lavu/opt: add a more general child class iteration API

Use opaque iteration state instead of the previous child class. This
mirrors similar changes done in lavf/lavc.

Deprecate the av_opt_child_class_next() API.

doc/APIchanges

@@ -15,6 +15,10 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2020-06-xx - xxxxxxxxxx - lavu 56.53.100 - log.h opt.h
+  Add av_opt_child_class_iterate() and AVClass.child_class_iterate().
+  Deprecate av_opt_child_class_next() and AVClass.child_class_next().
+
 2020-06-05 - ec39c2276a - lavu 56.50.100 - buffer.h
   Passing NULL as alloc argument to av_buffer_pool_init2() is now allowed.
 

libavutil/log.h

@@ -112,6 +112,7 @@ typedef struct AVClass {
      */
     void* (*child_next)(void *obj, void *prev);
 
+#if FF_API_CHILD_CLASS_NEXT
     /**
      * Return an AVClass corresponding to the next potential
      * AVOptions-enabled child.
@@ -120,7 +121,9 @@ typedef struct AVClass {
      * child_next iterates over _already existing_ objects, while
      * child_class_next iterates over _all possible_ children.
      */
+    attribute_deprecated
     const struct AVClass* (*child_class_next)(const struct AVClass *prev);
+#endif
 
     /**
      * Category used for visualization (like color)
@@ -140,6 +143,21 @@ typedef struct AVClass {
      * available since version (52.12)
      */
     int (*query_ranges)(struct AVOptionRanges **, void *obj, const char *key, int flags);
+
+    /**
+     * Iterate over the AVClasses corresponding to potential AVOptions-enabled
+     * children.
+     *
+     * @param iter pointer to opaque iteration state. The caller must initialize
+     *             *iter to NULL before the first call.
+     * @return AVClass for the next AVOptions-enabled child or NULL if there are
+     *         no more such children.
+     *
+     * @note The difference between child_next and this is that child_next
+     *       iterates over _already existing_ objects, while child_class_iterate
+     *       iterates over _all possible_ children.
+     */
+    const struct AVClass* (*child_class_iterate)(void **iter);
 } AVClass;
 
 /**

libavutil/opt.c

@@ -1679,8 +1679,9 @@ const AVOption *av_opt_find2(void *obj, const char *name, const char *unit,
 
     if (search_flags & AV_OPT_SEARCH_CHILDREN) {
         if (search_flags & AV_OPT_SEARCH_FAKE_OBJ) {
-            const AVClass *child = NULL;
-            while (child = av_opt_child_class_next(c, child))
+            void *iter = NULL;
+            const AVClass *child;
+            while (child = av_opt_child_class_iterate(c, &iter))
                 if (o = av_opt_find2(&child, name, unit, opt_flags, search_flags, NULL))
                     return o;
         } else {
@@ -1715,12 +1716,31 @@ void *av_opt_child_next(void *obj, void *prev)
     return NULL;
 }
 
+#if FF_API_CHILD_CLASS_NEXT
+FF_DISABLE_DEPRECATION_WARNINGS
 const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *prev)
 {
     if (parent->child_class_next)
         return parent->child_class_next(prev);
     return NULL;
 }
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
+
+const AVClass *av_opt_child_class_iterate(const AVClass *parent, void **iter)
+{
+    if (parent->child_class_iterate)
+        return parent->child_class_iterate(iter);
+#if FF_API_CHILD_CLASS_NEXT
+FF_DISABLE_DEPRECATION_WARNINGS
+    if (parent->child_class_next) {
+        *iter = parent->child_class_next(*iter);
+        return *iter;
+    }
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
+    return NULL;
+}
 
 void *av_opt_ptr(const AVClass *class, void *obj, const char *name)
 {

libavutil/opt.h

@@ -114,7 +114,7 @@
  *      libavcodec exports generic options, while its priv_data field exports
  *      codec-specific options). In such a case, it is possible to set up the
  *      parent struct to export a child's options. To do that, simply
- *      implement AVClass.child_next() and AVClass.child_class_next() in the
+ *      implement AVClass.child_next() and AVClass.child_class_iterate() in the
  *      parent struct's AVClass.
  *      Assuming that the test_struct from above now also contains a
  *      child_struct field:
@@ -143,23 +143,25 @@
  *              return t->child_struct;
  *          return NULL
  *      }
- *      const AVClass child_class_next(const AVClass *prev)
+ *      const AVClass child_class_iterate(void **iter)
  *      {
- *          return prev ? NULL : &child_class;
+ *          const AVClass *c = *iter ? NULL : &child_class;
+ *          *iter = (void*)(uintptr_t)c;
+ *          return c;
  *      }
  *      @endcode
- *      Putting child_next() and child_class_next() as defined above into
+ *      Putting child_next() and child_class_iterate() as defined above into
  *      test_class will now make child_struct's options accessible through
  *      test_struct (again, proper setup as described above needs to be done on
  *      child_struct right after it is created).
  *
  *      From the above example it might not be clear why both child_next()
- *      and child_class_next() are needed. The distinction is that child_next()
- *      iterates over actually existing objects, while child_class_next()
+ *      and child_class_iterate() are needed. The distinction is that child_next()
+ *      iterates over actually existing objects, while child_class_iterate()
  *      iterates over all possible child classes. E.g. if an AVCodecContext
  *      was initialized to use a codec which has private options, then its
  *      child_next() will return AVCodecContext.priv_data and finish
- *      iterating. OTOH child_class_next() on AVCodecContext.av_class will
+ *      iterating. OTOH child_class_iterate() on AVCodecContext.av_class will
  *      iterate over all available codecs with private options.
  *
  * @subsection avoptions_implement_named_constants Named constants
@@ -194,7 +196,7 @@
  * For enumerating there are basically two cases. The first is when you want to
  * get all options that may potentially exist on the struct and its children
  * (e.g.  when constructing documentation). In that case you should call
- * av_opt_child_class_next() recursively on the parent struct's AVClass.  The
+ * av_opt_child_class_iterate() recursively on the parent struct's AVClass.  The
  * second case is when you have an already initialized struct with all its
  * children and you want to get all options that can be actually written or read
  * from it. In that case you should call av_opt_child_next() recursively (and
@@ -646,13 +648,26 @@ const AVOption *av_opt_next(const void *obj, const AVOption *prev);
  */
 void *av_opt_child_next(void *obj, void *prev);
 
+#if FF_API_CHILD_CLASS_NEXT
 /**
  * Iterate over potential AVOptions-enabled children of parent.
  *
  * @param prev result of a previous call to this function or NULL
  * @return AVClass corresponding to next potential child or NULL
+ *
+ * @deprecated use av_opt_child_class_iterate
  */
+attribute_deprecated
 const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *prev);
+#endif
+
+/**
+ * Iterate over potential AVOptions-enabled children of parent.
+ *
+ * @param iter a pointer where iteration state is stored.
+ * @return AVClass corresponding to next potential child or NULL
+ */
+const AVClass *av_opt_child_class_iterate(const AVClass *parent, void **iter);
 
 /**
  * @defgroup opt_set_funcs Option setting functions

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  56
-#define LIBAVUTIL_VERSION_MINOR  52
+#define LIBAVUTIL_VERSION_MINOR  53
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
@@ -129,7 +129,9 @@
 #ifndef FF_API_PSEUDOPAL
 #define FF_API_PSEUDOPAL                (LIBAVUTIL_VERSION_MAJOR < 57)
 #endif
-
+#ifndef FF_API_CHILD_CLASS_NEXT
+#define FF_API_CHILD_CLASS_NEXT         (LIBAVUTIL_VERSION_MAJOR < 57)
+#endif
 
 /**
  * @}