avformat/hlsenc: allow dynamic encryption key rotation

Makes behaviour of 805ce25b optional, re-enables
HLS key rotation feature
Reviewed-by: Steven Liu <lq@onvideo.cn>
Signed-off-by: DHE <git@dehacked.net>

doc/muxers.texi

@@ -551,7 +551,7 @@ format. The optional third line specifies the initialization vector (IV) as a
 hexadecimal string to be used instead of the segment sequence number (default)
 for encryption. Changes to @var{key_info_file} will result in segment
 encryption with the new key/IV and an entry in the playlist for the new key
-URI/IV.
+URI/IV if @code{hls_flags periodic_rekey} is enabled.
 
 Key info file format:
 @example
@@ -665,6 +665,11 @@ first segment's information.
 @item omit_endlist
 Do not append the @code{EXT-X-ENDLIST} tag at the end of the playlist.
 
+@item periodic_rekey
+The file specified by @code{hls_key_info_file} will be checked periodically and
+detect updates to the encryption info. Be sure to replace this file atomically,
+including the file containing the AES encryption key.
+
 @item split_by_time
 Allow segments to start on frames other than keyframes. This improves
 behavior on some players when the time between keyframes is inconsistent,

libavformat/hlsenc.c

@@ -85,6 +85,7 @@ typedef enum HLSFlags {
     HLS_SECOND_LEVEL_SEGMENT_DURATION = (1 << 9), // include segment duration (microsec) in segment filenames when use_localtime  e.g.: %%09t
     HLS_SECOND_LEVEL_SEGMENT_SIZE = (1 << 10), // include segment size (bytes) in segment filenames when use_localtime  e.g.: %%014s
     HLS_TEMP_FILE = (1 << 11),
+    HLS_PERIODIC_REKEY = (1 << 12),
 } HLSFlags;
 
 typedef enum {
@@ -1236,7 +1237,7 @@ static int hls_start(AVFormatContext *s)
                   " will use -hls_key_info_file priority\n");
         }
 
-        if (c->number <= 1) {
+        if (c->number <= 1 || (c->flags & HLS_PERIODIC_REKEY)) {
             if (c->key_info_file) {
                 if ((err = hls_encryption_start(s)) < 0)
                     goto fail;
@@ -1804,6 +1805,7 @@ static const AVOption options[] = {
     {"second_level_segment_index", "include segment index in segment filenames when use_localtime", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_SECOND_LEVEL_SEGMENT_INDEX }, 0, UINT_MAX,   E, "flags"},
     {"second_level_segment_duration", "include segment duration in segment filenames when use_localtime", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_SECOND_LEVEL_SEGMENT_DURATION }, 0, UINT_MAX,   E, "flags"},
     {"second_level_segment_size", "include segment size in segment filenames when use_localtime", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_SECOND_LEVEL_SEGMENT_SIZE }, 0, UINT_MAX,   E, "flags"},
+    {"periodic_rekey", "reload keyinfo file periodically for re-keying", 0, AV_OPT_TYPE_CONST, {.i64 = HLS_PERIODIC_REKEY }, 0, UINT_MAX,   E, "flags"},
     {"use_localtime", "set filename expansion with strftime at segment creation", OFFSET(use_localtime), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, E },
     {"use_localtime_mkdir", "create last directory component in strftime-generated filename", OFFSET(use_localtime_mkdir), AV_OPT_TYPE_BOOL, {.i64 = 0 }, 0, 1, E },
     {"hls_playlist_type", "set the HLS playlist type", OFFSET(pl_type), AV_OPT_TYPE_INT, {.i64 = PLAYLIST_TYPE_NONE }, 0, PLAYLIST_TYPE_NB-1, E, "pl_type" },