ffmpeg: update to handle deprecation of `av_init_packet`

This has been a long standing annoyance - ffmpeg is removing
sizeof(AVPacket) from the API which means you cannot stack-allocate
AVPacket anymore. However, that is something we take advantage of
because we use short-lived AVPackets to bridge from native mpv packets
in our main decoding paths.

We don't think that switching these to `av_packet_alloc` is desirable,
given the cost of heap allocation, so this change takes a different
approach - allocating a single packet in the relevant context and
reusing it over and over.

That's fairly straight-forward, with the main caveat being that
re-initialising the packet is unintuitive. There is no function that
does exactly what we need (what `av_init_packet` did). The closest is
`av_packet_unref`, which additionally frees buffers and side-data.
However, we don't copy those things - we just assign them in from our
own packet, so we have to explicitly clear the pointers before calling
`av_packet_unref`. But at least we can make a wrapper function for
that.

The weirdest part of the change is the handling of the vtt subtitle
conversion. This requires two packets, so I had to pre-allocate two in
the context struct. That sounds excessive, but if allocating the
primary packet is too expensive, then allocating the secondary one for
vtt subtitles must also be too expensive.

This change is not conditional as heap allocated AVPackets were
available for years and years before the deprecation.

1 files changed, 10 insertions, 5 deletions

diff --git a/audio/decode/ad_spdif.c b/audio/decode/ad_spdif.c
index 520803c4b0..3b799660c1 100644
--- a/audio/decode/ad_spdif.c
+++ b/audio/decode/ad_spdif.c
@@ -41,6 +41,7 @@ struct spdifContext {
     struct mp_log   *log;
     enum AVCodecID   codec_id;
     AVFormatContext *lavf_ctx;
+    AVPacket        *avpkt;
     int              out_buffer_len;
     uint8_t          out_buffer[OUTBUF_SIZE];
     bool             need_close;
@@ -82,6 +83,7 @@ static void destroy(struct mp_filter *da)
         avformat_free_context(lavf_ctx);
         spdif_ctx->lavf_ctx = NULL;
     }
+    mp_free_av_packet(&spdif_ctx->avpkt);
 }
 
 static void determine_codec_params(struct mp_filter *da, AVPacket *pkt,
@@ -295,15 +297,14 @@ static void process(struct mp_filter *da)
     struct mp_aframe *out = NULL;
     double pts = mpkt->pts;
 
-    AVPacket pkt;
-    mp_set_av_packet(&pkt, mpkt, NULL);
-    pkt.pts = pkt.dts = 0;
+    mp_set_av_packet(spdif_ctx->avpkt, mpkt, NULL);
+    spdif_ctx->avpkt->pts = spdif_ctx->avpkt->dts = 0;
     if (!spdif_ctx->lavf_ctx) {
-        if (init_filter(da, &pkt) < 0)
+        if (init_filter(da, spdif_ctx->avpkt) < 0)
             goto done;
     }
     spdif_ctx->out_buffer_len  = 0;
-    int ret = av_write_frame(spdif_ctx->lavf_ctx, &pkt);
+    int ret = av_write_frame(spdif_ctx->lavf_ctx, spdif_ctx->avpkt);
     avio_flush(spdif_ctx->lavf_ctx->pb);
     if (ret < 0) {
         MP_ERR(da, "spdif mux error: '%s'\n", mp_strerror(AVUNERROR(ret)));
@@ -424,6 +425,10 @@ static struct mp_decoder *create(struct mp_filter *parent,
         talloc_free(da);
         return NULL;
     }
+
+    spdif_ctx->avpkt = av_packet_alloc();
+    MP_HANDLE_OOM(spdif_ctx->avpkt);
+
     return &spdif_ctx->public;
 }