matroskaenc: add an option to put the index at the start of the file

Changelog

@@ -16,6 +16,7 @@ version 10:
 - new trim and atrim filters
 - avconv -t and -ss (output-only) options are now sample-accurate when
   transcoding audio
+- Matroska muxer can now put the index at the beginning of the file.
 
 
 version 9:

doc/muxers.texi

@@ -373,6 +373,27 @@ For example a 3D WebM clip can be created using the following command line:
 avconv -i sample_left_right_clip.mpg -an -c:v libvpx -metadata STEREO_MODE=left_right -y stereo_clip.webm
 @end example
 
+This muxer supports the following options:
+
+@table @option
+
+@item reserve_index_space
+By default, this muxer writes the index for seeking (called cues in Matroska
+terms) at the end of the file, because it cannot know in advance how much space
+to leave for the index at the beginning of the file. However for some use cases
+-- e.g.  streaming where seeking is possible but slow -- it is useful to put the
+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.
+
+Note that cues are only written if the output is seekable and this option will
+have no effect if it is not.
+
+@end table
+
 @section segment
 
 Basic stream segmenter.

libavformat/matroskaenc.c

@@ -34,6 +34,7 @@
 #include "libavutil/intreadwrite.h"
 #include "libavutil/lfg.h"
 #include "libavutil/mathematics.h"
+#include "libavutil/opt.h"
 #include "libavutil/random_seed.h"
 #include "libavutil/samplefmt.h"
 
@@ -79,6 +80,7 @@ typedef struct {
 #define MODE_WEBM       0x02
 
 typedef struct MatroskaMuxContext {
+    const AVClass  *class;
     int             mode;
     AVIOContext   *dyn_bc;
     ebml_master     segment;
@@ -95,6 +97,9 @@ typedef struct MatroskaMuxContext {
     AVPacket        cur_audio_pkt;
 
     int have_attachments;
+
+    int reserve_cues_space;
+    int64_t cues_pos;
 } MatroskaMuxContext;
 
 
@@ -968,6 +973,11 @@ static int mkv_write_header(AVFormatContext *s)
     if (mkv->cues == NULL)
         return AVERROR(ENOMEM);
 
+    if (pb->seekable && mkv->reserve_cues_space) {
+        mkv->cues_pos = avio_tell(pb);
+        put_ebml_void(pb, mkv->reserve_cues_space);
+    }
+
     av_init_packet(&mkv->cur_audio_pkt);
     mkv->cur_audio_pkt.size = 0;
 
@@ -1250,7 +1260,28 @@ static int mkv_write_trailer(AVFormatContext *s)
 
     if (pb->seekable) {
         if (mkv->cues->num_entries) {
-            cuespos = mkv_write_cues(pb, mkv->cues, s->nb_streams);
+            if (mkv->reserve_cues_space) {
+                int64_t cues_end;
+
+                currentpos = avio_tell(pb);
+                avio_seek(pb, mkv->cues_pos, SEEK_SET);
+
+                cuespos = mkv_write_cues(pb, mkv->cues, 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);
+                }
+
+                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);
+            } else {
+                cuespos = mkv_write_cues(pb, mkv->cues, s->nb_streams);
+            }
 
             ret = mkv_add_seekhead_entry(mkv->main_seekhead, MATROSKA_ID_CUES, cuespos);
             if (ret < 0) return ret;
@@ -1291,7 +1322,22 @@ static int mkv_query_codec(enum AVCodecID codec_id, int std_compliance)
     return 0;
 }
 
+#define OFFSET(x) offsetof(MatroskaMuxContext, x)
+#define FLAGS AV_OPT_FLAG_ENCODING_PARAM
+static const AVOption options[] = {
+    { "reserve_index_space", "Reserve a given amount of space (in bytes) at the beginning "
+        "of the file for the index (cues).", OFFSET(reserve_cues_space), AV_OPT_TYPE_INT, { .i64 = 0 }, 0, INT_MAX, FLAGS },
+    { NULL },
+};
+
 #if CONFIG_MATROSKA_MUXER
+static const AVClass matroska_class = {
+    .class_name = "matroska muxer",
+    .item_name  = av_default_item_name,
+    .option     = options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
+
 AVOutputFormat ff_matroska_muxer = {
     .name              = "matroska",
     .long_name         = NULL_IF_CONFIG_SMALL("Matroska"),
@@ -1312,10 +1358,18 @@ AVOutputFormat ff_matroska_muxer = {
     },
     .subtitle_codec    = AV_CODEC_ID_SSA,
     .query_codec       = mkv_query_codec,
+    .priv_class        = &matroska_class,
 };
 #endif
 
 #if CONFIG_WEBM_MUXER
+static const AVClass webm_class = {
+    .class_name = "webm muxer",
+    .item_name  = av_default_item_name,
+    .option     = options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
+
 AVOutputFormat ff_webm_muxer = {
     .name              = "webm",
     .long_name         = NULL_IF_CONFIG_SMALL("WebM"),
@@ -1329,10 +1383,17 @@ AVOutputFormat ff_webm_muxer = {
     .write_trailer     = mkv_write_trailer,
     .flags             = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |
                          AVFMT_TS_NONSTRICT,
+    .priv_class        = &webm_class,
 };
 #endif
 
 #if CONFIG_MATROSKA_AUDIO_MUXER
+static const AVClass mka_class = {
+    .class_name = "matroska audio muxer",
+    .item_name  = av_default_item_name,
+    .option     = options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
 AVOutputFormat ff_matroska_audio_muxer = {
     .name              = "matroska",
     .long_name         = NULL_IF_CONFIG_SMALL("Matroska"),
@@ -1347,5 +1408,6 @@ AVOutputFormat ff_matroska_audio_muxer = {
     .write_trailer     = mkv_write_trailer,
     .flags             = AVFMT_GLOBALHEADER | AVFMT_TS_NONSTRICT,
     .codec_tag         = (const AVCodecTag* const []){ ff_codec_wav_tags, 0 },
+    .priv_class        = &mka_class,
 };
 #endif

libavformat/version.h

@@ -31,7 +31,7 @@
 
 #define LIBAVFORMAT_VERSION_MAJOR 55
 #define LIBAVFORMAT_VERSION_MINOR  0
-#define LIBAVFORMAT_VERSION_MICRO  0
+#define LIBAVFORMAT_VERSION_MICRO  1
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
                                                LIBAVFORMAT_VERSION_MINOR, \