movenc: add faststart option for web streaming

Faststart moves the moov atom to the beginning of the file and rewrites
the rest of the file after muxing is complete.
Signed-off-by: Martin Storsjö <martin@martin.st>

doc/muxers.texi

@@ -239,6 +239,10 @@ more efficient), but with this option set, the muxer writes one moof/mdat
 pair for each track, making it easier to separate tracks.
 
 This option is implicitly set when writing ismv (Smooth Streaming) files.
+@item -movflags faststart
+Run a second pass moving the index (moov atom) to the beginning of the file.
+This operation can take a while, and will not work in various situations such
+as fragmented output, thus it is not enabled by default.
 @end table
 
 Smooth Streaming content can be pushed in real time to a publishing

libavformat/movenc.c

@@ -51,6 +51,7 @@ static const AVOption options[] = {
     { "separate_moof", "Write separate moof/mdat atoms for each track", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_SEPARATE_MOOF}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     { "frag_custom", "Flush fragments on caller requests", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_FRAG_CUSTOM}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     { "isml", "Create a live smooth streaming feed (for pushing to a publishing point)", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_ISML}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
+    { "faststart", "Run a second pass to put the index (moov atom) at the beginning of the file", 0, AV_OPT_TYPE_CONST, {.i64 = FF_MOV_FLAG_FASTSTART}, INT_MIN, INT_MAX, AV_OPT_FLAG_ENCODING_PARAM, "movflags" },
     FF_RTP_FLAG_OPTS(MOVMuxContext, rtp_flags),
     { "skip_iods", "Skip writing iods atom.", offsetof(MOVMuxContext, iods_skip), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 1, AV_OPT_FLAG_ENCODING_PARAM},
     { "iods_audio_profile", "iods audio profile atom.", offsetof(MOVMuxContext, iods_audio_profile), AV_OPT_TYPE_INT, {.i64 = -1}, -1, 255, AV_OPT_FLAG_ENCODING_PARAM},
@@ -3027,6 +3028,13 @@ static int mov_write_header(AVFormatContext *s)
                       FF_MOV_FLAG_FRAG_CUSTOM))
         mov->flags |= FF_MOV_FLAG_FRAGMENT;
 
+    /* faststart: moov at the beginning of the file, if supported */
+    if (mov->flags & FF_MOV_FLAG_FASTSTART) {
+        if ((mov->flags & FF_MOV_FLAG_FRAGMENT) ||
+            (s->flags & AVFMT_FLAG_CUSTOM_IO))
+            mov->flags &= ~FF_MOV_FLAG_FASTSTART;
+    }
+
     /* Non-seekable output is ok if using fragmentation. If ism_lookahead
      * is enabled, we don't support non-seekable output at all. */
     if (!s->pb->seekable &&
@@ -3169,8 +3177,11 @@ static int mov_write_header(AVFormatContext *s)
                       FF_MOV_FLAG_FRAGMENT;
     }
 
-    if (!(mov->flags & FF_MOV_FLAG_FRAGMENT))
+    if (!(mov->flags & FF_MOV_FLAG_FRAGMENT)) {
+        if (mov->flags & FF_MOV_FLAG_FASTSTART)
+            mov->reserved_moov_pos = avio_tell(pb);
         mov_write_mdat_tag(pb, mov);
+    }
 
     if (t = av_dict_get(s->metadata, "creation_time", NULL, 0))
         mov->time = ff_iso8601_to_unix_time(t->value);
@@ -3208,6 +3219,115 @@ static int mov_write_header(AVFormatContext *s)
     return -1;
 }
 
+static int get_moov_size(AVFormatContext *s)
+{
+    int ret;
+    uint8_t *buf;
+    AVIOContext *moov_buf;
+    MOVMuxContext *mov = s->priv_data;
+
+    if ((ret = avio_open_dyn_buf(&moov_buf)) < 0)
+        return ret;
+    mov_write_moov_tag(moov_buf, mov, s);
+    ret = avio_close_dyn_buf(moov_buf, &buf);
+    av_free(buf);
+    return ret;
+}
+
+/*
+ * This function gets the moov size if moved to the top of the file: the chunk
+ * offset table can switch between stco (32-bit entries) to co64 (64-bit
+ * entries) when the moov is moved to the beginning, so the size of the moov
+ * would change. It also updates the chunk offset tables.
+ */
+static int compute_moov_size(AVFormatContext *s)
+{
+    int i, moov_size, moov_size2;
+    MOVMuxContext *mov = s->priv_data;
+
+    moov_size = get_moov_size(s);
+    if (moov_size < 0)
+        return moov_size;
+
+    for (i = 0; i < mov->nb_streams; i++)
+        mov->tracks[i].data_offset += moov_size;
+
+    moov_size2 = get_moov_size(s);
+    if (moov_size2 < 0)
+        return moov_size2;
+
+    /* if the size changed, we just switched from stco to co64 and need to
+     * update the offsets */
+    if (moov_size2 != moov_size)
+        for (i = 0; i < mov->nb_streams; i++)
+            mov->tracks[i].data_offset += moov_size2 - moov_size;
+
+    return moov_size2;
+}
+
+static int shift_data(AVFormatContext *s)
+{
+    int ret = 0, moov_size;
+    MOVMuxContext *mov = s->priv_data;
+    int64_t pos, pos_end = avio_tell(s->pb);
+    uint8_t *buf, *read_buf[2];
+    int read_buf_id = 0;
+    int read_size[2];
+    AVIOContext *read_pb;
+
+    moov_size = compute_moov_size(s);
+    if (moov_size < 0)
+        return moov_size;
+
+    buf = av_malloc(moov_size * 2);
+    if (!buf)
+        return AVERROR(ENOMEM);
+    read_buf[0] = buf;
+    read_buf[1] = buf + moov_size;
+
+    /* Shift the data: the AVIO context of the output can only be used for
+     * writing, so we re-open the same output, but for reading. It also avoids
+     * a read/seek/write/seek back and forth. */
+    avio_flush(s->pb);
+    ret = avio_open(&read_pb, s->filename, AVIO_FLAG_READ);
+    if (ret < 0) {
+        av_log(s, AV_LOG_ERROR, "Unable to re-open %s output file for "
+               "the second pass (faststart)\n", s->filename);
+        goto end;
+    }
+
+    /* mark the end of the shift to up to the last data we wrote, and get ready
+     * for writing */
+    pos_end = avio_tell(s->pb);
+    avio_seek(s->pb, mov->reserved_moov_pos + moov_size, SEEK_SET);
+
+    /* start reading at where the new moov will be placed */
+    avio_seek(read_pb, mov->reserved_moov_pos, SEEK_SET);
+    pos = avio_tell(read_pb);
+
+#define READ_BLOCK do {                                                             \
+    read_size[read_buf_id] = avio_read(read_pb, read_buf[read_buf_id], moov_size);  \
+    read_buf_id ^= 1;                                                               \
+} while (0)
+
+    /* shift data by chunk of at most moov_size */
+    READ_BLOCK;
+    do {
+        int n;
+        READ_BLOCK;
+        n = read_size[read_buf_id];
+        if (n <= 0)
+            break;
+        avio_write(s->pb, read_buf[read_buf_id], n);
+        pos += n;
+    } while (pos < pos_end);
+    avio_close(read_pb);
+
+end:
+    av_free(buf);
+    return ret;
+}
+
 static int mov_write_trailer(AVFormatContext *s)
 {
     MOVMuxContext *mov = s->priv_data;
@@ -3226,9 +3346,9 @@ static int mov_write_trailer(AVFormatContext *s)
         }
     }
 
+    if (!(mov->flags & FF_MOV_FLAG_FRAGMENT)) {
         moov_pos = avio_tell(pb);
 
-    if (!(mov->flags & FF_MOV_FLAG_FRAGMENT)) {
         /* Write size of mdat tag */
         if (mov->mdat_size + 8 <= UINT32_MAX) {
             avio_seek(pb, mov->mdat_pos, SEEK_SET);
@@ -3244,7 +3364,16 @@ static int mov_write_trailer(AVFormatContext *s)
         }
         avio_seek(pb, moov_pos, SEEK_SET);
 
+        if (mov->flags & FF_MOV_FLAG_FASTSTART) {
+            av_log(s, AV_LOG_INFO, "Starting second pass: moving the moov atom to the beginning of the file\n");
+            res = shift_data(s);
+            if (res == 0) {
+                avio_seek(s->pb, mov->reserved_moov_pos, SEEK_SET);
+                mov_write_moov_tag(pb, mov, s);
+            }
+        } else {
             mov_write_moov_tag(pb, mov, s);
+        }
     } else {
         mov_flush_fragment(s);
         mov_write_mfra_tag(pb, mov);

libavformat/movenc.h

@@ -156,6 +156,8 @@ typedef struct MOVMuxContext {
     int max_fragment_size;
     int ism_lookahead;
     AVIOContext *mdat_buf;
+
+    int64_t reserved_moov_pos;
 } MOVMuxContext;
 
 #define FF_MOV_FLAG_RTP_HINT 1
@@ -165,6 +167,7 @@ typedef struct MOVMuxContext {
 #define FF_MOV_FLAG_SEPARATE_MOOF 16
 #define FF_MOV_FLAG_FRAG_CUSTOM 32
 #define FF_MOV_FLAG_ISML 64
+#define FF_MOV_FLAG_FASTSTART 128
 
 int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt);