Re: [FFmpeg-devel] [PATCH 1/2] avutil/dict: Add av_dict_iterate

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Marvin Scholz:
> 
> 
> On 22 Sep 2022, at 13:33, Andreas Rheinhardt wrote:
> 
>> Marvin Scholz:
>>> This is a more explicit iteration API rather than using the "magic"
>>> av_dict_get(d, "", t, AV_DICT_IGNORE_SUFFIX) which is not really
>>> trivial to grasp what it does when casually reading through code.
>>> ---
>>>  libavutil/dict.c    | 19 +++++++++++++++++++
>>>  libavutil/dict.h    | 27 +++++++++++++++++++++++++++
>>>  libavutil/version.h |  2 +-
>>>  3 files changed, 47 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/libavutil/dict.c b/libavutil/dict.c
>>> index 14ad780a79..2f690a5b8e 100644
>>> --- a/libavutil/dict.c
>>> +++ b/libavutil/dict.c
>>> @@ -20,6 +20,7 @@
>>>
>>>  #include <string.h>
>>>
>>> +#include "avassert.h"
>>>  #include "avstring.h"
>>>  #include "dict.h"
>>>  #include "dict_internal.h"
>>> @@ -38,6 +39,24 @@ int av_dict_count(const AVDictionary *m)
>>>      return m ? m->count : 0;
>>>  }
>>>
>>> +AVDictionaryEntry *av_dict_iterate(const AVDictionary *m,
>>> +                                   const AVDictionaryEntry *prev)
>>> +{
>>> +    int i = 0;
>>> +
>>> +    if (!m)
>>> +        return NULL;
>>> +
>>> +    if (prev)
>>> +        i = prev - m->elems + 1;
>>> +
>>> +    av_assert2(i >= 0);
>>> +    if (i >= m->count)
>>> +        return NULL;
>>> +
>>> +    return &m->elems[i];
>>> +}
>>> +
>>>  AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key,
>>>                                 const AVDictionaryEntry *prev, int flags)
>>>  {
>>> diff --git a/libavutil/dict.h b/libavutil/dict.h
>>> index 0d1afc6c64..b42b3f07fd 100644
>>> --- a/libavutil/dict.h
>>> +++ b/libavutil/dict.h
>>> @@ -32,6 +32,8 @@
>>>
>>>  #include <stdint.h>
>>>
>>> +#include "attributes.h"
>>> +
>>>  /**
>>>   * @addtogroup lavu_dict AVDictionary
>>>   * @ingroup lavu_data
>>> @@ -101,6 +103,31 @@ typedef struct AVDictionary AVDictionary;
>>>  AVDictionaryEntry *av_dict_get(const AVDictionary *m, const char *key,
>>>                                 const AVDictionaryEntry *prev, int flags);
>>>
>>> +/**
>>> + * Iterate over a dictionary
>>> + *
>>> + * Iterates through all entries in the dictionary.
>>> + *
>>> + * @warning The returned AVDictionaryEntry key/value must not be changed.
>>> + *
>>> + * @param m     The dictionary to iterate over
>>> + * @param prev  Pointer to the previous AVDictionaryEntry, NULL initially
>>> + *
>>> + * @retval AVDictionaryEntry* The next element in the dictionary
>>> + * @retval NULL               No more elements in the dictionary
>>> + *
>>> + * Typical usage:
>>> + * @code
>>> + * AVDictionaryEntry *e = NULL;
>>> + * while (e = av_dict_iterate(m, e)) {
>>> + *     // ...
>>> + * }
>>> + * @endcode
>>> + */
>>> +av_warn_unused_result
>>> +AVDictionaryEntry *av_dict_iterate(const AVDictionary *m,
>>> +                                   const AVDictionaryEntry *prev);
>>
>> The user is not allowed to modify the returned AVDictionaryEntries, so
>> you should return a const AVDictionaryEntry here.
> 
> Ok.
> 
> Shouldn't _get return const as well then? If so, can that be changed or would
> it break ABI?
> 

It is an API break and therefore has to wait until a major bump.

>> And there is no reason
>> to use av_warn_unused_result at all; nothing bad happens if you ignore
>> the result (except that you called av_dict_iterate unnecessarily).
> 
> Yeah nothing bad happens but it is probably still wrong use of the API
> that seemed beneficial to highlight or is there any valid use-case
> to call it without using the return? I could not think of any…
> 

By that philosophy, basically all our API functions would need the
av_warn_unused_result attribute. I think this attribute should stand out
to alert the user that he is doing something with grave consequences if
he ignores the result.

>>
>> Furthermore, av_dict_set's documentation contains "Adding a new entry to
>> a dictionary invalidates all existing entries previously returned with
>> av_dict_get." This needs to be updated, too.
> 
> Sure, I will update the docs more, I just wanted to have some general
> feedback if this is acceptable addition before I invest more time to
> polish it.
> 
>>
>>> +
>>>  /**
>>>   * Get number of entries in dictionary.
>>>   *
>>> diff --git a/libavutil/version.h b/libavutil/version.h
>>> index 0585fa7b80..6fd07ed2a4 100644
>>> --- a/libavutil/version.h
>>> +++ b/libavutil/version.h
>>> @@ -80,7 +80,7 @@
>>>
>>>  #define LIBAVUTIL_VERSION_MAJOR  57
>>>  #define LIBAVUTIL_VERSION_MINOR  36
>>> -#define LIBAVUTIL_VERSION_MICRO 102
>>> +#define LIBAVUTIL_VERSION_MICRO 103
>>
>> New API additions need a minor bump (and need to reset micro).
> 
> Oh indeed, sorry, will fix that in the next version.
> 
>>
>>>
>>>  #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
>>>                                                 LIBAVUTIL_VERSION_MINOR, \
>>
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