Re: [Qemu-devel] [PATCH for-2.12 1/3] qapi: Add qdict_is_null()

[Markus Armbruster <armbru@redhat.com>]

Max Reitz <mreitz@redhat.com> writes:

> On 2017-11-14 15:57, Markus Armbruster wrote:
>> Max Reitz <mreitz@redhat.com> writes:
>> 
>>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>>> ---
>>>  include/qapi/qmp/qdict.h |  1 +
>>>  qobject/qdict.c          | 10 ++++++++++
>>>  2 files changed, 11 insertions(+)
>>>
>>> diff --git a/include/qapi/qmp/qdict.h b/include/qapi/qmp/qdict.h
>>> index fc218e7be6..c65ebfc748 100644
>>> --- a/include/qapi/qmp/qdict.h
>>> +++ b/include/qapi/qmp/qdict.h
>>> @@ -76,6 +76,7 @@ int64_t qdict_get_try_int(const QDict *qdict, const char *key,
>>>                            int64_t def_value);
>>>  bool qdict_get_try_bool(const QDict *qdict, const char *key, bool def_value);
>>>  const char *qdict_get_try_str(const QDict *qdict, const char *key);
>>> +bool qdict_is_qnull(const QDict *qdict, const char *key);
>>>  
>>>  void qdict_copy_default(QDict *dst, QDict *src, const char *key);
>>>  void qdict_set_default_str(QDict *dst, const char *key, const char *val);
>>> diff --git a/qobject/qdict.c b/qobject/qdict.c
>>> index e8f15f1132..a032ea629a 100644
>>> --- a/qobject/qdict.c
>>> +++ b/qobject/qdict.c
>>> @@ -294,6 +294,16 @@ const char *qdict_get_try_str(const QDict *qdict, const char *key)
>>>  }
>>>  
>>>  /**
>>> + * qdict_is_qnull(): Return true if the value for 'key' is QNull
>>> + */
>>> +bool qdict_is_qnull(const QDict *qdict, const char *key)
>>> +{
>>> +    QObject *value = qdict_get(qdict, key);
>>> +
>>> +    return value && value->type == QTYPE_QNULL;
>>> +}
>>> +
>>> +/**
>>>   * qdict_iter(): Iterate over all the dictionary's stored values.
>>>   *
>>>   * This function allows the user to provide an iterator, which will be
>> 
>> As far as I can tell, the new helper function is going to be used just
>> once, by bdrv_open_inherit() in PATCH 2:
>> 
>>     qdict_is_qnull(options, "backing")
>> 
>> I dislike abstracting from just one concrete instance.
>> 
>> Perhaps a more general helper could be more generally useful.  Something
>> like:
>> 
>>     qobject_is(qdict_get(options, "backing", QTYPE_QNULL))
>> 
>> There are numerous instances of
>> 
>>     !obj || qobject_type(obj) == T
>> 
>> in the tree, which could then be replaced by
>> 
>>     qobject_is(obj, T)
>> 
>> An alternative helper: macro qobject_dynamic_cast(obj, T) that returns
>> (T *)obj if obj is a T, else null.  Leads to something like
>> 
>>     qobject_dynamic_cast(qdict_get(options, "backing", QNull))
>
> If you think that's good, then that's good -- you know the QAPI code
> better then me, after all.

I'll play with it today.

> To explain myself: I thought it would be the natural extension of the
> qdict_get_try_*() functions for the QNull type.

I see now.  The name qdict_get_try_null() would make it obvious, but
what to return on success...