libavformat/segment: add an option to write the header to a separate file

This permits some interesting segmenting techniques with formats like Matroska,
where you can concatenate the header and segments [N, nb_segments) and get
a working file that starts at segment N's start time.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/segment.c

@@ -105,6 +105,7 @@ typedef struct SegmentContext {
     int64_t time_delta;
     int  individual_header_trailer; /**< Set by a private option. */
     int  write_header_trailer; /**< Set by a private option. */
+    char *header_filename;  ///< filename to write the output header to
 
     int reset_timestamps;  ///< reset timestamps at the begin of each segment
     int64_t initial_offset;    ///< initial timestamps offset, expressed in microseconds
@@ -593,6 +594,11 @@ static int seg_write_header(AVFormatContext *s)
     if (!seg->write_header_trailer)
         seg->individual_header_trailer = 0;
 
+    if (seg->header_filename) {
+        seg->write_header_trailer = 1;
+        seg->individual_header_trailer = 0;
+    }
+
     if (!!seg->time_str + !!seg->times_str + !!seg->frames_str > 1) {
         av_log(s, AV_LOG_ERROR,
                "segment_time, segment_times, and segment_frames options "
@@ -669,7 +675,7 @@ static int seg_write_header(AVFormatContext *s)
         goto fail;
 
     if (seg->write_header_trailer) {
-        if ((ret = avio_open2(&oc->pb, oc->filename, AVIO_FLAG_WRITE,
+        if ((ret = avio_open2(&oc->pb, seg->header_filename ? seg->header_filename : oc->filename, AVIO_FLAG_WRITE,
                               &s->interrupt_callback, NULL)) < 0) {
             av_log(s, AV_LOG_ERROR, "Failed to open segment '%s'\n", oc->filename);
             goto fail;
@@ -704,8 +710,13 @@ static int seg_write_header(AVFormatContext *s)
     if (oc->avoid_negative_ts > 0 && s->avoid_negative_ts < 0)
         s->avoid_negative_ts = 1;
 
-    if (!seg->write_header_trailer) {
-        close_null_ctxp(&oc->pb);
+    if (!seg->write_header_trailer || seg->header_filename) {
+        if (seg->header_filename) {
+            av_write_frame(oc, NULL);
+            avio_closep(&oc->pb);
+        } else {
+            close_null_ctxp(&oc->pb);
+        }
         if ((ret = avio_open2(&oc->pb, oc->filename, AVIO_FLAG_WRITE,
                               &s->interrupt_callback, NULL)) < 0)
             goto fail;
@@ -877,6 +888,7 @@ static const AVOption options[] = {
     { "segment_format",    "set container format used for the segments", OFFSET(format),  AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       E },
     { "segment_format_options", "set list of options for the container format used for the segments", OFFSET(format_options_str), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, E },
     { "segment_list",      "set the segment list filename",              OFFSET(list),    AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       E },
+    { "segment_header_filename", "write a single file containing the header", OFFSET(header_filename), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, E },
 
     { "segment_list_flags","set flags affecting segment list generation", OFFSET(list_flags), AV_OPT_TYPE_FLAGS, {.i64 = SEGMENT_LIST_FLAG_CACHE }, 0, UINT_MAX, E, "list_flags"},
     { "cache",             "allow list caching",                                    0, AV_OPT_TYPE_CONST, {.i64 = SEGMENT_LIST_FLAG_CACHE }, INT_MIN, INT_MAX,   E, "list_flags"},