David Hildenbrand <da...@redhat.com> writes:
>>> Would it be valid to do something like this (skipping elements without a
>>> proper visit_type_int)
>>>
>>> visit_start_list();
>>> visit_next_list(); more input, returns "there's more"
>>> visit_next_list(); parses "1-3,", buffers 2-3, skips over 1
>>> visit_type_int(); returns 2
>>> ...
>>
>> Excellent question!
>>
>> Here's the relevant part of visit_start_list()'s contract in visitor.h:
>>
>> * After visit_start_list() succeeds, the caller may visit its members
>> * one after the other. A real visit (where @obj is non-NULL) uses
>> * visit_next_list() for traversing the linked list, while a virtual
>> * visit (where @obj is NULL) uses other means. For each list
>> * element, call the appropriate visit_type_FOO() with name set to
>> * NULL and obj set to the address of the value member of the list
>> * element. Finally, visit_end_list() needs to be called with the
>> * same @list to clean up, even if intermediate visits fail. See the
>> * examples above.
>>
>> So, you *may* visit members, and you *must* call visit_end_list().
>>
>> But what are "real" and "virtual" visits? Again, the contract:
>>
>> * @list must be non-NULL for a real walk, in which case @size
>> * determines how much memory an input or clone visitor will allocate
>> * into *@list (at least sizeof(GenericList)). Some visitors also
>> * allow @list to be NULL for a virtual walk, in which case @size is
>> * ignored.
>>
>> I'm not sure whether I just decreased or increased global confusion ;)
>
> I was reading over these comments and got slightly confused :)
>
>>
>> The file comment explains:
>>
>> * The QAPI schema defines both a set of C data types, and a QMP wire
>> * format. QAPI objects can contain references to other QAPI objects,
>> * resulting in a directed acyclic graph. QAPI also generates visitor
>> * functions to walk these graphs. This file represents the interface
>> * for doing work at each node of a QAPI graph; it can also be used
>> * for a virtual walk, where there is no actual QAPI C struct.
>>
>> A real walk with an output visitor works like this (error handling
>> omitted for now):
>>
>> Error *err = NULL;
>> intList *tail;
>> size_t size = sizeof(**obj);
>>
>> // fetch list's head into *obj, start the list in output
>> visit_start_list(v, name, (GenericList **)obj, size, &err);
>>
>> // iterate over list's tails
>> // below the hood, visit_next_list() iterates over the GenericList
>> for (tail = *obj; tail;
>> tail = (intList *)visit_next_list(v, (GenericList *)tail, size)) {
>> // visit current tail's first member, add it to output
>> visit_type_int(v, NULL, &tail->value, &err);
>> }
>>
>> // end the list in output
>> visit_end_list(v, (void **)obj);
>>
>> The exact same code works for a real walk with an input visitor, where
>> visit_next_list() iterates over the *input* and builds up the
>> GenericList. Compare qobject_input_next_list() and
>> qobject_output_next_list().
>>
>> The code above is a straight copy of generated visit_type_intList() with
>> error handling cut and comments added.
>>
>> A real walk works on a QAPI-generated C type. QAPI generates a real
>> walk for each such type. Open-coding a real walk would senselessly
>> duplicate the generated one, so we don't. Thus, a real walk always
>> visits each member.
>>
>> Okay, I lied: it visits each member until it runs into an error and
>> bails out. See generated visit_type_intList() in
>> qapi/qapi-builtin-visit.c.
>>
>> A virtual walk doesn't work with a GenericList *. Calling
>> visit_next_list() makes no sense then. visitor.h gives this example of
>> a virtual walk:
>
> Alright, so we must not support virtual walks. But supporting it would
> not harm :)
Hmm, let me check something... aha! Both string-input-visitor.h and
string-output-visitor.h have this comment:
* The string input visitor does not implement support for visiting
* QAPI structs, alternates, null, or arbitrary QTypes. It also
* requires a non-null list argument to visit_start_list().
The last sentence means virtual walks are not supported. We owe thanks
to Eric Blake for his commit d9f62dde130 :)
>>
>> * Thus, a virtual walk corresponding to '{ "list": [1, 2] }' looks
>> * like:
>> *
>> * <example>
>> * Visitor *v;
>> * Error *err = NULL;
>> * int value;
>> *
>> * v = FOO_visitor_new(...);
>> * visit_start_struct(v, NULL, NULL, 0, &err);
>> * if (err) {
>> * goto out;
>> * }
>> * visit_start_list(v, "list", NULL, 0, &err);
>> * if (err) {
>> * goto outobj;
>> * }
>> * value = 1;
>> * visit_type_int(v, NULL, &value, &err);
>> * if (err) {
>> * goto outlist;
>> * }
>> * value = 2;
>> * visit_type_int(v, NULL, &value, &err);
>> * if (err) {
>> * goto outlist;
>> * }
>> * outlist:
>> * visit_end_list(v, NULL);
>> * if (!err) {
>> * visit_check_struct(v, &err);
>> * }
>> * outobj:
>> * visit_end_struct(v, NULL);
>> * out:
>> * error_propagate(errp, err);
>> * visit_free(v);
>>
>> Why could this be useful?
>>
>> 1. With the QObject input visitor, it's an alternative way to
>> destructure a QObject into a bunch of C variables. The "obvious" way
>> would be calling the QObject accessors. By using the visitors you
>> get much the error checking code for free. YMMV.
>>
>> 2. With the QObject output visitor, it's an alternative way to build up
>> a QObject. The "obvious" way would be calling the constructors
>> directly.
>>
>> 3. With the string input / output visitors, it's a way to parse / format
>> string visitor syntax without having to construct the C type first.
>>
>> Right now, we have no virtual list walks outside tests. We do have
>> virtual struct walks outside tests.
>>
>>> Or mixing types
>>>
>>> visit_start_list();
>>> visit_next_list();
>>> visit_type_int64();
>>> visit_next_list();
>>> visit_type_uint64();
>>
>> Another excellent question.
>>
>> QAPI can only express homogeneous lists, i.e. lists of some type T.
>>
>> The generated visit_type_TList call the same visit_type_T() for all list
>> members.
>
> Okay, my point would be: Somebody coding its own visit code should not
> assume this to work.
>
>>
>> QAPI type 'any' is the top type, but visit_type_anyList() still calls
>> visit_type_any() for all list members.
>>
>> Virtual walks could of course do anything. Since they don't exist
>> outside tests, we can outlaw doing things that cause us trouble.
>>
>> The virtual walks we currently have in tests/ seem to walk only
>> homogeneous lists, i.e. always call the same visit_type_T().
>
> Okay, so bailing out if types are switched (e.g. just about to report a
> range of uin64_t and somebody asks for a int64_t) is valid.
I think that would be okay.
>>
>>> IOW, can I assume that after every visit_next_list(), visit_type_X is
>>> called, and that X remains the same for one pass over the list?
>>
>> As far as I can tell, existing code is just fine with that assumption.
>> We'd have to write it into the contract, though.
>>
>>> Also, I assume it is supposed to work like this
>>>
>>> visit_start_list();
>>> visit_next_list(); more input, returns "there's more"
>>> visit_type_int(); returns 1 (parses 1-3, buffers 1-3)
>>> visit_type_int(); returns 1
>>> visit_type_int(); returns 1
>>> visit_next_list(); more input, unbuffers 1
>>> visit_type_int(); returns 2
>>>
>>> So unbuffering actually happens on visit_next_list()?
>>
>> I believe this violates the contract.
>>
>> If it's a real walk, the contract wants you to call visit_next_list()
>> between subsequent visit_type_int().
>>
>> If it's a virtual walk, calling visit_next_list() makes no sense.
>>
>> More questions?
>>
>
> Thanks for the excessive answer! I think that should be enough to come
> up with a RFC of a *rewrite* of the string input visitor :)
You're welcome! I love great questions, they make me *think*.
Besides, if something's worth doing, it's probably worth overdoing ;)
