cbs: Describe allocate/free methods in tabular form

Unit types are split into three categories, depending on how their
content is managed:
* POD structure - these require no special treatment.
* Structure containing references to refcounted buffers - these can use
  a common free function when the offsets of all the internal references
  are known.
* More complex structures - these still require ad-hoc treatment.

For each codec we can then maintain a table of descriptors for each set of
equivalent unit types, defining the mechanism needed to allocate/free that
unit content.  This is not required to be used immediately - a new alloc
function supports this, but does not replace the old one which works without
referring to these tables.

libavcodec/cbs.c

@@ -802,3 +802,72 @@ void ff_cbs_delete_unit(CodedBitstreamFragment *frag,
                 frag->units + position + 1,
                 (frag->nb_units - position) * sizeof(*frag->units));
 }
+
+static void cbs_default_free_unit_content(void *opaque, uint8_t *data)
+{
+    const CodedBitstreamUnitTypeDescriptor *desc = opaque;
+    if (desc->content_type == CBS_CONTENT_TYPE_INTERNAL_REFS) {
+        int i;
+        for (i = 0; i < desc->nb_ref_offsets; i++) {
+            void **ptr = (void**)(data + desc->ref_offsets[i]);
+            av_buffer_unref((AVBufferRef**)(ptr + 1));
+        }
+    }
+    av_free(data);
+}
+
+static const CodedBitstreamUnitTypeDescriptor
+    *cbs_find_unit_type_desc(CodedBitstreamContext *ctx,
+                             CodedBitstreamUnit *unit)
+{
+    const CodedBitstreamUnitTypeDescriptor *desc;
+    int i, j;
+
+    if (!ctx->codec->unit_types)
+        return NULL;
+
+    for (i = 0;; i++) {
+        desc = &ctx->codec->unit_types[i];
+        if (desc->nb_unit_types == 0)
+            break;
+        if (desc->nb_unit_types == CBS_UNIT_TYPE_RANGE) {
+            if (unit->type >= desc->unit_type_range_start &&
+                unit->type <= desc->unit_type_range_end)
+                return desc;
+        } else {
+            for (j = 0; j < desc->nb_unit_types; j++) {
+                if (desc->unit_types[j] == unit->type)
+                    return desc;
+            }
+        }
+    }
+    return NULL;
+}
+
+int ff_cbs_alloc_unit_content2(CodedBitstreamContext *ctx,
+                               CodedBitstreamUnit *unit)
+{
+    const CodedBitstreamUnitTypeDescriptor *desc;
+
+    av_assert0(!unit->content && !unit->content_ref);
+
+    desc = cbs_find_unit_type_desc(ctx, unit);
+    if (!desc)
+        return AVERROR(ENOSYS);
+
+    unit->content = av_mallocz(desc->content_size);
+    if (!unit->content)
+        return AVERROR(ENOMEM);
+
+    unit->content_ref =
+        av_buffer_create(unit->content, desc->content_size,
+                         desc->content_free ? desc->content_free
+                                            : cbs_default_free_unit_content,
+                         (void*)desc, 0);
+    if (!unit->content_ref) {
+        av_freep(&unit->content);
+        return AVERROR(ENOMEM);
+    }
+
+    return 0;
+}

libavcodec/cbs.h

@@ -349,6 +349,15 @@ int ff_cbs_alloc_unit_content(CodedBitstreamUnit *unit,
                               size_t size,
                               void (*free)(void *opaque, uint8_t *content));
 
+/**
+ * Allocate a new internal content buffer matching the type of the unit.
+ *
+ * The content will be zeroed.
+ */
+int ff_cbs_alloc_unit_content2(CodedBitstreamContext *ctx,
+                               CodedBitstreamUnit *unit);
+
+
 /**
  * Allocate a new internal data buffer of the given size in the unit.
  *

libavcodec/cbs_internal.h

@@ -25,11 +25,72 @@
 #include "put_bits.h"
 
 
+enum CBSContentType {
+    // Unit content is a simple structure.
+    CBS_CONTENT_TYPE_POD,
+    // Unit content contains some references to other structures, but all
+    // managed via buffer reference counting.  The descriptor defines the
+    // structure offsets of every buffer reference.
+    CBS_CONTENT_TYPE_INTERNAL_REFS,
+    // Unit content is something more complex.  The descriptor defines
+    // special functions to manage the content.
+    CBS_CONTENT_TYPE_COMPLEX,
+};
+
+enum {
+      // Maximum number of unit types described by the same unit type
+      // descriptor.
+      CBS_MAX_UNIT_TYPES  = 3,
+      // Maximum number of reference buffer offsets in any one unit.
+      CBS_MAX_REF_OFFSETS = 2,
+      // Special value used in a unit type descriptor to indicate that it
+      // applies to a large range of types rather than a set of discrete
+      // values.
+      CBS_UNIT_TYPE_RANGE = -1,
+};
+
+typedef const struct CodedBitstreamUnitTypeDescriptor {
+    // Number of entries in the unit_types array, or the special value
+    // CBS_UNIT_TYPE_RANGE to indicate that the range fields should be
+    // used instead.
+    int nb_unit_types;
+
+    // Array of unit types that this entry describes.
+    const CodedBitstreamUnitType unit_types[CBS_MAX_UNIT_TYPES];
+
+    // Start and end of unit type range, used if nb_unit_types is
+    // CBS_UNIT_TYPE_RANGE.
+    const CodedBitstreamUnitType unit_type_range_start;
+    const CodedBitstreamUnitType unit_type_range_end;
+
+    // The type of content described.
+    enum CBSContentType content_type;
+    // The size of the structure which should be allocated to contain
+    // the decomposed content of this type of unit.
+    size_t content_size;
+
+    // Number of entries in the ref_offsets array.  Only used if the
+    // content_type is CBS_CONTENT_TYPE_INTERNAL_REFS.
+    int nb_ref_offsets;
+    // The structure must contain two adjacent elements:
+    //   type        *field;
+    //   AVBufferRef *field_ref;
+    // where field points to something in the buffer referred to by
+    // field_ref.  This offset is then set to offsetof(struct, field).
+    size_t ref_offsets[CBS_MAX_REF_OFFSETS];
+
+    void (*content_free)(void *opaque, uint8_t *data);
+} CodedBitstreamUnitTypeDescriptor;
+
 typedef struct CodedBitstreamType {
     enum AVCodecID codec_id;
 
     size_t priv_data_size;
 
+    // List of unit type descriptors for this codec.
+    // Terminated by a descriptor with nb_unit_types equal to zero.
+    const CodedBitstreamUnitTypeDescriptor *unit_types;
+
     // Split frag->data into coded bitstream units, creating the
     // frag->units array.  Fill data but not content on each unit.
     // The header argument should be set if the fragment came from