Kevin Wolf <kw...@redhat.com> writes: > 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!
Right! > 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. Agree. >> > >> > 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. You're right; I missed that. > The more interesting point I wasn't aware of is that virtual walks don't > need to call next_list(). visitor.h: * After visit_start_list() succeeds, the caller may visit its members * one after the other. A real visit (where @list is non-NULL) uses * visit_next_list() for traversing the linked list, while a virtual * visit (where @list 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. > 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. visitor.h: * A visitor should be used for exactly one top-level visit_type_FOO() * or virtual walk; if that is successful, the caller can optionally * call visit_complete() (useful only for output visits, but safe to * call on all visits). Then, regardless of success or failure, the * user should call visit_free() to clean up resources. It is okay to * free the visitor without completing the visit, if some other error * is detected in the meantime. The callers of property getters I can see look more or less like this: v = FOO_output_visitor_new(..., &ret); if (object_property_get(obj, name, v, errp)) { visit_complete(v, &ret); } visit_free(v); Such callers don't walk anything themselves. I think a virtual walk should be okay.