Serial has two purposes nowadays: the initial purpose in the commit was to be able to distinguish packets in the packet queue from before and after a seek. The demuxer (input for packet queue) runs in a separate thread. After a seek, we want to flush it, but we don't want to stop the producer thread because overhead. However, we also don't want to flush too few or too many packets. So, the serial field tells us which packets are pre- and post-flush and thus which packets to drop without having to stop the producer thread while we're dropping those packets.
The second purpose is your line of code: it tells us when EOF occurs. Finished is set to the last serial number of a packet from the packet queue used to decode a frame. If that serial number is also the tail of the packet queue (and no more packets are produced), it means we stopped producing packets and decoded the frame belonging to that packet. In other words: end-of-file. Elsewhere, you'll find a test along those lines, and then either playback stops or (if looping is enabled) we seek back to the beginning of the file (i.e. invoke the looping behaviour).
(This write-up was helpfully assisted by several FFmpeg developers on IRC.)

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The purpose of the serial field is to accompany the decoded data during the
decoding process to know if the decoded data belongs to the data stream after
the latest packet queue flush.

代码

 static int sws_flags = SWS_BICUBIC;
 
+typedef struct MyAVPacketList {
+    AVPacket pkt;
+    struct MyAVPacketList *next;
+    int serial;
+} MyAVPacketList;
+
 typedef struct PacketQueue {
-    AVPacketList *first_pkt, *last_pkt;
+    MyAVPacketList *first_pkt, *last_pkt;
     int nb_packets;
     int size;
     int abort_request;
+    int serial;
     SDL_mutex *mutex;
     SDL_cond *cond;
 } PacketQueue;
@@ -108,6 +115,7 @@ typedef struct VideoPicture {
     AVRational sample_aspect_ratio;
     int allocated;
     int reallocate;
+    int serial;
 
 #if CONFIG_AVFILTER
     AVFilterBufferRef *picref;
@@ -174,6 +182,7 @@ typedef struct VideoState {
     int audio_write_buf_size;
     AVPacket audio_pkt_temp;
     AVPacket audio_pkt;
+    int audio_pkt_temp_serial;
     struct AudioParams audio_src;
     struct AudioParams audio_tgt;
     struct SwrContext *swr_ctx;
@@ -305,16 +314,19 @@ static int packet_queue_put(PacketQueue *q, AVPacket *pkt);
 
 static int packet_queue_put_private(PacketQueue *q, AVPacket *pkt)
 {
-    AVPacketList *pkt1;
+    MyAVPacketList *pkt1;
 
     if (q->abort_request)
        return -1;
 
-    pkt1 = av_malloc(sizeof(AVPacketList));
+    pkt1 = av_malloc(sizeof(MyAVPacketList));
     if (!pkt1)
         return -1;
     pkt1->pkt = *pkt;
     pkt1->next = NULL;
+    if (pkt == &flush_pkt)
+        q->serial++;
+    pkt1->serial = q->serial;
 
     if (!q->last_pkt)
         q->first_pkt = pkt1;
@@ -357,7 +369,7 @@ static void packet_queue_init(PacketQueue *q)
 
 static void packet_queue_flush(PacketQueue *q)
 {
-    AVPacketList *pkt, *pkt1;
+    MyAVPacketList *pkt, *pkt1;
 
     SDL_LockMutex(q->mutex);
     for (pkt = q->first_pkt; pkt != NULL; pkt = pkt1) {
@@ -399,9 +411,9 @@ static void packet_queue_start(PacketQueue *q)
 }
 
 /* return < 0 if aborted, 0 if no packet and > 0 if packet.  */
-static int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block)
+static int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block, int *serial)
 {
-    AVPacketList *pkt1;
+    MyAVPacketList *pkt1;
     int ret;
 
     SDL_LockMutex(q->mutex);
@@ -420,6 +432,8 @@ static int packet_queue_get(PacketQueue *q, AVPacket *pkt, int block)
             q->nb_packets--;
             q->size -= pkt1->pkt.size + sizeof(*pkt1);
             *pkt = pkt1->pkt;
+            if (serial)
+                *serial = pkt1->serial;
             av_free(pkt1);
             ret = 1;
             break;
@@ -1169,7 +1183,7 @@ static void pictq_prev_picture(VideoState *is) {
     }
 }
 
-static void update_video_pts(VideoState *is, double pts, int64_t pos) {
+static void update_video_pts(VideoState *is, double pts, int64_t pos, int serial) {
     double time = av_gettime() / 1000000.0;
     /* update current video pts */
     is->video_current_pts = pts;
@@ -1195,7 +1209,7 @@ retry:
         if (is->pictq_size == 0) {
             SDL_LockMutex(is->pictq_mutex);
             if (is->frame_last_dropped_pts != AV_NOPTS_VALUE && is->frame_last_dropped_pts > is->frame_last_pts) {
-                update_video_pts(is, is->frame_last_dropped_pts, is->frame_last_dropped_pos);
+                update_video_pts(is, is->frame_last_dropped_pts, is->frame_last_dropped_pos, 0);
                 is->frame_last_dropped_pts = AV_NOPTS_VALUE;
             }
             SDL_UnlockMutex(is->pictq_mutex);
@@ -1229,7 +1243,7 @@ retry:
                 is->frame_timer += delay * FFMAX(1, floor((time-is->frame_timer) / delay));
 
             SDL_LockMutex(is->pictq_mutex);
-            update_video_pts(is, vp->pts, vp->pos);
+            update_video_pts(is, vp->pts, vp->pos, vp->serial);
             SDL_UnlockMutex(is->pictq_mutex);
 
             if (is->pictq_size > 1) {
@@ -1374,7 +1388,7 @@ static void alloc_picture(VideoState *is)
     SDL_UnlockMutex(is->pictq_mutex);
 }
 
-static int queue_picture(VideoState *is, AVFrame *src_frame, double pts1, int64_t pos)
+static int queue_picture(VideoState *is, AVFrame *src_frame, double pts1, int64_t pos, int serial)
 {
     VideoPicture *vp;
     double frame_delay, pts = pts1;
@@ -1495,6 +1509,7 @@ static int queue_picture(VideoState *is, AVFrame *src_frame, double pts1, int64_
         vp->pts = pts;
         vp->pos = pos;
         vp->skip = 0;
+        vp->serial = serial;
 
         /* now we can update the picture count */
         if (++is->pictq_windex == VIDEO_PICTURE_QUEUE_SIZE)
@@ -1506,11 +1521,11 @@ static int queue_picture(VideoState *is, AVFrame *src_frame, double pts1, int64_
     return 0;
 }
 
-static int get_video_frame(VideoState *is, AVFrame *frame, int64_t *pts, AVPacket *pkt)
+static int get_video_frame(VideoState *is, AVFrame *frame, int64_t *pts, AVPacket *pkt, int *serial)
 {
     int got_picture, i;
 
-    if (packet_queue_get(&is->videoq, pkt, 1) < 0)
+    if (packet_queue_get(&is->videoq, pkt, 1, serial) < 0)
         return -1;
 
     if (pkt->data == flush_pkt.data) {
@@ -1682,6 +1697,7 @@ static int video_thread(void *arg)
     int64_t pts_int = AV_NOPTS_VALUE, pos = -1;
     double pts;
     int ret;
+    int serial = 0;
 
 #if CONFIG_AVFILTER
     AVCodecContext *codec = is->video_st->codec;
@@ -1710,7 +1726,7 @@ static int video_thread(void *arg)
         avcodec_get_frame_defaults(frame);
         av_free_packet(&pkt);
 
-        ret = get_video_frame(is, frame, &pts_int, &pkt);
+        ret = get_video_frame(is, frame, &pts_int, &pkt, &serial);
         if (ret < 0)
             goto the_end;
 
@@ -1791,11 +1807,11 @@ static int video_thread(void *arg)
                         is->video_st->time_base.num, is->video_st->time_base.den, pts_int);
             }
             pts = pts_int * av_q2d(is->video_st->time_base);
-            ret = queue_picture(is, frame, pts, pos);
+            ret = queue_picture(is, frame, pts, pos, serial);
         }
 #else
         pts = pts_int * av_q2d(is->video_st->time_base);
-        ret = queue_picture(is, frame, pts, pkt.pos);
+        ret = queue_picture(is, frame, pts, pkt.pos, serial);
 #endif
 
         if (ret < 0)
@@ -1828,7 +1844,7 @@ static int subtitle_thread(void *arg)
         while (is->paused && !is->subtitleq.abort_request) {
             SDL_Delay(10);
         }
-        if (packet_queue_get(&is->subtitleq, pkt, 1) < 0)
+        if (packet_queue_get(&is->subtitleq, pkt, 1, NULL) < 0)
             break;
 
         if (pkt->data == flush_pkt.data) {
@@ -2079,7 +2095,7 @@ static int audio_decode_frame(VideoState *is, double *pts_ptr)
             SDL_CondSignal(is->continue_read_thread);
 
         /* read next packet */
-        if ((new_packet = packet_queue_get(&is->audioq, pkt, 1)) < 0)
+        if ((new_packet = packet_queue_get(&is->audioq, pkt, 1, &is->audio_pkt_temp_serial)) < 0)
             return -1;
 
         if (pkt->data == flush_pkt.data) {

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