Am 30.11.2023 um 14:11 hat Markus Armbruster geschrieben:
> I understand Stefan already took this patch. I'm looking at it anyway,
> because experience has taught me to be very afraid of the string
> visitors.
>
> Kevin Wolf <kw...@redhat.com> writes:
>
> > With the introduction of list-based array properties in qdev, the string
> > output visitor has to deal with lists of non-integer elements now ('info
> > qtree' prints all properties with the string output visitor).
> >
> > Currently there is no explicit support for such lists, and the resulting
> > output is only the last element because string_output_set() always
> > replaces the output with the latest value.
>
> Yes.
>
> The string visitors were created just for QOM's object_property_parse()
> and object_property_print(). At the time, QOM properties were limited
> to scalars, and the new visitors implemented just enough of the visitor
> API to be usable with scalars. This was a Really Bad Idea(tm).
>
> Commit a020f9809cf (qapi: add string-based visitors)
> and b2cd7dee86f (qom: add generic string parsing/printing).
>
> When we wanted a QOM property for "set of NUMA node number", we extended
> the visitors to support integer lists. With fancy range syntax. Except
> for 'size'. This was another Really Bad Idea(tm).
>
> Commit 659268ffbff (qapi: make string input visitor parse int list)
> and 69e255635d0 (qapi: make string output visitor parse int list)
>
> All the visitor stuff was scandalously under-documented (that's not even
> a bad idea, just a Really Bad Habit(tm)). When we added documentation
> much later, we missed the lack of support for lists with elements other
> than integers. We later fixed that oversight for the input visitor
> only.
>
> Commit adfb264c9ed (qapi: Document visitor interfaces, add assertions)
> and c9fba9de89d (qapi: Rewrite string-input-visitor's integer and list
> parsing)
>
> Your patch extends the string output visitor to support lists of
> arbitrary scalars.
>
> > Instead of replacing the old
> > value, append comma separated values in list context.
> >
> > The difference can be observed in 'info qtree' with a 'rocker' device
> > that has a 'ports' list with more than one element.
> >
> > Signed-off-by: Kevin Wolf <kw...@redhat.com>
> > ---
> > qapi/string-output-visitor.c | 24 ++++++++++++++++++++----
> > 1 file changed, 20 insertions(+), 4 deletions(-)
>
> Missing: update of string-output-visitor.h's comment
>
> * The string output 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().
>
> It is wrong before the patch: most lists do not work. After the patch,
> only lists of scalars work. Document that, please. Maybe:
>
> * The string output visitor does not implement support for visiting
> * QAPI structs, alternates, null, or arbitrary QTypes. Only flat lists
> * are supported. It also requires a non-null list argument to
> * visit_start_list().
>
> Stolen from string-input-visitor.h's comment.
>
> Could instead use "Only lists of scalars are supported."
>
> Follow-up patch would be fine.
I guess I'm lucky that the comment I missed already failed to point out
the limitations before, so at least I didn't make anything worse!
Adding a sentence makes sense to me. I find "list of scalars" easier to
understand than "flat lists" (in particular, I would have considered a
list of structs to be flat), so I'd prefer that wording.
> >
> > diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
> > index 71ddc92b7b..c0cb72dbe4 100644
> > --- a/qapi/string-output-visitor.c
> > +++ b/qapi/string-output-visitor.c
> > @@ -74,11 +74,27 @@ static StringOutputVisitor *to_sov(Visitor *v)
> >
> > static void string_output_set(StringOutputVisitor *sov, char *string)
> > {
> > - if (sov->string) {
> > - g_string_free(sov->string, true);
> > + switch (sov->list_mode) {
> > + case LM_STARTED:
> > + sov->list_mode = LM_IN_PROGRESS;
> > + /* fall through */
> > + case LM_NONE:
> > + if (sov->string) {
> > + g_string_free(sov->string, true);
> > + }
> > + sov->string = g_string_new(string);
> > + g_free(string);
> > + break;
> > +
> > + case LM_IN_PROGRESS:
> > + case LM_END:
> > + g_string_append(sov->string, ", ");
> > + g_string_append(sov->string, string);
> > + break;
> > +
> > + default:
> > + abort();
> > }
> > - sov->string = g_string_new(string);
> > - g_free(string);
> > }
> >
> > static void string_output_append(StringOutputVisitor *sov, int64_t a)
>
> The ->list_mode state machine was designed for parsing integer lists
> with fancy range syntax. Let me try to figure out how it works.
>
> Initial state is LM_NONE.
>
> On start_list():
> LM_NONE -> LM_STARTED.
>
> On end_list():
> any -> LM_NONE.
>
> On next_list():
> any -> LM_END.
>
> On print_type_int64():
> LM_STARTED -> LM_IN_PROGRESS
> LM_IN_PROGRESS -> LM_IN_PROGRESS
> LM_END -> LM_END
>
> The two states LM_SIGNED_INTERVAL and LM_UNSIGNED_INTERVAL have never
> been used. Copy-pasta from opts-visitor.c.
>
> Only real walks call next_list(), virtual walks do not. In a real walk,
> print_type_int64() executes its LM_END case for non-first elements. In
> a virtual walk, it executes its LM_IN_PROGRESS case. This can't be
> right.
>
> What a load of confused crap.
I won't try to argue that the string visitor isn't a load of confused
crap, but I don't see how LM_END is non-first elements? It only gets set
in next_list() for the last element.
The more interesting point I wasn't aware of is that virtual walks don't
need to call next_list(). If we can fix the string visitor, doing a
virtual walk might have made more sense for the array property getter
than construction a temporary real list?
Or can't you mix virtual and real with the same visitor? Because I
assume the callers of property getters are doing a real walk.
Kevin
