avformat/concatdec: add fallback for calculating file duration

If a file does not have a known duration, this leads to the timestamps
starting over for the next file, causing non-monotonic timestamps.
To prevent this, track the duration during demuxing and use it to
determine the current file duration before opening the next file.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/concatdec.c

@@ -44,6 +44,7 @@ typedef struct {
     int64_t file_start_time;
     int64_t file_inpoint;
     int64_t duration;
+    int64_t next_dts;
     ConcatStream *streams;
     int64_t inpoint;
     int64_t outpoint;
@@ -149,6 +150,7 @@ static int add_file(AVFormatContext *avf, char *filename, ConcatFile **rfile,
     file->url        = url;
     file->start_time = AV_NOPTS_VALUE;
     file->duration   = AV_NOPTS_VALUE;
+    file->next_dts   = AV_NOPTS_VALUE;
     file->inpoint    = AV_NOPTS_VALUE;
     file->outpoint   = AV_NOPTS_VALUE;
 
@@ -509,8 +511,14 @@ static int open_next_file(AVFormatContext *avf)
     ConcatContext *cat = avf->priv_data;
     unsigned fileno = cat->cur_file - cat->files;
 
-    if (cat->cur_file->duration == AV_NOPTS_VALUE)
-        cat->cur_file->duration = cat->avf->duration - (cat->cur_file->file_inpoint - cat->cur_file->file_start_time);
+    if (cat->cur_file->duration == AV_NOPTS_VALUE) {
+        if (cat->avf->duration > 0 || cat->cur_file->next_dts == AV_NOPTS_VALUE) {
+            cat->cur_file->duration = cat->avf->duration;
+        } else {
+            cat->cur_file->duration = cat->cur_file->next_dts;
+        }
+        cat->cur_file->duration -= (cat->cur_file->file_inpoint - cat->cur_file->file_start_time);
+    }
 
     if (++fileno >= cat->nb_files) {
         cat->eof = 1;
@@ -627,6 +635,14 @@ static int concat_read_packet(AVFormatContext *avf, AVPacket *pkt)
         memcpy(metadata, packed_metadata, metadata_len);
         av_freep(&packed_metadata);
     }
+
+    if (cat->cur_file->duration == AV_NOPTS_VALUE && st->cur_dts != AV_NOPTS_VALUE) {
+        int64_t next_dts = av_rescale_q(st->cur_dts, st->time_base, AV_TIME_BASE_Q);
+        if (cat->cur_file->next_dts == AV_NOPTS_VALUE || next_dts > cat->cur_file->next_dts) {
+            cat->cur_file->next_dts = next_dts;
+        }
+    }
+
     return ret;
 }