadowing in net_client_init()
>      4a4462ce4c hw/sd: drop extra whitespace in sdhci_sysbus_realize() header
>      d52d44e822 backends/cryptodev: drop local_err from cryptodev_backend_complete()
>      7e95d30766 hw/vfio/ap: drop local_err from vfio_ap_realize
> 
> Unusual Error ** parameters that can be made Error *const *errp should
> not also need a rename, neither to avoid confusion about @errp's role,
> nor to help Coccinelle.
> 
> Unusual Error ** parameters that can't be made Error *const *errp:
> 
>      nbd_iter_channel_error(): parameter is called @local_err.  Confusion
>      seems as unlikely without the rename as with it.  Coccinelle should
>      not need the rename.  No need to touch.  I'm willing to accept your
>      assertion change.

Hmm but you reverted it.. Will you keep an assertion?

> 
>      error_free_or_abort(): parameter is called @errp.  Confusion seems
>      as unlikely without the rename as with it.  Coccinelle should not
>      need the rename.  I'm willing to accept a rename to @local_err
>      regardless.

So, either we rename it to local_err, or coccinelle should distinguish it
by assertion at the top.

> 
> I pushed my fixups to git://repo.or.cz/qemu/armbru.git branch error-prep
> for your convenience.  The commit messages of the fixed up commits need
> rephrasing, of course.
> 

Looked through fixups, looks OK for me, thanks! What next?


-- 
Best regards,
Vladimir