vd_lavc: be more careful with flushing the decoder

Until now, we've relied on the following things:

- you can send flush packets to the decoder even if it's fully flushed,
- you can send new packets to a flushed decoder,
- you can send new packers to a partially flushed decoder.

("flushing" refers to sending flush packets to the decoder until the
decoder does not return new pictures, not avcodec_flush_buffers().)

All of these are questionable. The libavcodec API probably doesn't
guarantee that these work well or at all, even though most decoders have
no issue with these. But especially with hardware decoding wrappers
(like MMAL), real problems can be expected. Isolate us from these corner
cases by handling them explicitly.

video/decode/lavc.h

@@ -18,6 +18,7 @@ typedef struct lavc_ctx {
     enum AVPixelFormat pix_fmt;
     int best_csp;
     enum AVDiscard skip_frame;
+    bool flushing;
     const char *software_fallback_decoder;
     bool hwdec_failed;
     bool hwdec_notified;

video/decode/vd_lavc.c

@@ -443,6 +443,15 @@ error:
     uninit_avctx(vd);
 }
 
+static void reset_avctx(struct dec_video *vd)
+{
+    vd_ffmpeg_ctx *ctx = vd->priv;
+
+    if (ctx->avctx)
+        avcodec_flush_buffers(ctx->avctx);
+    ctx->flushing = false;
+}
+
 static void uninit_avctx(struct dec_video *vd)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
@@ -463,6 +472,7 @@ static void uninit_avctx(struct dec_video *vd)
 
     av_frame_free(&ctx->pic);
 
+    ctx->flushing = false;
     ctx->hwdec_failed = false;
     ctx->hwdec_fail_count = 0;
 }
@@ -614,9 +624,6 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
     struct vd_lavc_params *opts = ctx->opts->vd_lavc_params;
     AVPacket pkt;
 
-    if (ctx->hwdec_request_reinit)
-        avcodec_flush_buffers(avctx);
-
     if (flags) {
         // hr-seek framedrop vs. normal framedrop
         avctx->skip_frame = flags == 2 ? AVDISCARD_NONREF : opts->framedrop;
@@ -626,11 +633,21 @@ static void decode(struct dec_video *vd, struct demux_packet *packet,
     }
 
     mp_set_av_packet(&pkt, packet, NULL);
+    ctx->flushing |= !pkt.data;
+
+    // Reset decoder if hw state got reset, or new data comes during flushing.
+    if (ctx->hwdec_request_reinit || (pkt.data && ctx->flushing))
+        reset_avctx(vd);
 
     hwdec_lock(ctx);
     ret = avcodec_decode_video2(avctx, ctx->pic, &got_picture, &pkt);
     hwdec_unlock(ctx);
 
+    // Reset decoder if it was fully flushed. Caller might send more flush
+    // packets, or even new actual packets.
+    if (ctx->flushing && (ret < 0 || !got_picture))
+        reset_avctx(vd);
+
     if (ret < 0) {
         MP_WARN(vd, "Error while decoding frame!\n");
         if (ctx->hwdec) {
@@ -701,10 +718,9 @@ static struct mp_image *decode_with_fallback(struct dec_video *vd,
 static int control(struct dec_video *vd, int cmd, void *arg)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
-    AVCodecContext *avctx = ctx->avctx;
     switch (cmd) {
     case VDCTRL_RESET:
-        avcodec_flush_buffers(avctx);
+        reset_avctx(vd);
         return CONTROL_TRUE;
     case VDCTRL_GET_HWDEC: {
         int hwdec = ctx->hwdec ? ctx->hwdec->type : 0;