vd_lavc: ridiculous workaround for Libav 9 compatibility

This "sometimes" crashed when seeking. The fault apparently lies in
libavcodec: the decoder returns an unreferenced frame! This is
completely insane, but somehow I'm apparently still expected to
work this around. As a reaction, I will drop Libav 9 support in the
next commit. (While this commit will go into release/0.3.)

3 files changed, 23 insertions, 1 deletions

diff --git a/video/decode/lavc.h b/video/decode/lavc.h
index 867488de95..305c0b551d 100644
--- a/video/decode/lavc.h
+++ b/video/decode/lavc.h
@@ -91,5 +91,6 @@ void mp_buffer_ref(struct FrameBuffer *buffer);
 void mp_buffer_unref(struct FrameBuffer *buffer);
 bool mp_buffer_is_unique(struct FrameBuffer *buffer);
 void mp_buffer_pool_free(struct FramePool **pool);
+bool mp_buffer_check(struct FrameBuffer *buffer);
 
 #endif
diff --git a/video/decode/lavc_dr1.c b/video/decode/lavc_dr1.c
index 03e2b5b873..f730a6c231 100644
--- a/video/decode/lavc_dr1.c
+++ b/video/decode/lavc_dr1.c
@@ -204,6 +204,16 @@ void mp_buffer_ref(struct FrameBuffer *buf)
     pool_unlock();
 }
 
+bool mp_buffer_check(struct FrameBuffer *buf)
+{
+    pool_lock();
+    bool ok = buf->refcount > 0;
+    if (ok)
+        buf->refcount++;
+    pool_unlock();
+    return ok;
+}
+
 void mp_buffer_unref(struct FrameBuffer *buf)
 {
     FramePool *pool = buf->pool;
diff --git a/video/decode/vd_lavc.c b/video/decode/vd_lavc.c
index 355db26556..3533841502 100644
--- a/video/decode/vd_lavc.c
+++ b/video/decode/vd_lavc.c
@@ -706,7 +706,16 @@ static struct mp_image *image_from_decoder(struct dec_video *vd)
         mp_image_copy_attributes(mpi, &new);
     } else if (ctx->do_dr1 && pic->opaque) {
         struct FrameBuffer *fb = pic->opaque;
-        mp_buffer_ref(fb); // initial reference for mpi
+        // initial reference for mpi
+        if (!new.planes[0] || !mp_buffer_check(fb)) {
+            // Decoder returned an unreferenced buffer! Taking this would just
+            // lead to an eventual double-free. Nothing we can do about this.
+            // So just say "fuck you" in a nice way.
+            MP_FATAL(vd,
+    "Impossible condition detected! This version of Libav/FFmpeg is not\n"
+    "supported anymore. Please update.\n");
+            return NULL;
+        }
         mpi = mp_image_new_external_ref(&new, fb, fb_ref, fb_unref,
                                         fb_is_unique, NULL);
     } else {
@@ -752,6 +761,8 @@ static int decode(struct dec_video *vd, struct demux_packet *packet,
 
     // Note: potentially resets ctx->pic as it is transferred to mpi
     struct mp_image *mpi = image_from_decoder(vd);
+    if (!mpi)
+        return 0;
     assert(mpi->planes[0]);
     mp_image_set_params(mpi, &params);