segment: implement wrap around

Provide a way to wrap around the segment index so pseudostreaming
live through a web server and html5 browser is simpler.

Also ensure that 0 (disable) is a valid value across the options
providing wrap around.

doc/muxers.texi

@@ -353,6 +353,8 @@ Set segment duration to @var{t} seconds.
 Generate also a listfile named @var{name}.
 @item segment_list_size @var{size}
 Overwrite the listfile once it reaches @var{size} entries.
+@item segment_wrap @var{limit}
+Wrap around segment index once it reaches @var{limit}.
 @end table
 
 @example

libavformat/segment.c

@@ -39,6 +39,7 @@ typedef struct {
     char *list;            /**< Set by a private option. */
     float time;            /**< Set by a private option. */
     int  size;             /**< Set by a private option. */
+    int  wrap;             /**< Set by a private option. */
     int64_t offset_time;
     int64_t recording_time;
     int has_video;
@@ -51,6 +52,9 @@ static int segment_start(AVFormatContext *s)
     AVFormatContext *oc = c->avf;
     int err = 0;
 
+    if (c->wrap)
+        c->number %= c->wrap;
+
     if (av_get_frame_filename(oc->filename, sizeof(oc->filename),
                               s->filename, c->number++) < 0)
         return AVERROR(EINVAL);
@@ -206,12 +210,11 @@ static int seg_write_packet(AVFormatContext *s, AVPacket *pkt)
         if (seg->list) {
             avio_printf(seg->pb, "%s\n", oc->filename);
             avio_flush(seg->pb);
-            if (!(seg->number % seg->size)) {
+            if (seg->size && !(seg->number % seg->size)) {
                 avio_close(seg->pb);
                 if ((ret = avio_open2(&seg->pb, seg->list, AVIO_FLAG_WRITE,
                                       &s->interrupt_callback, NULL)) < 0)
                     goto fail;
-
             }
         }
     }
@@ -250,6 +253,7 @@ static const AVOption options[] = {
     { "segment_time",      "segment length in seconds",               OFFSET(time),    AV_OPT_TYPE_FLOAT,  {.dbl = 2},     0, FLT_MAX, E },
     { "segment_list",      "output the segment list",                 OFFSET(list),    AV_OPT_TYPE_STRING, {.str = NULL},  0, 0,       E },
     { "segment_list_size", "maximum number of playlist entries",      OFFSET(size),    AV_OPT_TYPE_INT,    {.dbl = 5},     0, INT_MAX, E },
+    { "segment_wrap",      "number after which the index wraps",      OFFSET(wrap),    AV_OPT_TYPE_INT,    {.dbl = 0},     0, INT_MAX, E },
     { NULL },
 };