[FFmpeg-devel] [PATCH 1/2 v2] avformat/avisynth: remove atexit() handler

[Ramiro Polla]

On Fri, Jul 19, 2024 at 7:51 PM Stephen Hutchinson <qyot27 at gmail.com> wrote:
>
> The atexit() handler in the avisynth demuxer was added because
> there was a conflict in AvxSynth that arose due to their use
> of C++ global objects, particularly in relation to having
> added a logging function relying on log4cpp.
>
> This conflict was responsible for causing a segfault on exit.
> It did not affect Windows with the (at the time) upstream
> AviSynth 2.5 and 2.6, nor does it affect AviSynth+.
>
> Unfortunately, none of this was actually shielded by ifdefs
> indicating the fact it was only needed for AvxSynth, so four
> years ago when AviSynth+ replaced AvxSynth as the handler
> for AviSynth scripts on Unix-like OSes, the fact that the
> atexit handler was no longer necessary was overlooked.
>
> Signed-off-by: Stephen Hutchinson <qyot27 at gmail.com>
> ---
> Changes compared to v1:
> * Added missing dlclose invocation to read_close
>
>  libavformat/avisynth.c | 46 +-----------------------------------------
>  1 file changed, 1 insertion(+), 45 deletions(-)
>
> diff --git a/libavformat/avisynth.c b/libavformat/avisynth.c
> index 625bdf7e3a..26747671c0 100644
> --- a/libavformat/avisynth.c
> +++ b/libavformat/avisynth.c
> @@ -115,9 +115,6 @@ typedef struct AviSynthContext {
>      int error;
>
>      uint32_t flags;
> -
> -    /* Linked list pointers. */
> -    struct AviSynthContext *next;
>  } AviSynthContext;
>
>  static const int avs_planes_packed[1] = { 0 };
> @@ -133,15 +130,7 @@ static const int avs_planes_rgba[4]   = { AVS_PLANAR_G, AVS_PLANAR_B,
>
>  static AVMutex avisynth_mutex = AV_MUTEX_INITIALIZER;
>
> -/* A conflict between C++ global objects, atexit, and dynamic loading requires
> - * us to register our own atexit handler to prevent double freeing. */
>  static AviSynthLibrary avs_library;
> -static int avs_atexit_called        = 0;
> -
> -/* Linked list of AviSynthContexts. An atexit handler destroys this list. */
> -static AviSynthContext *avs_ctx_list = NULL;
> -
> -static av_cold void avisynth_atexit_handler(void);
>
>  static av_cold int avisynth_load_library(void)
>  {
> @@ -185,7 +174,6 @@ static av_cold int avisynth_load_library(void)
>      LOAD_AVS_FUNC(avs_get_env_property, 1);
>  #undef LOAD_AVS_FUNC
>
> -    atexit(avisynth_atexit_handler);
>      return 0;
>
>  fail:
> @@ -214,30 +202,11 @@ static av_cold int avisynth_context_create(AVFormatContext *s)
>          }
>      }
>
> -    if (!avs_ctx_list) {
> -        avs_ctx_list = avs;
> -    } else {
> -        avs->next    = avs_ctx_list;
> -        avs_ctx_list = avs;
> -    }
> -
>      return 0;
>  }
>
>  static av_cold void avisynth_context_destroy(AviSynthContext *avs)
>  {
> -    if (avs_atexit_called)
> -        return;
> -
> -    if (avs == avs_ctx_list) {
> -        avs_ctx_list = avs->next;
> -    } else {
> -        AviSynthContext *prev = avs_ctx_list;
> -        while (prev->next != avs)
> -            prev = prev->next;
> -        prev->next = avs->next;
> -    }
> -
>      if (avs->clip) {
>          avs_library.avs_release_clip(avs->clip);
>          avs->clip = NULL;
> @@ -248,20 +217,6 @@ static av_cold void avisynth_context_destroy(AviSynthContext *avs)
>      }
>  }
>
> -static av_cold void avisynth_atexit_handler(void)
> -{
> -    AviSynthContext *avs = avs_ctx_list;
> -
> -    while (avs) {
> -        AviSynthContext *next = avs->next;
> -        avisynth_context_destroy(avs);
> -        avs = next;
> -    }
> -    dlclose(avs_library.library);
> -
> -    avs_atexit_called = 1;
> -}
> -
>  /* Create AVStream from audio and video data. */
>  static int avisynth_create_stream_video(AVFormatContext *s, AVStream *st)
>  {
> @@ -1134,6 +1089,7 @@ static av_cold int avisynth_read_close(AVFormatContext *s)
>          return AVERROR_UNKNOWN;
>
>      avisynth_context_destroy(s->priv_data);
> +    dlclose(avs_library.library);

Maybe it's best to wrap this around an if (avs_library.library).

[...]

Besides that, the patch looks ok.

Ramiro