[vlc-devel] [PATCH] libvlc: dump object types abd names when leaking

[Thomas Guillem]

On Thu, Feb 21, 2019, at 11:18, Rémi Denis-Courmont wrote:
> Hi,
> 
> Well I did debug that assertion failure using (ASan +) gdb just fine last time. The code has not changed. So this kludge is definitely not necessary for its alleged purpose.

Even if it works with gdb (please tell us how), evelopers don't always run VLC with gdb. This cause an issue if the leak is not reproducible...

> 
> Not only that but stderr is not universally the output for debug nor is it universally where assertion failures are dumped to, 
So if there is no stderr, you don't see the error. I don't see the problem at all.
> so this patch does not even work properly
Works on Windows, Android, Linux, macOS when VLC is run via command line. This is the case for most developers usecase.

> 
> And in most cases you can see the leak by stopping all inputs and dumping the objects tree before quitting, anyway.

What about leaks not related to playback ? Interface, service discovery, playlist that are only cleaned when vlc is closed.

> 
> Le 21 février 2019 11:24:57 GMT+02:00, Thomas Guillem <thomas at gllm.fr> a écrit :
>> Comment:
>> A lot of developers asked me to propose this patch again. It was previously
>> refused because developers could use proper debugging tools. I disagree with
>> that assumption. Even with Asan, when the leak assert fail, it will abort()
>> without prompting any leaks, so you'll have to edit libvlc.c to remove this
>> assert. If your leak was not reproducible, you'll lose a precious information.
>> And good luck finding the leak with GDB, you really need to know the internal
>> of vlc_object_t to iterate through the different list to get the culprit.
>> 
>> Commit log:
>> 
>> In case of a vlc_object_t leak, and if asserts are enabled, the error output
>> will be like the following (when leaking intentionally decoder_t objects):
>> 
>> === vlc_object LEAKS detected ===
>>   \ playlist
>>   |   \ input
>>   |   |   \ decoder avcodec
>>   |   |   | decoder faad
>>   |   |   | decoder stl src/libvlc.c | 30 ++++++++++++++++++++++++++++++
>>  1 file changed, 30 insertions(+)
>> 
>> diff --git a/src/libvlc.c b/src/libvlc.c
>> index 8e8d2ca8b2..d9ff96de84 100644
>> --- a/src/libvlc.c
>> +++ b/src/libvlc.c
>> @@ -436,6 +436,24 @@ void libvlc_InternalCleanup( libvlc_int_t *p_libvlc )
>>  #endif
>>  }
>>  
>> +#ifndef NDEBUG
>> +static void DumpObjectLeaks(vlc_object_internals_t *priv, unsigned level)
>> +{
>> +    bool first = true;
>> +    vlc_list_foreach(priv, &priv->children, siblings)
>> +    {
>> +        vlc_object_t *obj = vlc_externals(priv);
>> +        for (unsigned i = 0; i < level; i++)
>> +            fprintf(stderr, "  %s ", first && i == level -1 ? "\\" : "|");
>> +        char *name = vlc_object_get_name(obj);
>> +        fprintf(stderr, "%s %s\n", vlc_object_typename(obj), name ? name : "");
>> +        free(name);
>> +        DumpObjectLeaks(priv, level + 1);
>> +        first = false;
>> +    }
>> +}
>> +#endif
>> +
>>  /**
>>   * Destroy everything.
>>   * This function requests the running threads to finish, waits for their
>> @@ -449,6 +467,18 @@ void libvlc_InternalDestroy( libvlc_int_t *p_libvlc )
>>  
>>      vlc_ExitDestroy( &priv->exit );
>>  
>> +#ifndef NDEBUG
>> +    {
>> +        vlc_object_internals_t *internal = vlc_internals(p_libvlc);
>> +        if (atomic_load(&internal->refs) != 1)
>> +        {
>> +            vlc_mutex_lock(&internal->tree_lock);
>> +            fprintf(stderr, "=== vlc_object LEAKS detected ===\n");
>> +            DumpObjectLeaks(internal, 1);
>> +            vlc_mutex_unlock(&internal->tree_lock);
>> +        }
>> +    }
>> +#endif
>>      assert( atomic_load(&(vlc_internals(p_libvlc)->refs)) == 1 );
>>      vlc_object_release( p_libvlc );
>>  }
> 
> -- 
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