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

[Alexandre Janniaux]

Fundamentally, you're completely right, there are far better solutions.

But I don't think it fits every situation we encounter when developing
VLC, and working on ASan or even making similar tools works on platforms
like windows, whether it's memory debugging tools or full feature fast
debugger, might be a lot more work than a 30 lines patch.

There are also edge cases where it becomes very useful in current and
future works. The sandbox embodies this issue quite well: yes you can
attach gdb, have coredump, check memory. But debugging many processes
with many threads (and potentially many coredumps) is a lot harder with
gdb than just launching `gdb vlc`, especially for newcomers. Again we
could develop more sophisticated tools but it takes times, what do we
do until they are available, useful and easy to use?

I think having this tool could prove very very helpful in these cases
as a simple development sanity check. If it's not correct in the current
state, it might be a good idea to find a common ground on this.

It's printf debugging, but on a very specific kind of bug that happens
a lot in VLC on the spine of it's architecture. In some way you can
check memory issues with gdb too but it's a lot harder than compiling
and launching with ASan, which produces a printf-like output too. I see
this patch the same: you can hack the codebase to produce useful
information with ASan but most of the time it's harder than having a
dedicated tool to signal the issue and highlight which object is
responsible.


On 2019-02-21 11:37, Rémi Denis-Courmont wrote:
> Hi,
> 
> My GDB skills are bad so I just walked the objects tree by printing
> memory content on the abort. It would be much easier with VLC-specific
> GDB macros, but the point is that you can debug that assertion just as
> any.
> 
> It also works after the fact for pseudorandom failures, if you enable
> core dumps. Much better than this patch since you have access to all
> memory, not just the objects tree.
> 
> Adding special snow flake debug code for assertion is a bad idea that
> sets a very bad precedent - as opposed to improving integration with
> proper debugging tools.
> 
> Le 21 février 2019 12:26:47 GMT+02:00, Thomas Guillem <thomas at gllm.fr> 
> a écrit :
>> 
>> 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 );
>>>>  }
>>> 
>>> --
>>> Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez
>> excuser ma brièveté.
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