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Re: [Qemu-ppc] [Qemu-devel] [PATCH v9 34/37] qapi: Simplify semantics of
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From: |
Markus Armbruster |
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Subject: |
Re: [Qemu-ppc] [Qemu-devel] [PATCH v9 34/37] qapi: Simplify semantics of visit_next_list() |
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Date: |
Thu, 28 Jan 2016 14:37:41 +0100 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
Eric Blake <address@hidden> writes:
> We have two uses of list visits in the entire code base: one in
> spapr_drc (which completely avoids visit_next_list(), feeding in
> integers from a different source than uint8List), and one in
> qapi-visit.py (that is, all other list visitors are generated
> in qapi-visit.c, and share the same paradigm based on a qapi
> FooList type). What's more, the semantics of the list visit are
> somewhat baroque, with the following pseudocode when FooList is
> used:
>
> start()
> prev = head
> while (cur = next(prev)) {
> visit(cur)
Actually, we pass &cur->value to the element visit.
> prev = &cur
> }
>
> Note that these semantics (advance before visit) requires that
> the first call to next() return the list head, while all other
> calls return the next element of the list; that is, every visitor
> implementation is required to track extra state to decide whether
> to return the input as-is, or to advance. It also requires an
> argument of 'GenericList **' to next(), solely because the first
> iteration might need to modify the caller's GenericList head, so
> that all other calls have to do a layer of dereferencing.
>
> We can greatly simplify things by hoisting the special case
> into the start() routine, and flipping the order in the loop
> to visit before advance:
>
> start(head)
> element = *head
> while (element) {
> visit(element)
> element = next(element)
> }
@element isn't a list element, it's a list node. Suggest
start(head)
tail = *head
while (tail) {
visit(&tail->value)
tail = next(tail)
}
Of course, this pseudo-code just screams to be a for-loop instead:
for (tail = *head; tail; tail = next(tail)) ...
May or may not be an improvement of the real code. The pseudo-code
should follow the real code.
> With the simpler semantics, visitors have less state to track,
> the argument to next() is reduced to 'GenericList *', and it
> also becomes obvious whether an input visitor is allocating a
> FooList during visit_start_list() (rather than the old way of
> not knowing if an allocation happened until the first
> visit_next_list()).
>
> The signature of visit_start_list() is chosen to match
> visit_start_struct(), with the new parameter after 'name'.
>
> The spapr_drc case requires that visit_start_list() has to pay
> attention to whether visit_next_list() will even be used to
> visit a FooList qapi struct; this is done by passing NULL for
> list, similarly to how NULL is passed to visit_start_struct()
> when a qapi type is not used in those visits. It was easy to
> provide these semantics for qmp-output and dealloc visitors,
> and a bit harder for qmp-input (it required hoisting the
> advance of the current qlist entry out of qmp_input_next_list()
> into qmp_input_get_object()). But it turned out that the
> string and opts visitors munge enough state during
> visit_next_list() to make those conversions simpler if they
> require a GenericList visit for now; an assertion will remind
> us to adjust things if we need the semantics in the future.
>
> Signed-off-by: Eric Blake <address@hidden>
> Reviewed-by: Marc-André Lureau <address@hidden>
>
> ---
> v9: no change
> v8: consistent parameter order, fix qmp_input_get_next_type() to not
> skip list entries
> v7: new patch
> ---
> hw/ppc/spapr_drc.c | 2 +-
> include/qapi/visitor-impl.h | 5 ++--
> include/qapi/visitor.h | 45 +++++++++++++++++--------------
> qapi/opts-visitor.c | 32 +++++++++-------------
> qapi/qapi-dealloc-visitor.c | 29 +++++---------------
> qapi/qapi-visit-core.c | 7 ++---
> qapi/qmp-input-visitor.c | 64
> +++++++++++++++++++++-----------------------
> qapi/qmp-output-visitor.c | 21 +++------------
> qapi/string-input-visitor.c | 34 +++++++++++------------
> qapi/string-output-visitor.c | 36 ++++++++-----------------
> scripts/qapi-visit.py | 21 ++++++++-------
> 11 files changed, 126 insertions(+), 170 deletions(-)
Diffstat suggests it's indeed a simplification.
>
> diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
> index 3b27caa..41f2da0 100644
> --- a/hw/ppc/spapr_drc.c
> +++ b/hw/ppc/spapr_drc.c
> @@ -299,7 +299,7 @@ static void prop_get_fdt(Object *obj, Visitor *v, const
> char *name,
> int i;
> prop = fdt_get_property_by_offset(fdt, fdt_offset, &prop_len);
> name = fdt_string(fdt, fdt32_to_cpu(prop->nameoff));
> - visit_start_list(v, name, &err);
> + visit_start_list(v, name, NULL, &err);
> if (err) {
> error_propagate(errp, err);
> return;
> diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
> index 248b1e5..acbe7d6 100644
> --- a/include/qapi/visitor-impl.h
> +++ b/include/qapi/visitor-impl.h
> @@ -40,9 +40,10 @@ struct Visitor
> void (*end_implicit_struct)(Visitor *v);
>
> /* Must be set */
> - void (*start_list)(Visitor *v, const char *name, Error **errp);
> + void (*start_list)(Visitor *v, const char *name, GenericList **list,
> + Error **errp);
> /* Must be set */
> - GenericList *(*next_list)(Visitor *v, GenericList **list, Error **errp);
> + GenericList *(*next_list)(Visitor *v, GenericList *element, Error
> **errp);
You rename the parameter here, but not in the implementations.
The parameter isn't a list element, it's a list node. If we want to
rename it, what about tail?
> /* Must be set */
> void (*end_list)(Visitor *v);
>
> diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
> index e5dcde4..4638863 100644
> --- a/include/qapi/visitor.h
> +++ b/include/qapi/visitor.h
> @@ -1,6 +1,7 @@
> /*
> * Core Definitions for QAPI Visitor Classes
> *
> + * Copyright (C) 2015 Red Hat, Inc.
It's 2016 now. I'd make it 2012-2016.
> * Copyright IBM, Corp. 2011
> *
> * Authors:
> @@ -111,32 +112,36 @@ void visit_end_implicit_struct(Visitor *v);
> /**
> * Prepare to visit a list tied to an object key @name.
> * @name will be NULL if this is visited as part of another list.
> - * After calling this, the elements must be collected until
> - * visit_next_list() returns NULL, then visit_end_list() must be
> - * used to complete the visit.
> - */
> -void visit_start_list(Visitor *v, const char *name, Error **errp);
> -/**
> - * Iterate over a GenericList during a list visit.
> - * @list must not be NULL; on the first call, @list contains the
> - * address of the list head, and on subsequent calls address@hidden must be
> - * the previously returned value. Must be called in a loop until a
> - * NULL return or error occurs; for each non-NULL return, the caller
> - * must then call the appropriate visit_type_*() for the element type
> - * of the list, with that function's name parameter set to NULL.
> + * Input visitors malloc a qapi List struct into address@hidden,
QAPI
What's a QAPI list struct? I guess you mean GenericList.
> or set it to
> + * NULL if there are no elements in the list;
So start_list() now needs to know whether the list is empty.
visit_start_struct() has an "if @obj is not NULL" clause here. Do we
need the equivalent "if @list is not NULL"? Ah, you do that further
down! Can we structure the two comments the same way?
> and output visitors
> + * expect address@hidden to point to the start of the list, if any.
Perhaps "to the first list node, if any", and give GenericList a better
comment in PATCH 21:
/*
* Generic list node
* Any generated QAPI FOOList struct pointer can be safely cast to
* GenericList * and dereferenced.
*/
Of course, this actually assumes uniform pointer representation, which
is not guaranteed by the standard.
Should we mention output visitors don't change address@hidden Same for
visit_start_struct(), by the way.
What about the dealloc visitor?
> On
> + * return, if address@hidden is non-NULL, the caller should enter a loop
> + * visiting the current element, then using visit_next_list() to
> + * advance to the next element, until that returns NULL; then
> + * visit_end_list() must be used to complete the visit.
For visit_start_struct() this part reads:
The
* caller then makes a series of visit calls for each key expected in
* the object, where those visits set their respective obj parameter
* to the address of a member of the qapi struct, and follows
* everything by a call to visit_end_struct() to clean up resources.
Following that pattern here, I get:
The caller then visits the list elements in turn, where those visits
get passed the address of the list element within the QAPI list node.
The caller normally uses visit_next_list() to step through the list.
When done, it must call visit_end_list() to clean up.
> *
> - * Note that for some visitors (qapi-dealloc and qmp-output), when a
> - * qapi GenericList linked list is not being used (comparable to when
> - * a NULL obj is used for visit_start_struct()), it is acceptable to
> - * bypass the use of visit_next_list() and just directly call the
> - * appropriate visit_type_*() for each element in between the
> - * visit_start_list() and visit_end_list() calls.
> + * If supported by a visitor, @list can be NULL to indicate that there
by the visitor
> + * is no qapi List struct, and that the upcoming visit calls are
> + * parsing input to or creating output from some other representation;
> + * in this case, visit_next_list() will not be needed, but
> + * visit_end_list() is still mandatory.
You first explain normal usage, then add this paragraph to explain
special usage. I like that better than the visit_start_struct()
comment, where the special usage comes earlier.
> *
> * FIXME: For input visitors, address@hidden can be assigned here even if
> * later visits will fail; this can lead to memory leaks if clients
> * aren't careful.
> */
> -GenericList *visit_next_list(Visitor *v, GenericList **list, Error **errp);
> +void visit_start_list(Visitor *v, const char *name, GenericList **list,
> + Error **errp);
> +/**
> + * Iterate over a GenericList during a list visit.
> + * Before calling this function, the caller should use the appropriate
> + * visit_type_FOO() for the current list element at @element->value, and
> + * check for errors. @element must not be NULL; on the first iteration,
> + * it should be the value in *list after visit_start_list(); on other
> + * calls it should be the previous return value. This function
> + * returns NULL once there are no further list elements.
> + */
I feel if we get visit_start_list()'s comment right, then this one can
concentrate on what this function does, and leave intended usage of the
start/next/end team to visit_start_list()'s comment.
> +GenericList *visit_next_list(Visitor *v, GenericList *element, Error **errp);
> /**
> * Complete the list started earlier.
> * Must be called after any successful use of visit_start_list(),
> diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
> index b469573..c5a7396 100644
> --- a/qapi/opts-visitor.c
> +++ b/qapi/opts-visitor.c
> @@ -21,9 +21,8 @@
> enum ListMode
> {
> LM_NONE, /* not traversing a list of repeated options */
> - LM_STARTED, /* opts_start_list() succeeded */
>
> - LM_IN_PROGRESS, /* opts_next_list() has been called.
> + LM_IN_PROGRESS, /* opts_next_list() ready to be called.
> *
> * Generating the next list link will consume the
> most
> * recently parsed QemuOpt instance of the repeated
> @@ -212,35 +211,32 @@ lookup_distinct(const OptsVisitor *ov, const char
> *name, Error **errp)
>
>
> static void
> -opts_start_list(Visitor *v, const char *name, Error **errp)
> +opts_start_list(Visitor *v, const char *name, GenericList **list, Error
> **errp)
I'd break this line before Error.
> {
> OptsVisitor *ov = to_ov(v);
>
> /* we can't traverse a list in a list */
> assert(ov->list_mode == LM_NONE);
> + /* we don't support visits without GenericList, yet */
> + assert(list);
Aha, this is why you wrote "If supported by a visitor". The visitors
better document what they support then.
> ov->repeated_opts = lookup_distinct(ov, name, errp);
> - if (ov->repeated_opts != NULL) {
> - ov->list_mode = LM_STARTED;
> + if (ov->repeated_opts) {
> + ov->list_mode = LM_IN_PROGRESS;
> + *list = g_new0(GenericList, 1);
> + } else {
> + *list = NULL;
> }
> }
>
>
> static GenericList *
> -opts_next_list(Visitor *v, GenericList **list, Error **errp)
> +opts_next_list(Visitor *v, GenericList *list, Error **errp)
> {
> OptsVisitor *ov = to_ov(v);
> - GenericList **link;
>
> switch (ov->list_mode) {
> - case LM_STARTED:
> - ov->list_mode = LM_IN_PROGRESS;
> - link = list;
> - break;
> -
> case LM_SIGNED_INTERVAL:
> case LM_UNSIGNED_INTERVAL:
> - link = &(*list)->next;
> -
> if (ov->list_mode == LM_SIGNED_INTERVAL) {
> if (ov->range_next.s < ov->range_limit.s) {
> ++ov->range_next.s;
> @@ -261,7 +257,6 @@ opts_next_list(Visitor *v, GenericList **list, Error
> **errp)
> g_hash_table_remove(ov->unprocessed_opts, opt->name);
> return NULL;
> }
> - link = &(*list)->next;
> break;
> }
>
> @@ -269,8 +264,8 @@ opts_next_list(Visitor *v, GenericList **list, Error
> **errp)
> abort();
> }
>
> - *link = g_malloc0(sizeof **link);
> - return *link;
> + list->next = g_new0(GenericList, 1);
> + return list->next;
> }
>
>
> @@ -279,8 +274,7 @@ opts_end_list(Visitor *v)
> {
> OptsVisitor *ov = to_ov(v);
>
> - assert(ov->list_mode == LM_STARTED ||
> - ov->list_mode == LM_IN_PROGRESS ||
> + assert(ov->list_mode == LM_IN_PROGRESS ||
> ov->list_mode == LM_SIGNED_INTERVAL ||
> ov->list_mode == LM_UNSIGNED_INTERVAL);
> ov->repeated_opts = NULL;
Only slight simplification for this visitor: we lose LM_STARTED.
> diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
> index a89e6d1..839049e 100644
> --- a/qapi/qapi-dealloc-visitor.c
> +++ b/qapi/qapi-dealloc-visitor.c
> @@ -20,7 +20,6 @@
> typedef struct StackEntry
> {
> void *value;
> - bool is_list_head;
> QTAILQ_ENTRY(StackEntry) node;
> } StackEntry;
>
> @@ -41,10 +40,6 @@ static void qapi_dealloc_push(QapiDeallocVisitor *qov,
> void *value)
>
> e->value = value;
>
> - /* see if we're just pushing a list head tracker */
> - if (value == NULL) {
> - e->is_list_head = true;
> - }
> QTAILQ_INSERT_HEAD(&qov->stack, e, node);
> }
>
> @@ -92,31 +87,19 @@ static void qapi_dealloc_end_implicit_struct(Visitor *v)
> }
> }
>
> -static void qapi_dealloc_start_list(Visitor *v, const char *name, Error
> **errp)
> +static void qapi_dealloc_start_list(Visitor *v, const char *name,
> + GenericList **list, Error **errp)
> {
> QapiDeallocVisitor *qov = to_qov(v);
> qapi_dealloc_push(qov, NULL);
Do we still need to push/pop for a list?
If yes, can we push list instead of NULL? Pointers always become more
complicated when they can be null...
> }
>
> -static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp,
> +static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList *list,
> Error **errp)
> {
> - GenericList *list = *listp;
> - QapiDeallocVisitor *qov = to_qov(v);
> - StackEntry *e = QTAILQ_FIRST(&qov->stack);
> -
> - if (e && e->is_list_head) {
> - e->is_list_head = false;
> - return list;
> - }
> -
> - if (list) {
> - list = list->next;
> - g_free(*listp);
> - return list;
> - }
> -
> - return NULL;
> + GenericList *next = list->next;
> + g_free(list);
> + return next;
> }
Okay, this is actually a more worthwhile simplification.
>
> static void qapi_dealloc_end_list(Visitor *v)
> diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> index 9506a02..f391a70 100644
> --- a/qapi/qapi-visit-core.c
> +++ b/qapi/qapi-visit-core.c
> @@ -67,12 +67,13 @@ void visit_end_implicit_struct(Visitor *v)
> }
> }
>
> -void visit_start_list(Visitor *v, const char *name, Error **errp)
> +void visit_start_list(Visitor *v, const char *name, GenericList **list,
> + Error **errp)
> {
> - v->start_list(v, name, errp);
> + v->start_list(v, name, list, errp);
> }
>
> -GenericList *visit_next_list(Visitor *v, GenericList **list, Error **errp)
> +GenericList *visit_next_list(Visitor *v, GenericList *list, Error **errp)
> {
> assert(list);
> return v->next_list(v, list, errp);
> diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
> index f256d9e..82f9333 100644
> --- a/qapi/qmp-input-visitor.c
> +++ b/qapi/qmp-input-visitor.c
> @@ -44,16 +44,20 @@ static QObject *qmp_input_get_object(QmpInputVisitor *qiv,
> const char *name,
> bool consume)
> {
> - QObject *qobj = qiv->stack[qiv->nb_stack - 1].obj;
> + StackObject *so = &qiv->stack[qiv->nb_stack - 1];
> + QObject *qobj = so->obj;
>
> if (qobj) {
> if (name && qobject_type(qobj) == QTYPE_QDICT) {
> - if (qiv->stack[qiv->nb_stack - 1].h && consume) {
> - g_hash_table_remove(qiv->stack[qiv->nb_stack - 1].h, name);
> + if (so->h && consume) {
> + g_hash_table_remove(so->h, name);
> + }
> + qobj = qdict_get(qobject_to_qdict(qobj), name);
> + } else if (so->entry) {
> + qobj = qlist_entry_obj(so->entry);
> + if (consume) {
> + so->entry = qlist_next(so->entry);
> }
> - return qdict_get(qobject_to_qdict(qobj), name);
> - } else if (qiv->stack[qiv->nb_stack - 1].entry) {
> - return qlist_entry_obj(qiv->stack[qiv->nb_stack - 1].entry);
> }
> }
>
Much easier to read if split into two steps:
1. Capture the pointer to the top of the stack in a variable
@@ -44,16 +44,17 @@ static QObject *qmp_input_get_object(QmpInputVisitor *qiv,
const char *name,
bool consume)
{
- QObject *qobj = qiv->stack[qiv->nb_stack - 1].obj;
+ StackObject *so = &qiv->stack[qiv->nb_stack - 1];
+ QObject *qobj = so->obj;
if (qobj) {
if (name && qobject_type(qobj) == QTYPE_QDICT) {
- if (qiv->stack[qiv->nb_stack - 1].h && consume) {
- g_hash_table_remove(qiv->stack[qiv->nb_stack - 1].h, name);
+ if (so->h && consume) {
+ g_hash_table_remove(so->h, name);
}
return qdict_get(qobject_to_qdict(qobj), name);
- } else if (qiv->stack[qiv->nb_stack - 1].entry) {
- return qlist_entry_obj(qiv->stack[qiv->nb_stack - 1].entry);
+ } else if (so->entry) {
+ return qlist_entry_obj(so->entry);
}
}
2. Make the actual change
@@ -52,9 +52,12 @@ static QObject *qmp_input_get_object(QmpInputVisitor *qiv,
if (so->h && consume) {
g_hash_table_remove(so->h, name);
}
- return qdict_get(qobject_to_qdict(qobj), name);
+ qobj = qdict_get(qobject_to_qdict(qobj), name);
} else if (so->entry) {
- return qlist_entry_obj(so->entry);
+ qobj = qlist_entry_obj(so->entry);
+ if (consume) {
+ so->entry = qlist_next(so->entry);
+ }
}
}
return qobj;
I'd call the new variable tos (top of stack) instead of so. Needs a
matching rename in qmp_input_next_list().
Note that @consume is true unless we're called from
qmp_input_get_next_type(). What is it supposed to do? Can it be true
in the place where you add a use?
Since we're cleaning up the function anyway in step 1, I'd be tempted to
reduce its nesting:
if (!qobj) {
return NULL;
} else if (name && qobject_type(qobj) == QTYPE_QDICT) {
if (tos->h && consume) {
g_hash_table_remove(tos->h, name);
}
return qdict_get(qobject_to_qdict(qobj), name);
} else if (tos->entry) {
return qlist_entry_obj(tos->entry);
} else {
return qobj;
}
Immediately begs the question what the four cases mean. Unobvious
enough to justify comments, I think.
What's the stack's contents? You cleaned that up and documented it in
qmp-output-visitor.c. Same treatment here?
> @@ -66,7 +70,8 @@ static void qdict_add_key(const char *key, QObject *obj,
> void *opaque)
> g_hash_table_insert(h, (gpointer) key, NULL);
> }
>
> -static void qmp_input_push(QmpInputVisitor *qiv, QObject *obj, Error **errp)
> +static void qmp_input_push(QmpInputVisitor *qiv, QObject *obj,
> + const QListEntry *entry, Error **errp)
> {
> GHashTable *h;
>
if (qiv->nb_stack >= QIV_STACK_SIZE) {
error_setg(errp, "An internal buffer overran");
return;
}
Aside: this is stupid, realloc() exists. It's less stupid than the
qmp-output-visitor.c's tail queue, though.
> @@ -76,7 +81,7 @@ static void qmp_input_push(QmpInputVisitor *qiv, QObject
> *obj, Error **errp)
> }
>
> qiv->stack[qiv->nb_stack].obj = obj;
> - qiv->stack[qiv->nb_stack].entry = NULL;
> + qiv->stack[qiv->nb_stack].entry = entry;
> qiv->stack[qiv->nb_stack].h = NULL;
>
> if (qiv->strict && qobject_type(obj) == QTYPE_QDICT) {
> @@ -138,7 +143,7 @@ static void qmp_input_start_struct(Visitor *v, const char
> *name, void **obj,
> return;
> }
>
> - qmp_input_push(qiv, qobj, &err);
> + qmp_input_push(qiv, qobj, NULL, &err);
> if (err) {
> error_propagate(errp, err);
> return;
The stack entry for a struct being visited has obj = its source QDict,
entry = NULL.
> @@ -158,10 +163,12 @@ static void qmp_input_start_implicit_struct(Visitor *v,
> void **obj,
> }
> }
>
> -static void qmp_input_start_list(Visitor *v, const char *name, Error **errp)
> +static void qmp_input_start_list(Visitor *v, const char *name,
> + GenericList **list, Error **errp)
> {
> QmpInputVisitor *qiv = to_qiv(v);
> QObject *qobj = qmp_input_get_object(qiv, name, true);
> + const QListEntry *entry;
>
> if (!qobj || qobject_type(qobj) != QTYPE_QLIST) {
> error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
> @@ -169,37 +176,28 @@ static void qmp_input_start_list(Visitor *v, const char
> *name, Error **errp)
> return;
> }
>
> - qmp_input_push(qiv, qobj, errp);
> + entry = qlist_first(qobject_to_qlist(qobj));
> + qmp_input_push(qiv, qobj, entry, errp);
The stack entry for a list being visited has obj = its source QList,
entry = NULL.
> + if (list) {
> + if (entry) {
> + *list = g_new0(GenericList, 1);
> + } else {
> + *list = NULL;
> + }
> + }
> }
>
> -static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
> +static GenericList *qmp_input_next_list(Visitor *v, GenericList *list,
> Error **errp)
> {
> QmpInputVisitor *qiv = to_qiv(v);
> - GenericList *entry;
> StackObject *so = &qiv->stack[qiv->nb_stack - 1];
> - bool first;
>
> - if (so->entry == NULL) {
> - so->entry = qlist_first(qobject_to_qlist(so->obj));
> - first = true;
> - } else {
> - so->entry = qlist_next(so->entry);
> - first = false;
> - }
> -
> - if (so->entry == NULL) {
> + if (!so->entry) {
> return NULL;
> }
> -
> - entry = g_malloc0(sizeof(*entry));
> - if (first) {
> - *list = entry;
> - } else {
> - (*list)->next = entry;
> - }
> -
> - return entry;
> + list->next = g_new0(GenericList, 1);
> + return list->next;
> }
>
>
> @@ -375,7 +373,7 @@ QmpInputVisitor *qmp_input_visitor_new(QObject *obj)
> v->visitor.optional = qmp_input_optional;
> v->visitor.get_next_type = qmp_input_get_next_type;
>
> - qmp_input_push(v, obj, NULL);
> + qmp_input_push(v, obj, NULL, NULL);
The stack entry for the root value being visited has obj = its source
QObject, entry = NULL.
> qobject_incref(obj);
>
> return v;
> diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
> index 5376948..913f378 100644
> --- a/qapi/qmp-output-visitor.c
> +++ b/qapi/qmp-output-visitor.c
> @@ -21,7 +21,6 @@
> typedef struct QStackEntry
> {
> QObject *value;
> - bool is_list_head;
> QTAILQ_ENTRY(QStackEntry) node;
> } QStackEntry;
>
> @@ -51,9 +50,6 @@ static void qmp_output_push_obj(QmpOutputVisitor *qov,
> QObject *value)
> assert(qov->root);
> assert(value);
> e->value = value;
> - if (qobject_type(e->value) == QTYPE_QLIST) {
> - e->is_list_head = true;
> - }
> QTAILQ_INSERT_HEAD(&qov->stack, e, node);
> }
>
> @@ -122,7 +118,8 @@ static void qmp_output_end_struct(Visitor *v)
> qmp_output_pop(qov, QTYPE_QDICT);
> }
>
> -static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
> +static void qmp_output_start_list(Visitor *v, const char *name,
> + GenericList **listp, Error **errp)
> {
> QmpOutputVisitor *qov = to_qov(v);
> QList *list = qlist_new();
> @@ -131,20 +128,10 @@ static void qmp_output_start_list(Visitor *v, const
> char *name, Error **errp)
> qmp_output_push(qov, list);
> }
>
> -static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp,
> +static GenericList *qmp_output_next_list(Visitor *v, GenericList *list,
> Error **errp)
> {
> - GenericList *list = *listp;
> - QmpOutputVisitor *qov = to_qov(v);
> - QStackEntry *e = QTAILQ_FIRST(&qov->stack);
> -
> - assert(e);
> - if (e->is_list_head) {
> - e->is_list_head = false;
> - return list;
> - }
> -
> - return list ? list->next : NULL;
> + return list->next;
> }
>
> static void qmp_output_end_list(Visitor *v)
A simple one, for a change.
> diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
> index 610c233..582a62a 100644
> --- a/qapi/string-input-visitor.c
> +++ b/qapi/string-input-visitor.c
> @@ -23,8 +23,6 @@ struct StringInputVisitor
> {
> Visitor visitor;
>
> - bool head;
> -
> GList *ranges;
> GList *cur_range;
> int64_t cur;
> @@ -123,11 +121,19 @@ error:
> }
>
> static void
> -start_list(Visitor *v, const char *name, Error **errp)
> +start_list(Visitor *v, const char *name, GenericList **list, Error **errp)
> {
> StringInputVisitor *siv = to_siv(v);
> + Error *err = NULL;
>
> - parse_str(siv, errp);
> + /* We don't support visits without a GenericList, yet */
> + assert(list);
> +
> + parse_str(siv, &err);
> + if (err) {
> + error_propagate(errp, err);
> + return;
> + }
Should you set address@hidden = NULL on error?
Does this handle parse_str() error correctly before your patch?
>
> siv->cur_range = g_list_first(siv->ranges);
> if (siv->cur_range) {
> @@ -135,14 +141,16 @@ start_list(Visitor *v, const char *name, Error **errp)
> if (r) {
> siv->cur = r->begin;
> }
> + *list = g_new0(GenericList, 1);
> + } else {
> + *list = NULL;
> }
> }
If it does, then this must be the reason you have to bail out on error.
>
> static GenericList *
> -next_list(Visitor *v, GenericList **list, Error **errp)
> +next_list(Visitor *v, GenericList *list, Error **errp)
> {
> StringInputVisitor *siv = to_siv(v);
> - GenericList **link;
> Range *r;
>
> if (!siv->ranges || !siv->cur_range) {
> @@ -166,22 +174,13 @@ next_list(Visitor *v, GenericList **list, Error **errp)
> siv->cur = r->begin;
> }
>
> - if (siv->head) {
> - link = list;
> - siv->head = false;
> - } else {
> - link = &(*list)->next;
> - }
> -
> - *link = g_malloc0(sizeof **link);
> - return *link;
> + list->next = g_new0(GenericList, 1);
> + return list->next;
> }
>
> static void
> end_list(Visitor *v)
> {
> - StringInputVisitor *siv = to_siv(v);
> - siv->head = true;
> }
>
> static void parse_type_int64(Visitor *v, const char *name, int64_t *obj,
> @@ -361,6 +360,5 @@ StringInputVisitor *string_input_visitor_new(const char
> *str)
> v->visitor.optional = parse_optional;
>
> v->string = str;
> - v->head = true;
> return v;
> }
> diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
> index fd917a4..7209d80 100644
> --- a/qapi/string-output-visitor.c
> +++ b/qapi/string-output-visitor.c
> @@ -57,7 +57,6 @@ struct StringOutputVisitor
> Visitor visitor;
> bool human;
> GString *string;
> - bool head;
> ListMode list_mode;
> union {
> int64_t s;
> @@ -265,40 +264,29 @@ static void print_type_number(Visitor *v, const char
> *name, double *obj,
> }
>
> static void
> -start_list(Visitor *v, const char *name, Error **errp)
> +start_list(Visitor *v, const char *name, GenericList **list, Error **errp)
> {
> StringOutputVisitor *sov = to_sov(v);
>
> /* we can't traverse a list in a list */
> assert(sov->list_mode == LM_NONE);
> - sov->list_mode = LM_STARTED;
> - sov->head = true;
> + /* We don't support visits without a GenericList, yet */
> + assert(list);
> + /* List handling is only needed if there are at least two elements */
> + if (*list && (*list)->next) {
> + sov->list_mode = LM_STARTED;
> + }
Contradicts the comment next to LM_STARTED: /* start_list() succeeded */
Why do you need to stay in state LM_NONE for shorter lists now?
> }
>
> static GenericList *
> -next_list(Visitor *v, GenericList **list, Error **errp)
> +next_list(Visitor *v, GenericList *list, Error **errp)
> {
> StringOutputVisitor *sov = to_sov(v);
> - GenericList *ret = NULL;
> - if (*list) {
> - if (sov->head) {
> - ret = *list;
> - } else {
> - ret = (*list)->next;
> - }
> + GenericList *ret = list->next;
>
> - if (sov->head) {
> - if (ret && ret->next == NULL) {
> - sov->list_mode = LM_NONE;
> - }
> - sov->head = false;
> - } else {
> - if (ret && ret->next == NULL) {
> - sov->list_mode = LM_END;
> - }
> - }
> + if (ret && !ret->next) {
> + sov->list_mode = LM_END;
> }
> -
> return ret;
> }
What does state LM_END mean? It has no comment...
>
> @@ -312,8 +300,6 @@ end_list(Visitor *v)
> sov->list_mode == LM_NONE ||
> sov->list_mode == LM_IN_PROGRESS);
> sov->list_mode = LM_NONE;
> - sov->head = true;
> -
> }
>
> char *string_output_get_string(StringOutputVisitor *sov)
Unless I'm blind, no next_list() implementation sets an error. Drop
parameter errp and simplify?
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index 8039b97..6016734 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -125,20 +125,23 @@ def gen_visit_list(name, element_type):
> void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj,
> Error **errp)
> {
> Error *err = NULL;
> - GenericList *i, **prev;
> + %(c_name)s *elt;
This isn't an element, it's a list node. I'd call it tail.
Sure changing its type from GenericList to the specific one saves casts?
>
> - visit_start_list(v, name, &err);
> + visit_start_list(v, name, (GenericList **)obj, &err);
> if (err) {
> goto out;
> }
> -
> - for (prev = (GenericList **)obj;
> - !err && (i = visit_next_list(v, prev, &err)) != NULL;
> - prev = &i) {
> - %(c_name)s *native_i = (%(c_name)s *)i;
> - visit_type_%(c_elt_type)s(v, NULL, &native_i->value, &err);
> + elt = *obj;
> + while (elt) {
> + visit_type_%(c_elt_type)s(v, NULL, &elt->value, &err);
> + if (err) {
> + break;
> + }
> + elt = (%(c_name)s *)visit_next_list(v, (GenericList *)elt, &err);
> + if (err) {
> + break;
> + }
> }
With a simplified next_list(), this could be a nice for-loop:
for (tail = *obj; tail; tail = visit_next_list(v, tail, &err)) ...
> -
> visit_end_list(v);
> out:
> error_propagate(errp, err);