Re: [Qemu-devel] [PATCH v9 35/37] qapi: Change visit_type_FOO() to no longer return partial objects

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> Returning a partial object on error is an invitation for a careless
> caller to leak memory.  As no one outside the testsuite was actually
> relying on these semantics, it is cleaner to just document and
> guarantee that ALL pointer-based visit_type_FOO() functions always
> leave a safe value in *obj during an input visitor (either the new
> object on success, or NULL if an error is encountered).
>
> Since input visitors have blind assignment semantics, we have to
> track the result of whether an assignment is made all the way down
> to each visitor callback implementation, to avoid making decisions
> based on potentially uninitialized storage.

I'm not sure I get this paragraph.  What's "blind assignment semantics"?

> Note that we still leave *obj unchanged after a scalar-based
> visit_type_FOO(); I did not feel like auditing all uses of
> visit_type_Enum() to see if the callers would tolerate a specific
> sentinel value (not to mention having to decide whether it would
> be better to use 0 or ENUM__MAX as that sentinel).

The assigning input visitor functions (core and generated) all assign
either a pointer to a newly allocated object, or a non-pointer scalar
value.

Possible behaviors on error:

(0) What we have now: assign something that must be cleaned up with the
    dealloc visitor if it's a pointer, but is otherwise useless

    CON: callers have to clean up
    CON: exposes careless callers to useless values

(1) Don't assign anything

    PRO: consistent
    CON: exposes careless callers to uninitialized values

(2) Assign zero bits

    PRO: consistent
    CON: exposes careless callers to bogus zero values

(3) Assign null pointer, else don't assign anything

    CON: inconsistent
    CON: mix of (1)'s and (2)'s CON

(4) Other ideas?

Note that *obj is almost always null on entry, because we allocate
objects zero-initialized.  Only root visits can expose their caller to
uninitialized values.

> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v9: fix bug in use of errp
> v8: rebase to earlier changes
> v7: rebase to earlier changes, enhance commit message, also fix
> visit_type_str() and visit_type_any()
> v6: rebase on top of earlier doc and formatting improvements, mention
> that *obj can be uninitialized on entry to an input visitor, rework
> semantics to keep valgrind happy on uninitialized input, break some
> long lines
> ---
>  include/qapi/visitor-impl.h    |  6 ++---
>  include/qapi/visitor.h         | 53 
> ++++++++++++++++++++++++++++--------------
>  qapi/opts-visitor.c            | 11 ++++++---
>  qapi/qapi-dealloc-visitor.c    |  9 ++++---
>  qapi/qapi-visit-core.c         | 45 ++++++++++++++++++++++++++++-------
>  qapi/qmp-input-visitor.c       | 18 +++++++++-----
>  qapi/qmp-output-visitor.c      |  6 +++--
>  qapi/string-input-visitor.c    |  6 +++--
>  qapi/string-output-visitor.c   |  3 ++-
>  scripts/qapi-visit.py          | 40 +++++++++++++++++++++++--------
>  tests/test-qmp-commands.c      | 13 +++++------
>  tests/test-qmp-input-strict.c  | 19 +++++++--------
>  tests/test-qmp-input-visitor.c | 10 ++------
>  13 files changed, 157 insertions(+), 82 deletions(-)
>
> diff --git a/include/qapi/visitor-impl.h b/include/qapi/visitor-impl.h
> index acbe7d6..8df4ba1 100644
> --- a/include/qapi/visitor-impl.h
> +++ b/include/qapi/visitor-impl.h
> @@ -26,7 +26,7 @@ struct Visitor
>  {
>      /* Must be provided to visit structs (the string visitors do not
>       * currently visit structs). */
> -    void (*start_struct)(Visitor *v, const char *name, void **obj,
> +    bool (*start_struct)(Visitor *v, const char *name, void **obj,
>                           size_t size, Error **errp);
>      /* May be NULL; most useful for input visitors. */
>      void (*check_struct)(Visitor *v, Error **errp);
> @@ -34,13 +34,13 @@ struct Visitor
>      void (*end_struct)(Visitor *v);
>
>      /* May be NULL; most useful for input visitors. */
> -    void (*start_implicit_struct)(Visitor *v, void **obj, size_t size,
> +    bool (*start_implicit_struct)(Visitor *v, void **obj, size_t size,
>                                    Error **errp);
>      /* May be NULL */
>      void (*end_implicit_struct)(Visitor *v);
>
>      /* Must be set */
> -    void (*start_list)(Visitor *v, const char *name, GenericList **list,
> +    bool (*start_list)(Visitor *v, const char *name, GenericList **list,
>                         Error **errp);
>      /* Must be set */
>      GenericList *(*next_list)(Visitor *v, GenericList *element, Error 
> **errp);
> diff --git a/include/qapi/visitor.h b/include/qapi/visitor.h
> index 4638863..4eae633 100644
> --- a/include/qapi/visitor.h
> +++ b/include/qapi/visitor.h
> @@ -31,6 +31,27 @@
>   * visitor-impl.h.
>   */
>
> +/* All qapi types have a corresponding function with a signature
> + * roughly compatible with this:
> + *
> + * void visit_type_FOO(Visitor *v, void *obj, const char *name, Error 
> **errp);
> + *
> + * where address@hidden is itself a pointer or a scalar.  The visit 
> functions for
> + * built-in types are declared here, while the functions for qapi-defined
> + * struct, union, enum, and list types are generated (see qapi-visit.h).
> + * Input visitors may receive an uninitialized address@hidden, and guarantee 
> a
> + * safe value is assigned (a new object on success, or NULL on failure).

This specifies the safe value: NULL.  Makes no sense for non-pointer
scalars.

May input visitors really receive uninitialized address@hidden  As far as I can
see, we routinely store a newly allocated object to address@hidden on success,
and store nothing on failure.  To be able to pass this to the dealloc
visitor, address@hidden must have been null initially, mustn't it?

> + * Output visitors expect address@hidden to be properly initialized on entry.

What about the dealloc visitor?

> + *
> + * Additionally, all qapi structs have a generated function compatible
> + * with this:
> + *
> + * void qapi_free_FOO(void *obj);
> + *
> + * which behaves like free(), even if @obj is NULL or was only partially
> + * allocated before encountering an error.

Will partially allocated QAPI objects remain visible outside input
visitors?

> + */
> +
>  /* This struct is layout-compatible with all other *List structs
>   * created by the qapi generator. */
>  typedef struct GenericList
> @@ -62,11 +83,12 @@ typedef struct GenericList
>   * Set address@hidden on failure; for example, if the input stream does not
>   * have a member @name or if the member is not an object.
>   *
> - * 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.
> + * Returns true if address@hidden was allocated; if that happens, and an 
> error
> + * occurs any time before the matching visit_end_struct(), then the
> + * caller (usually a visit_type_FOO() function) knows to undo the
> + * allocation before returning control further.
>   */
> -void visit_start_struct(Visitor *v, const char *name, void **obj,
> +bool visit_start_struct(Visitor *v, const char *name, void **obj,
>                          size_t size, Error **errp);

Need to see how this is used before I can judge whether I like it :)

>  /**
>   * Prepare for completing a struct visit.
> @@ -85,19 +107,15 @@ void visit_end_struct(Visitor *v);
>
>  /**
>   * Prepare to visit an implicit struct.
> - * Similar to visit_start_struct(), except that an implicit struct
> - * represents a subset of keys that are present at the same nesting level
> - * of a common object but as a separate qapi C struct, rather than a new
> - * object at a deeper nesting level.
> + * Similar to visit_start_struct(), including return semantics, except
> + * that an implicit struct represents a subset of keys that are present
> + * at the same nesting level of a common object but as a separate qapi
> + * C struct, rather than a new object at a deeper nesting level.
>   *
>   * @obj must not be NULL, since this function is only called when
>   * visiting with qapi structs.
> - *
> - * 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.
>   */
> -void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
> +bool visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
>                                   Error **errp);
>  /**
>   * Complete an implicit struct visit started earlier.
> @@ -126,11 +144,12 @@ void visit_end_implicit_struct(Visitor *v);
>   * in this case, visit_next_list() will not be needed, but
>   * visit_end_list() is still mandatory.
>   *
> - * 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.
> + * Returns true if address@hidden was allocated; if that happens, and an 
> error
> + * occurs any time before the matching visit_end_list(), then the
> + * caller (usually a visit_type_FOO() function) knows to undo the
> + * allocation before returning control further.
>   */
> -void visit_start_list(Visitor *v, const char *name, GenericList **list,
> +bool visit_start_list(Visitor *v, const char *name, GenericList **list,
>                        Error **errp);
>  /**
>   * Iterate over a GenericList during a list visit.
> diff --git a/qapi/opts-visitor.c b/qapi/opts-visitor.c
> index c5a7396..38d1e68 100644
> --- a/qapi/opts-visitor.c
> +++ b/qapi/opts-visitor.c
> @@ -122,18 +122,20 @@ opts_visitor_insert(GHashTable *unprocessed_opts, const 
> QemuOpt *opt)
>  }
>
>
> -static void
> +static bool
>  opts_start_struct(Visitor *v, const char *name, void **obj,
>                    size_t size, Error **errp)
>  {
>      OptsVisitor *ov = to_ov(v);
>      const QemuOpt *opt;
> +    bool result = false;
>
>      if (obj) {
>          *obj = g_malloc0(size > 0 ? size : 1);
> +        result = true;
>      }
>      if (ov->depth++ > 0) {
> -        return;
> +        return result;
>      }
>
>      ov->unprocessed_opts = g_hash_table_new_full(&g_str_hash, &g_str_equal,
> @@ -152,6 +154,7 @@ opts_start_struct(Visitor *v, const char *name, void 
> **obj,
>          ov->fake_id_opt->str = g_strdup(ov->opts_root->id);
>          opts_visitor_insert(ov->unprocessed_opts, ov->fake_id_opt);
>      }
> +    return result;
>  }

Stores the newly allocated object in address@hidden on success, doesn't store
anything on failure.

To make cleanup possible, address@hidden must be null initially.  Then the 
return
value is true iff address@hidden is non-null on return.

>
>
> @@ -210,7 +213,7 @@ lookup_distinct(const OptsVisitor *ov, const char *name, 
> Error **errp)
>  }
>
>
> -static void
> +static bool
>  opts_start_list(Visitor *v, const char *name, GenericList **list, Error 
> **errp)
>  {
>      OptsVisitor *ov = to_ov(v);
> @@ -223,8 +226,10 @@ opts_start_list(Visitor *v, const char *name, 
> GenericList **list, Error **errp)
>      if (ov->repeated_opts) {
>          ov->list_mode = LM_IN_PROGRESS;
>          *list = g_new0(GenericList, 1);
> +        return true;
>      } else {
>          *list = NULL;
> +        return false;
>      }
>  }

This one stores null on failure, unlike opts_start_struct().

Again, the return value is true iff address@hidden is non-null on return.

>
> diff --git a/qapi/qapi-dealloc-visitor.c b/qapi/qapi-dealloc-visitor.c
> index 839049e..0990199 100644
> --- a/qapi/qapi-dealloc-visitor.c
> +++ b/qapi/qapi-dealloc-visitor.c
> @@ -53,11 +53,12 @@ static void *qapi_dealloc_pop(QapiDeallocVisitor *qov)
>      return value;
>  }
>
> -static void qapi_dealloc_start_struct(Visitor *v, const char *name, void 
> **obj,
> +static bool qapi_dealloc_start_struct(Visitor *v, const char *name, void 
> **obj,
>                                        size_t unused, Error **errp)
>  {
>      QapiDeallocVisitor *qov = to_qov(v);
>      qapi_dealloc_push(qov, obj);
> +    return false;
>  }
>

Not an input visitor, always returns false.

>  static void qapi_dealloc_end_struct(Visitor *v)
> @@ -69,13 +70,14 @@ static void qapi_dealloc_end_struct(Visitor *v)
>      }
>  }
>
> -static void qapi_dealloc_start_implicit_struct(Visitor *v,
> +static bool qapi_dealloc_start_implicit_struct(Visitor *v,
>                                                 void **obj,
>                                                 size_t size,
>                                                 Error **errp)
>  {
>      QapiDeallocVisitor *qov = to_qov(v);
>      qapi_dealloc_push(qov, obj);
> +    return false;
>  }
>

Likewise.

>  static void qapi_dealloc_end_implicit_struct(Visitor *v)
> @@ -87,11 +89,12 @@ static void qapi_dealloc_end_implicit_struct(Visitor *v)
>      }
>  }
>
> -static void qapi_dealloc_start_list(Visitor *v, const char *name,
> +static bool qapi_dealloc_start_list(Visitor *v, const char *name,
>                                      GenericList **list, Error **errp)
>  {
>      QapiDeallocVisitor *qov = to_qov(v);
>      qapi_dealloc_push(qov, NULL);
> +    return false;
>  }
>

Likewise.

>  static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList *list,
> diff --git a/qapi/qapi-visit-core.c b/qapi/qapi-visit-core.c
> index f391a70..259e0cb 100644
> --- a/qapi/qapi-visit-core.c
> +++ b/qapi/qapi-visit-core.c
> @@ -2,7 +2,7 @@
>   * Core Definitions for QAPI Visitor Classes
>   *
>   * Copyright IBM, Corp. 2011
> - * Copyright (C) 2015 Red Hat, Inc.
> + * Copyright (C) 2015-2016 Red Hat, Inc.
>   *
>   * Authors:
>   *  Anthony Liguori   <address@hidden>
> @@ -26,14 +26,20 @@ static bool visit_is_output(Visitor *v)
>      return v->type_enum == output_type_enum;
>  }
>
> -void visit_start_struct(Visitor *v, const char *name, void **obj,
> +bool visit_start_struct(Visitor *v, const char *name, void **obj,
>                          size_t size, Error **errp)
>  {
> +    bool result;
> +
>      assert(obj ? size : !size);
>      if (obj && visit_is_output(v)) {
>          assert(*obj);
>      }
> -    v->start_struct(v, name, obj, size, errp);
> +    result = v->start_struct(v, name, obj, size, errp);
> +    if (result) {
> +        assert(obj && *obj);
> +    }

Roundabout way to write assert(!result || (obj && *obj));

Heh, you even assert one half of what I'm trying to prove.

Can we make it assert(result == (visit_is_input(v) && obj && *obj) ?

Similarly for the other functions.

> +    return result;
>  }
>
>  void visit_check_struct(Visitor *v, Error **errp)
> @@ -48,16 +54,22 @@ void visit_end_struct(Visitor *v)
>      v->end_struct(v);
>  }
>
> -void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
> +bool visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
>                                   Error **errp)
>  {
> +    bool result = false;
> +
>      assert(obj && size);
>      if (visit_is_output(v)) {
>          assert(*obj);
>      }
>      if (v->start_implicit_struct) {
> -        v->start_implicit_struct(v, obj, size, errp);
> +        result = v->start_implicit_struct(v, obj, size, errp);
>      }
> +    if (result) {
> +        assert(*obj);
> +    }
> +    return result;
>  }
>
>  void visit_end_implicit_struct(Visitor *v)
> @@ -67,10 +79,14 @@ void visit_end_implicit_struct(Visitor *v)
>      }
>  }
>
> -void visit_start_list(Visitor *v, const char *name, GenericList **list,
> +bool visit_start_list(Visitor *v, const char *name, GenericList **list,
>                        Error **errp)
>  {
> -    v->start_list(v, name, list, errp);
> +    bool result = v->start_list(v, name, list, errp);
> +    if (result) {
> +        assert(list && *list);
> +    }
> +    return result;
>  }
>
>  GenericList *visit_next_list(Visitor *v, GenericList *list, Error **errp)
> @@ -229,6 +245,7 @@ void visit_type_bool(Visitor *v, const char *name, bool 
> *obj, Error **errp)
>
>  void visit_type_str(Visitor *v, const char *name, char **obj, Error **errp)
>  {
> +    Error *err = NULL;
>      assert(obj);
>      /* TODO: Fix callers to not pass NULL when they mean "", so that we
>       * can enable:
> @@ -236,7 +253,11 @@ void visit_type_str(Visitor *v, const char *name, char 
> **obj, Error **errp)
>          assert(*obj);
>      }
>       */
> -    v->type_str(v, name, obj, errp);
> +    v->type_str(v, name, obj, &err);
> +    if (!visit_is_output(v) && err) {
> +        *obj = NULL;

Shouldn't storing null on error be left to v->type_str(), like we do for
structs and lists?  Not least because it begs the question whether this
leaks a string stored by it.

!visit_is_output(v) covers input visitors and the dealloc visitor.

> +    }
> +    error_propagate(errp, err);
>  }
>
>  void visit_type_number(Visitor *v, const char *name, double *obj,
> @@ -248,11 +269,17 @@ void visit_type_number(Visitor *v, const char *name, 
> double *obj,
>
>  void visit_type_any(Visitor *v, const char *name, QObject **obj, Error 
> **errp)
>  {
> +    Error *err = NULL;
> +
>      assert(obj);
>      if (visit_is_output(v)) {
>          assert(*obj);
>      }
> -    v->type_any(v, name, obj, errp);
> +    v->type_any(v, name, obj, &err);
> +    if (!visit_is_output(v) && err) {
> +        *obj = NULL;

Likewise.

> +    }
> +    error_propagate(errp, err);
>  }
>
>  void visit_type_null(Visitor *v, const char *name, Error **errp)
> diff --git a/qapi/qmp-input-visitor.c b/qapi/qmp-input-visitor.c
> index 82f9333..6b4ad68 100644
> --- a/qapi/qmp-input-visitor.c
> +++ b/qapi/qmp-input-visitor.c
> @@ -130,7 +130,7 @@ static void qmp_input_pop(Visitor *v)
>      qiv->nb_stack--;
>  }
>
> -static void qmp_input_start_struct(Visitor *v, const char *name, void **obj,
> +static bool qmp_input_start_struct(Visitor *v, const char *name, void **obj,
>                                     size_t size, Error **errp)
>  {
>      QmpInputVisitor *qiv = to_qiv(v);
> @@ -140,30 +140,34 @@ static void qmp_input_start_struct(Visitor *v, const 
> char *name, void **obj,
>      if (!qobj || qobject_type(qobj) != QTYPE_QDICT) {
>          error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
>                     "QDict");
> -        return;
> +        return false;
>      }
>
>      qmp_input_push(qiv, qobj, NULL, &err);
>      if (err) {
>          error_propagate(errp, err);
> -        return;
> +        return false;
>      }
>
>      if (obj) {
>          *obj = g_malloc0(size);
> +        return true;
>      }
> +    return false;

Stores the newly allocated object in address@hidden on success, doesn't store
anything on failure.

To make cleanup possible, address@hidden must be null initially.  Then the 
return
value is true iff address@hidden is non-null on return.

Just like opts-visitor.c.

>  }
>
>
> -static void qmp_input_start_implicit_struct(Visitor *v, void **obj,
> +static bool qmp_input_start_implicit_struct(Visitor *v, void **obj,
>                                              size_t size, Error **errp)
>  {
>      if (obj) {
>          *obj = g_malloc0(size);
> +        return true;
>      }
> +    return false;
>  }

Likewise.

>
> -static void qmp_input_start_list(Visitor *v, const char *name,
> +static bool qmp_input_start_list(Visitor *v, const char *name,
>                                   GenericList **list, Error **errp)
>  {
>      QmpInputVisitor *qiv = to_qiv(v);
> @@ -173,7 +177,7 @@ static void qmp_input_start_list(Visitor *v, const char 
> *name,
>      if (!qobj || qobject_type(qobj) != QTYPE_QLIST) {
>          error_setg(errp, QERR_INVALID_PARAMETER_TYPE, name ? name : "null",
>                     "list");
> -        return;
> +        return false;
>      }
>
>      entry = qlist_first(qobject_to_qlist(qobj));
> @@ -181,10 +185,12 @@ static void qmp_input_start_list(Visitor *v, const char 
> *name,
>      if (list) {
>          if (entry) {
>              *list = g_new0(GenericList, 1);
> +            return true;
>          } else {
>              *list = NULL;
>          }
>      }
> +    return false;
>  }

This one stores null on failure, unlike start_struct().

Again, the return value is true iff address@hidden is non-null on return.

Just like opts-visitor.c.

>
>  static GenericList *qmp_input_next_list(Visitor *v, GenericList *list,
> diff --git a/qapi/qmp-output-visitor.c b/qapi/qmp-output-visitor.c
> index 913f378..ce592d2 100644
> --- a/qapi/qmp-output-visitor.c
> +++ b/qapi/qmp-output-visitor.c
> @@ -102,7 +102,7 @@ static void qmp_output_add_obj(QmpOutputVisitor *qov, 
> const char *name,
>      }
>  }
>
> -static void qmp_output_start_struct(Visitor *v, const char *name, void **obj,
> +static bool qmp_output_start_struct(Visitor *v, const char *name, void **obj,
>                                      size_t unused, Error **errp)
>  {
>      QmpOutputVisitor *qov = to_qov(v);
> @@ -110,6 +110,7 @@ static void qmp_output_start_struct(Visitor *v, const 
> char *name, void **obj,
>
>      qmp_output_add(qov, name, dict);
>      qmp_output_push(qov, dict);
> +    return false;
>  }

Not an input visitor, always returns false.

>
>  static void qmp_output_end_struct(Visitor *v)
> @@ -118,7 +119,7 @@ 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,
> +static bool qmp_output_start_list(Visitor *v, const char *name,
>                                    GenericList **listp, Error **errp)
>  {
>      QmpOutputVisitor *qov = to_qov(v);
> @@ -126,6 +127,7 @@ static void qmp_output_start_list(Visitor *v, const char 
> *name,
>
>      qmp_output_add(qov, name, list);
>      qmp_output_push(qov, list);
> +    return false;
>  }

Likewise.

>
>  static GenericList *qmp_output_next_list(Visitor *v, GenericList *list,
> diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
> index 582a62a..0e4a07c 100644
> --- a/qapi/string-input-visitor.c
> +++ b/qapi/string-input-visitor.c
> @@ -120,7 +120,7 @@ error:
>      siv->ranges = NULL;
>  }
>
> -static void
> +static bool
>  start_list(Visitor *v, const char *name, GenericList **list, Error **errp)
>  {
>      StringInputVisitor *siv = to_siv(v);
> @@ -132,7 +132,7 @@ start_list(Visitor *v, const char *name, GenericList 
> **list, Error **errp)
>      parse_str(siv, &err);
>      if (err) {
>          error_propagate(errp, err);
> -        return;
> +        return false;
>      }
>
>      siv->cur_range = g_list_first(siv->ranges);
> @@ -142,8 +142,10 @@ start_list(Visitor *v, const char *name, GenericList 
> **list, Error **errp)
>              siv->cur = r->begin;
>          }
>          *list = g_new0(GenericList, 1);
> +        return true;
>      } else {
>          *list = NULL;
> +        return false;
>      }
>  }

This one stores null on failure, like the start_list() we've seen
before, and unlike the start_struct().

Again, the return value is true iff address@hidden is non-null on return.

>
> diff --git a/qapi/string-output-visitor.c b/qapi/string-output-visitor.c
> index 7209d80..2666802 100644
> --- a/qapi/string-output-visitor.c
> +++ b/qapi/string-output-visitor.c
> @@ -263,7 +263,7 @@ static void print_type_number(Visitor *v, const char 
> *name, double *obj,
>      string_output_set(sov, g_strdup_printf("%f", *obj));
>  }
>
> -static void
> +static bool
>  start_list(Visitor *v, const char *name, GenericList **list, Error **errp)
>  {
>      StringOutputVisitor *sov = to_sov(v);
> @@ -276,6 +276,7 @@ start_list(Visitor *v, const char *name, GenericList 
> **list, Error **errp)
>      if (*list && (*list)->next) {
>          sov->list_mode = LM_STARTED;
>      }
> +    return false;
>  }

Not an input visitor, always returns false.

>
>  static GenericList *
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index 6016734..eebb5f7 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -58,8 +58,10 @@ def gen_visit_implicit_struct(typ):
>  static void visit_type_implicit_%(c_type)s(Visitor *v, %(c_type)s **obj, 
> Error **errp)
>  {
>      Error *err = NULL;
> +    bool allocated;
>
> -    visit_start_implicit_struct(v, (void **)obj, sizeof(%(c_type)s), &err);
> +    allocated = visit_start_implicit_struct(v, (void **)obj,
> +                                            sizeof(%(c_type)s), &err);
>      if (err) {
>          goto out;
>      }
> @@ -69,6 +71,10 @@ static void visit_type_implicit_%(c_type)s(Visitor *v, 
> %(c_type)s **obj, Error *
>      visit_type_%(c_type)s_fields(v, obj, &err);
>  out_obj:
>      visit_end_implicit_struct(v);
> +    if (allocated && err) {
> +        g_free(*obj);
> +        *obj = NULL;
> +    }
>  out:
>      error_propagate(errp, err);
>  }
> @@ -116,18 +122,15 @@ out:
>
>
>  def gen_visit_list(name, element_type):
> -    # FIXME: if *obj is NULL on entry, and the first visit_next_list()
> -    # assigns to *obj, while a later one fails, we should clean up *obj
> -    # rather than leaving it non-NULL. As currently written, the caller must
> -    # call qapi_free_FOOList() to avoid a memory leak of the partial FOOList.
>      return mcgen('''
>
>  void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, 
> Error **errp)
>  {
>      Error *err = NULL;
>      %(c_name)s *elt;
> +    bool allocated;
>
> -    visit_start_list(v, name, (GenericList **)obj, &err);
> +    allocated = visit_start_list(v, name, (GenericList **)obj, &err);
>      if (err) {
>          goto out;
>      }
> @@ -144,6 +147,10 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, 
> %(c_name)s **obj, Error
>      }
>      visit_end_list(v);
>  out:
> +    if (allocated && err) {
> +        qapi_free_%(c_name)s(*obj);
> +        *obj = NULL;
> +    }
>      error_propagate(errp, err);
>  }
>  ''',
> @@ -174,8 +181,10 @@ def gen_visit_alternate(name, variants):
>  void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, 
> Error **errp)
>  {
>      Error *err = NULL;
> +    bool allocated;
>
> -    visit_start_implicit_struct(v, (void**) obj, sizeof(%(c_name)s), &err);
> +    allocated = visit_start_implicit_struct(v, (void **)obj,
> +                                            sizeof(%(c_name)s), &err);
>      if (err) {
>          goto out;
>      }
> @@ -204,11 +213,15 @@ void visit_type_%(c_name)s(Visitor *v, const char 
> *name, %(c_name)s **obj, Error
>      }
>  out_obj:
>      visit_end_implicit_struct(v);
> +    if (allocated && err) {
> +        qapi_free_%(c_name)s(*obj);
> +        *obj = NULL;
> +    }
>  out:
>      error_propagate(errp, err);
>  }
>  ''',
> -                 name=name)
> +                 name=name, c_name=c_name(name))
>
>      return ret
>
> @@ -236,8 +249,10 @@ def gen_visit_object(name, base, members, variants):
>  void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, 
> Error **errp)
>  {
>      Error *err = NULL;
> +    bool allocated;
>
> -    visit_start_struct(v, name, (void **)obj, sizeof(%(c_name)s), &err);
> +    allocated = visit_start_struct(v, name, (void **)obj, sizeof(%(c_name)s),
> +                                   &err);
>      if (err) {
>          goto out;
>      }
> @@ -301,10 +316,15 @@ void visit_type_%(c_name)s(Visitor *v, const char 
> *name, %(c_name)s **obj, Error
>      visit_check_struct(v, &err);
>  out_obj:
>      visit_end_struct(v);
> +    if (allocated && err) {
> +        qapi_free_%(c_name)s(*obj);
> +        *obj = NULL;
> +    }
>  out:
>      error_propagate(errp, err);
>  }
> -''')
> +''',
> +                 c_name=c_name(name))
>
>      return ret
>

Hmm.

This grows qapi-visit.c by 14% for me.

Can we do the deallocation in the visitor core somehow?  We'd have to
pass "how to deallocate" information: the appropriate qapi_free_FOO()
function.  We already pass in "how to allocate" information: size.

> diff --git a/tests/test-qmp-commands.c b/tests/test-qmp-commands.c
> index bc8085d..d3466a4 100644
> --- a/tests/test-qmp-commands.c
> +++ b/tests/test-qmp-commands.c
> @@ -227,14 +227,13 @@ static void test_dealloc_partial(void)
>          QDECREF(ud2_dict);
>      }
>
> -    /* verify partial success */
> -    assert(ud2 != NULL);
> -    assert(ud2->string0 != NULL);
> -    assert(strcmp(ud2->string0, text) == 0);
> -    assert(ud2->dict1 == NULL);
> -
> -    /* confirm & release construction error */
> +    /* verify that visit_type_XXX() cleans up properly on error */
>      error_free_or_abort(&err);
> +    assert(!ud2);
> +
> +    /* Manually create a partial object, leaving ud2->dict1 at NULL */
> +    ud2 = g_new0(UserDefTwo, 1);
> +    ud2->string0 = g_strdup(text);
>
>      /* tear down partial object */
>      qapi_free_UserDefTwo(ud2);
> diff --git a/tests/test-qmp-input-strict.c b/tests/test-qmp-input-strict.c
> index 775ad39..4db35dd 100644
> --- a/tests/test-qmp-input-strict.c
> +++ b/tests/test-qmp-input-strict.c
> @@ -181,10 +181,7 @@ static void 
> test_validate_fail_struct(TestInputVisitorData *data,
>
>      visit_type_TestStruct(v, NULL, &p, &err);
>      error_free_or_abort(&err);
> -    if (p) {
> -        g_free(p->string);
> -    }
> -    g_free(p);
> +    g_assert(!p);
>  }
>
>  static void test_validate_fail_struct_nested(TestInputVisitorData *data,
> @@ -198,7 +195,7 @@ static void 
> test_validate_fail_struct_nested(TestInputVisitorData *data,
>
>      visit_type_UserDefTwo(v, NULL, &udp, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefTwo(udp);
> +    g_assert(!udp);
>  }
>
>  static void test_validate_fail_list(TestInputVisitorData *data,
> @@ -212,7 +209,7 @@ static void test_validate_fail_list(TestInputVisitorData 
> *data,
>
>      visit_type_UserDefOneList(v, NULL, &head, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefOneList(head);
> +    g_assert(!head);
>  }
>
>  static void test_validate_fail_union_native_list(TestInputVisitorData *data,
> @@ -227,7 +224,7 @@ static void 
> test_validate_fail_union_native_list(TestInputVisitorData *data,
>
>      visit_type_UserDefNativeListUnion(v, NULL, &tmp, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefNativeListUnion(tmp);
> +    g_assert(!tmp);
>  }
>
>  static void test_validate_fail_union_flat(TestInputVisitorData *data,
> @@ -241,7 +238,7 @@ static void 
> test_validate_fail_union_flat(TestInputVisitorData *data,
>
>      visit_type_UserDefFlatUnion(v, NULL, &tmp, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefFlatUnion(tmp);
> +    g_assert(!tmp);
>  }
>
>  static void test_validate_fail_union_flat_no_discrim(TestInputVisitorData 
> *data,
> @@ -256,13 +253,13 @@ static void 
> test_validate_fail_union_flat_no_discrim(TestInputVisitorData *data,
>
>      visit_type_UserDefFlatUnion2(v, NULL, &tmp, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefFlatUnion2(tmp);
> +    g_assert(!tmp);
>  }
>
>  static void test_validate_fail_alternate(TestInputVisitorData *data,
>                                           const void *unused)
>  {
> -    UserDefAlternate *tmp = NULL;
> +    UserDefAlternate *tmp;

Are you sure this is sound?

>      Visitor *v;
>      Error *err = NULL;
>
> @@ -270,7 +267,7 @@ static void 
> test_validate_fail_alternate(TestInputVisitorData *data,
>
>      visit_type_UserDefAlternate(v, NULL, &tmp, &err);
>      error_free_or_abort(&err);
> -    qapi_free_UserDefAlternate(tmp);
> +    g_assert(!tmp);
>  }
>
>  static void do_test_validate_qmp_introspect(TestInputVisitorData *data,
> diff --git a/tests/test-qmp-input-visitor.c b/tests/test-qmp-input-visitor.c
> index cd5e765..0eef7e0 100644
> --- a/tests/test-qmp-input-visitor.c
> +++ b/tests/test-qmp-input-visitor.c
> @@ -733,18 +733,12 @@ static void test_visitor_in_errors(TestInputVisitorData 
> *data,
>
>      visit_type_TestStruct(v, NULL, &p, &err);
>      error_free_or_abort(&err);
> -    /* FIXME - a failed parse should not leave a partially-allocated p
> -     * for us to clean up; this could cause callers to leak memory. */
> -    g_assert(p->string == NULL);
> -
> -    g_free(p->string);
> -    g_free(p);
> +    g_assert(!p);
>
>      v = visitor_input_test_init(data, "[ '1', '2', false, '3' ]");
>      visit_type_strList(v, NULL, &q, &err);
>      error_free_or_abort(&err);
> -    assert(q);
> -    qapi_free_strList(q);
> +    assert(!q);
>  }
>
>  static void test_visitor_in_wrong_type(TestInputVisitorData *data,

Now I've seen the complete patch, two more remarks:

* I think all the allocating methods should behave the same.  Right now,
  some store null on failure, and some don't.  For the latter to work,
  the value must be null initially (or else the dealloc visitor runs
  amok).

* Do we really need the new return value?  It looks like it's always
  equal to visit_is_input(v) && obj && *obj.