Re: Questionable aspects of QEMU Error's design

[Markus Armbruster]

Vladimir Sementsov-Ogievskiy <address@hidden> writes:

> 28.04.2020 08:20, Vladimir Sementsov-Ogievskiy wrote:
>> 27.04.2020 18:36, Markus Armbruster wrote:
>>> Markus Armbruster <address@hidden> writes:
>>>
>>>> Markus Armbruster <address@hidden> writes:
>>>>
>>>>> QEMU's Error was patterned after GLib's GError.  Differences include:
>>>> [...]
>>>>> * Return value conventions
>>>>>
>>>>>    Common: non-void functions return a distinct error value on failure
>>>>>    when such a value can be defined.  Patterns:
>>>>>
>>>>>    - Functions returning non-null pointers on success return null pointer
>>>>>      on failure.
>>>>>
>>>>>    - Functions returning non-negative integers on success return a
>>>>>      negative error code on failure.
>>>>>
>>>>>    Different: GLib discourages void functions, because these lead to
>>>>>    awkward error checking code.  We have tons of them, and tons of
>>>>>    awkward error checking code:
>>>>>
>>>>>      Error *err = NULL;
>>>>>      frobnicate(arg, &err);
>>>>>      if (err) {
>>>>>          ... recover ...
>>>>>          error_propagate(errp, err);
>>>>>      }
>>>>>
>>>>>    instead of
>>>>>
>>>>>      if (!frobnicate(arg, errp))
>>>>>          ... recover ...
>>>>>      }
>>>>>
>>>>>    Can also lead to pointless creation of Error objects.
>>>>>
>>>>>    I consider this a design mistake.  Can we still fix it?  We have more
>>>>>    than 2000 void functions taking an Error ** parameter...
>>>>>
>>>>>    Transforming code that receives and checks for errors with Coccinelle
>>>>>    shouldn't be hard.  Transforming code that returns errors seems more
>>>>>    difficult.  We need to transform explicit and implicit return to
>>>>>    either return true or return false, depending on what we did to the
>>>>>    @errp parameter on the way to the return.  Hmm.
>>>> [...]
>>>>
>>>> To figure out what functions with an Error ** parameter return, I used
>>>> Coccinelle to find such function definitions and print the return types.
>>>> Summary of results:
>>>>
>>>>     2155 void
>>>>      873 signed integer
>>>>      494 pointer
>>>>      153 bool
>>>>       33 unsigned integer
>>>>        6 enum
>>>>     ---------------------
>>>>     3714 total
>>>>
>>>> I then used Coccinelle to find checked calls of void functions (passing
>>>> &error_fatal or &error_abort is not considered "checking" here).  These
>>>> calls become simpler if we make the functions return a useful value.  I
>>>> found a bit under 600 direct calls, and some 50 indirect calls.
>>>>
>>>> Most frequent direct calls:
>>>>
>>>>      127 object_property_set_bool
>>>>       27 qemu_opts_absorb_qdict
>>>>       16 visit_type_str
>>>>       14 visit_type_int
>>>>       10 visit_type_uint32
>>>>
>>>> Let's have a closer look at object_property_set() & friends.  Out of
>>>> almost 1000 calls, some 150 are checked.  While I'm sure many of the
>>>> unchecked calls can't actually fail, I am concerned some unchecked calls
>>>> can.
>>>>
>>>> If we adopt the convention to return a value that indicates success /
>>>> failure, we should consider converting object.h to it sooner rather than
>>>> later.
>>>>
>>>> Please understand these are rough numbers from quick & dirty scripts.
>>>
>>> FYI, I'm working on converting QemuOpts, QAPI visitors and QOM.  I keep
>>> running into bugs.  So far:
>>>
>>>      [PATCH v2 for-5.1 0/9] qemu-option: Fix corner cases and clean up
>>>      [PATCH for-5.1 0/5] qobject: Minor spring cleaning
>>>      [PATCH v2 00/14] Miscellaneous error handling fixes
>>>      [PATCH 0/4] Subject: [PATCH 0/4] smbus: SPD fixes
>>>      [PATCH 0/3] fuzz: Probably there is a better way to do this
>>>      [PATCH v2 00/15] qapi: Spring cleaning
>>>      [PATCH 00/11] More miscellaneous error handling fixes
>>>
>>> I got another one coming for QOM and qdev before I can post the
>>> conversion.
>>>
>>> Vladimir, since the conversion will mess with error_propagate(), I'd
>>> like to get it in before your auto-propagation work.
>>>
>>
>> OK, just let me know when to regenerate the series, it's not hard.
>>
>
> Hi! Is all that merged? Should I resend now?

I ran into many bugs and fell into a few rabbit holes.  I'm busy
finishing and flushing the patches.