lavf/webm_chunk: Use dyn_buf to write chunks

Use dyn_duf to write chunks so that we create the actual chunk
file only after the entire chunk data is available. This will help
not confuse other software looking at the chunk file (e.g. a web
server) by seeing a zero length file when ffmpeg is writing into
it.
Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/webm_chunk.c

@@ -41,12 +41,13 @@
 #include "libavutil/time_internal.h"
 #include "libavutil/timestamp.h"
 
+#define MAX_FILENAME_SIZE 1024
+
 typedef struct WebMChunkContext {
     const AVClass *class;
     int chunk_start_index;
     char *header_filename;
     int chunk_duration;
-    int chunk_count;
     int chunk_index;
     uint64_t duration_written;
     int prev_pts;
@@ -86,18 +87,21 @@ static int chunk_mux_init(AVFormatContext *s)
     return 0;
 }
 
-static int set_chunk_filename(AVFormatContext *s, int is_header)
+static int get_chunk_filename(AVFormatContext *s, int is_header, char *filename)
 {
     WebMChunkContext *wc = s->priv_data;
     AVFormatContext *oc = wc->avf;
+    if (!filename) {
+        return AVERROR(EINVAL);
+    }
     if (is_header) {
         if (!wc->header_filename) {
             return AVERROR(EINVAL);
         }
-        av_strlcpy(oc->filename, wc->header_filename, strlen(wc->header_filename) + 1);
+        av_strlcpy(filename, wc->header_filename, strlen(wc->header_filename) + 1);
     } else {
-        if (av_get_frame_filename(oc->filename, sizeof(oc->filename),
-                                  s->filename, wc->chunk_index) < 0) {
+        if (av_get_frame_filename(filename, MAX_FILENAME_SIZE,
+                                  s->filename, wc->chunk_index - 1) < 0) {
             av_log(oc, AV_LOG_ERROR, "Invalid chunk filename template '%s'\n", s->filename);
             return AVERROR(EINVAL);
         }
@@ -114,7 +118,6 @@ static int webm_chunk_write_header(AVFormatContext *s)
     // DASH Streams can only have either one track per file.
     if (s->nb_streams != 1) { return AVERROR_INVALIDDATA; }
 
-    wc->chunk_count = 0;
     wc->chunk_index = wc->chunk_start_index;
     wc->oformat = av_guess_format("webm", s->filename, "video/webm");
     if (!wc->oformat)
@@ -124,8 +127,8 @@ static int webm_chunk_write_header(AVFormatContext *s)
     if (ret < 0)
         return ret;
     oc = wc->avf;
-    ret = set_chunk_filename(s, 1);
-    if (ret< 0)
+    ret = get_chunk_filename(s, 1, oc->filename);
+    if (ret < 0)
         return ret;
     ret = avio_open2(&oc->pb, oc->filename, AVIO_FLAG_WRITE,
                      &s->interrupt_callback, NULL);
@@ -146,15 +149,10 @@ static int chunk_start(AVFormatContext *s)
     AVFormatContext *oc = wc->avf;
     int ret;
 
-    ret = set_chunk_filename(s, 0);
+    ret = avio_open_dyn_buf(&oc->pb);
     if (ret < 0)
         return ret;
     wc->chunk_index++;
-    ret = avio_open2(&oc->pb, oc->filename, AVIO_FLAG_WRITE,
-                     &s->interrupt_callback, NULL);
-    if (ret < 0)
-        return ret;
-    oc->pb->seekable = 0;
     return 0;
 }
 
@@ -163,16 +161,30 @@ static int chunk_end(AVFormatContext *s)
     WebMChunkContext *wc = s->priv_data;
     AVFormatContext *oc = wc->avf;
     int ret;
+    int buffer_size;
+    uint8_t *buffer;
+    AVIOContext *pb;
+    char filename[MAX_FILENAME_SIZE];
 
     if (wc->chunk_start_index == wc->chunk_index)
         return 0;
     // Flush the cluster in WebM muxer.
     oc->oformat->write_packet(oc, NULL);
-    ret = avio_close(oc->pb);
+    buffer_size = avio_close_dyn_buf(oc->pb, &buffer);
+    ret = get_chunk_filename(s, 0, filename);
     if (ret < 0)
-        return ret;
-    wc->chunk_count++;
-    return 0;
+        goto fail;
+    ret = avio_open2(&pb, filename, AVIO_FLAG_WRITE, &s->interrupt_callback, NULL);
+    if (ret < 0)
+        goto fail;
+    avio_write(pb, buffer, buffer_size);
+    ret = avio_close(pb);
+    if (ret < 0)
+        goto fail;
+    oc->pb = NULL;
+fail:
+    av_free(buffer);
+    return (ret < 0) ? ret : 0;
 }
 
 static int webm_chunk_write_packet(AVFormatContext *s, AVPacket *pkt)