vd_lavc: complicated improved fallback behavior for --hwdec=cuda

The ffmpeg cuda wrappers need more than 1 packet for determining whether
hw decoding will work. So do something complicated and keep up to 32
packets when trying to do hw decoding, and replay the packets on the
software decoder if it doesn't work.

This code was written in a delirious state, testing for regressions and
determining whether this is worth the trouble will follow later. All mpv
git users are alpha testers as of this moment.

Fixes #3914.

video/decode/lavc.h

@@ -30,7 +30,12 @@ typedef struct lavc_ctx {
     // For HDR side-data caching
     double cached_hdr_peak;
 
-    struct demux_packet *prev_packet;
+    bool hw_probing;
+    struct demux_packet **sent_packets;
+    int num_sent_packets;
+
+    struct demux_packet **requeue_packets;
+    int num_requeue_packets;
 
     struct mp_image **delay_queue;
     int num_delay_queue;

video/decode/vd_lavc.c

@@ -514,6 +514,7 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
         if (ctx->hwdec->init && ctx->hwdec->init(ctx) < 0)
             goto error;
         ctx->max_delay_queue = ctx->hwdec->delay_queue;
+        ctx->hw_probing = true;
     } else {
         mp_set_avcodec_threads(vd->log, avctx, lavc_param->threads);
     }
@@ -578,8 +579,13 @@ static void flush_all(struct dec_video *vd)
         talloc_free(ctx->delay_queue[n]);
     ctx->num_delay_queue = 0;
 
-    talloc_free(ctx->prev_packet);
-    ctx->prev_packet = NULL;
+    for (int n = 0; n < ctx->num_sent_packets; n++)
+        talloc_free(ctx->sent_packets[n]);
+    ctx->num_sent_packets = 0;
+
+    for (int n = 0; n < ctx->num_requeue_packets; n++)
+        talloc_free(ctx->requeue_packets[n]);
+    ctx->num_requeue_packets = 0;
 
     reset_avctx(vd);
 }
@@ -607,6 +613,7 @@ static void uninit_avctx(struct dec_video *vd)
     ctx->hwdec_failed = false;
     ctx->hwdec_fail_count = 0;
     ctx->max_delay_queue = 0;
+    ctx->hw_probing = false;
 }
 
 static void update_image_params(struct dec_video *vd, AVFrame *frame,
@@ -748,13 +755,16 @@ static int get_buffer2_hwdec(AVCodecContext *avctx, AVFrame *pic, int flags)
     return 0;
 }
 
-static struct mp_image *read_output(struct dec_video *vd)
+static struct mp_image *read_output(struct dec_video *vd, bool eof)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
 
     if (!ctx->num_delay_queue)
         return NULL;
 
+    if (ctx->num_delay_queue <= ctx->max_delay_queue && !eof)
+        return NULL;
+
     struct mp_image *res = ctx->delay_queue[0];
     MP_TARRAY_REMOVE_AT(ctx->delay_queue, ctx->num_delay_queue, 0);
 
@@ -773,6 +783,11 @@ static struct mp_image *read_output(struct dec_video *vd)
             MP_INFO(vd, "Using software decoding.\n");
         }
         ctx->hwdec_notified = true;
+
+        ctx->hw_probing = false;
+        for (int n = 0; n < ctx->num_sent_packets; n++)
+            talloc_free(ctx->sent_packets[n]);
+        ctx->num_sent_packets = 0;
     }
 
     return res;
@@ -818,7 +833,7 @@ static void handle_err(struct dec_video *vd)
     }
 }
 
-static bool send_packet(struct dec_video *vd, struct demux_packet *pkt)
+static bool do_send_packet(struct dec_video *vd, struct demux_packet *pkt)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
     AVCodecContext *avctx = ctx->avctx;
@@ -836,14 +851,29 @@ static bool send_packet(struct dec_video *vd, struct demux_packet *pkt)
     if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
         return false;
 
-    talloc_free(ctx->prev_packet);
-    ctx->prev_packet = pkt ? demux_copy_packet(pkt) : NULL;
+    if (ctx->hw_probing && ctx->num_sent_packets < 32) {
+        pkt = pkt ? demux_copy_packet(pkt) : NULL;
+        MP_TARRAY_APPEND(ctx, ctx->sent_packets, ctx->num_sent_packets, pkt);
+    }
 
     if (ret < 0)
         handle_err(vd);
     return true;
 }
 
+static bool send_packet(struct dec_video *vd, struct demux_packet *pkt)
+{
+    vd_ffmpeg_ctx *ctx = vd->priv;
+
+    if (ctx->num_requeue_packets) {
+        if (do_send_packet(vd, ctx->requeue_packets[0]))
+            MP_TARRAY_REMOVE_AT(ctx->requeue_packets, ctx->num_requeue_packets, 0);
+        return false;
+    }
+
+    return do_send_packet(vd, pkt);
+}
+
 // Returns EOF state.
 static bool decode_frame(struct dec_video *vd)
 {
@@ -912,20 +942,34 @@ static struct mp_image *receive_frame(struct dec_video *vd)
 
     if (ctx->hwdec_failed) {
         // Failed hardware decoding? Try again in software.
-        struct demux_packet *pkt = ctx->prev_packet;
-        ctx->prev_packet = NULL;
+        struct demux_packet **pkts = ctx->sent_packets;
+        int num_pkts = ctx->num_sent_packets;
+        ctx->sent_packets = NULL;
+        ctx->num_sent_packets = 0;
 
         force_fallback(vd);
-        if (pkt)
-            send_packet(vd, pkt);
-        talloc_free(pkt);
 
-        eof = decode_frame(vd);
+        struct mp_image *img = NULL;
+
+        while (num_pkts > 0) {
+            if (send_packet(vd, pkts[0])) {
+                talloc_free(pkts[0]);
+                MP_TARRAY_REMOVE_AT(pkts, num_pkts, 0);
+            }
+            if (decode_frame(vd)) {
+                eof = true;
+                break;
+            }
+            img = read_output(vd, eof);
+            if (img)
+                break;
+        }
+
+        ctx->requeue_packets = pkts;
+        ctx->num_requeue_packets = num_pkts;
     }
 
-    if (eof || ctx->num_delay_queue > ctx->max_delay_queue)
-        return read_output(vd);
-    return NULL;
+    return read_output(vd, eof);
 }
 
 static int control(struct dec_video *vd, int cmd, void *arg)