Re: [Qemu-devel] [PATCH v3 1/7] qapi: use qemu_strtoi64() in parse_str

[David Hildenbrand]

>> 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 address@hidden (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 :)

> 
>  * 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.

> 
>> 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 :)

-- 

Thanks,

David / dhildenb