avformat/matroskaenc: Don't fail if reserved Cues space doesn't suffice

When the user opted to write the Cues at the beginning, the Cues were
simply written without checking in advance whether enough space has been
reserved for them. If it wasn't enough, the data following the space
reserved for the Cues was simply overwritten, corrupting the file.

This commit changes this by checking whether enough space has been
reserved for the Cues before outputting anything. If it isn't enough,
no Cues will be output at all and the file will be finalized normally,
yet writing the trailer will nevertheless return an error to notify
the user that his wish of having Cues at the front of the file hasn't
been fulfilled.

This change opens new usecases for this option: It is now safe to use
this option to e.g. record live streams or to use it when muxing the
output of an expensive encoding, because when the reserved space turns
out to be insufficient, one ends up with a file that just lacks Cues
but is otherwise fine.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

doc/muxers.texi

@@ -1352,8 +1352,9 @@ index at the beginning of the file.
 
 If this option is set to a non-zero value, the muxer will reserve a given amount
 of space in the file header and then try to write the cues there when the muxing
-finishes. If the available space does not suffice, muxing will fail. A safe size
-for most use cases should be about 50kB per hour of video.
+finishes. If the reserved space does not suffice, no Cues will be written, the
+file will be finalized and writing the trailer will return an error.
+A safe size for most use cases should be about 50kB per hour of video.
 
 Note that cues are only written if the output is seekable and this option will
 have no effect if it is not.

libavformat/matroskaenc.c

@@ -500,23 +500,15 @@ static int mkv_add_cuepoint(MatroskaMuxContext *mkv, int stream, int tracknum, i
     return 0;
 }
 
-static int64_t mkv_write_cues(AVFormatContext *s, mkv_cues *cues, mkv_track *tracks, int num_tracks)
+static int mkv_assemble_cues(AVStream **streams, AVIOContext *dyn_cp,
+                             mkv_cues *cues, mkv_track *tracks, int num_tracks)
 {
-    MatroskaMuxContext *mkv = s->priv_data;
-    AVIOContext *dyn_cp, *pb = s->pb, *cuepoint;
-    int64_t currentpos;
+    AVIOContext *cuepoint;
     int ret;
 
-    currentpos = avio_tell(pb);
-    ret = start_ebml_master_crc32(&dyn_cp, mkv);
-    if (ret < 0)
-        return ret;
-
     ret = avio_open_dyn_buf(&cuepoint);
-    if (ret < 0) {
-        ffio_free_dyn_buf(&dyn_cp);
+    if (ret < 0)
         return ret;
-    }
 
     for (mkv_cuepoint *entry = cues->entries, *end = entry + cues->num_entries;
          entry < end;) {
@@ -535,7 +527,7 @@ static int64_t mkv_write_cues(AVFormatContext *s, mkv_cues *cues, mkv_track *tra
             int idx = entry->stream_idx;
 
             av_assert0(idx >= 0 && idx < num_tracks);
-            if (tracks[idx].has_cue && s->streams[idx]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
+            if (tracks[idx].has_cue && streams[idx]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
                 continue;
             tracks[idx].has_cue = 1;
             track_positions = start_ebml_master(cuepoint, MATROSKA_ID_CUETRACKPOSITION, MAX_CUETRACKPOS_SIZE);
@@ -551,9 +543,8 @@ static int64_t mkv_write_cues(AVFormatContext *s, mkv_cues *cues, mkv_track *tra
         ffio_reset_dyn_buf(cuepoint);
     }
     ffio_free_dyn_buf(&cuepoint);
-    end_ebml_master_crc32(pb, &dyn_cp, mkv, MATROSKA_ID_CUES);
 
-    return currentpos;
+    return 0;
 }
 
 static int put_xiph_codecpriv(AVFormatContext *s, AVIOContext *pb, AVCodecParameters *par)
@@ -2464,7 +2455,7 @@ static int mkv_write_trailer(AVFormatContext *s)
 {
     MatroskaMuxContext *mkv = s->priv_data;
     AVIOContext *pb = s->pb;
-    int64_t currentpos, cuespos;
+    int64_t currentpos;
     int ret;
 
     // check if we have an audio packet cached
@@ -2487,35 +2478,52 @@ static int mkv_write_trailer(AVFormatContext *s)
 
 
     if ((pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
+        int64_t ret64;
+
         if (mkv->cues.num_entries) {
-            if (mkv->reserve_cues_space) {
-                int64_t cues_end;
+            AVIOContext *cues;
+            uint64_t size;
 
-                currentpos = avio_tell(pb);
-                avio_seek(pb, mkv->cues_pos, SEEK_SET);
+            ret = start_ebml_master_crc32(&cues, mkv);
+            if (ret < 0)
+                return ret;
 
-                cuespos  = mkv_write_cues(s, &mkv->cues, mkv->tracks, s->nb_streams);
-                cues_end = avio_tell(pb);
-                if (cues_end > cuespos + mkv->reserve_cues_space) {
-                    av_log(s, AV_LOG_ERROR,
-                           "Insufficient space reserved for cues: %d "
-                           "(needed: %" PRId64 ").\n",
-                           mkv->reserve_cues_space, cues_end - cuespos);
-                    return AVERROR(EINVAL);
+            ret = mkv_assemble_cues(s->streams, cues, &mkv->cues,
+                                    mkv->tracks, s->nb_streams);
+            if (ret < 0) {
+                ffio_free_dyn_buf(&cues);
+                return ret;
             }
 
-                if (cues_end < cuespos + mkv->reserve_cues_space)
-                    put_ebml_void(pb, mkv->reserve_cues_space -
-                                  (cues_end - cuespos));
-
-                avio_seek(pb, currentpos, SEEK_SET);
+            if (mkv->reserve_cues_space) {
+                size  = avio_tell(cues);
+                size += 4 + ebml_num_size(size);
+                if (mkv->reserve_cues_space < size) {
+                    av_log(s, AV_LOG_WARNING,
+                           "Insufficient space reserved for Cues: "
+                           "%d < %"PRIu64". No Cues will be output.\n",
+                           mkv->reserve_cues_space, size);
+                    mkv->reserve_cues_space = -1;
+                    ffio_free_dyn_buf(&cues);
+                    goto after_cues;
                 } else {
-                cuespos = mkv_write_cues(s, &mkv->cues, mkv->tracks, s->nb_streams);
+                    currentpos = avio_tell(pb);
+                    if ((ret64 = avio_seek(pb, mkv->cues_pos, SEEK_SET)) < 0) {
+                        ffio_free_dyn_buf(&cues);
+                        return ret64;
+                    }
+                }
+            }
+            mkv_add_seekhead_entry(mkv, MATROSKA_ID_CUES, avio_tell(pb));
+            end_ebml_master_crc32(pb, &cues, mkv, MATROSKA_ID_CUES);
+            if (mkv->reserve_cues_space) {
+                if (size < mkv->reserve_cues_space)
+                    put_ebml_void(pb, mkv->reserve_cues_space - size);
+                avio_seek(pb, currentpos, SEEK_SET);
             }
-
-            mkv_add_seekhead_entry(mkv, MATROSKA_ID_CUES, cuespos);
         }
 
+    after_cues:
         currentpos = avio_tell(pb);
 
         ret = mkv_write_seekhead(pb, mkv, 1, mkv->info_pos);
@@ -2570,7 +2578,7 @@ static int mkv_write_trailer(AVFormatContext *s)
         end_ebml_master(pb, mkv->segment);
     }
 
-    return 0;
+    return mkv->reserve_cues_space < 0 ? AVERROR(EINVAL) : 0;
 }
 
 static int mkv_query_codec(enum AVCodecID codec_id, int std_compliance)