[FFmpeg-cvslog] avformat/mp3dec: improve junk skipping heuristic

[wm4]

ffmpeg | branch: release/2.8 | wm4 <nfxjfg at googlemail.com> | Tue Oct 20 12:17:21 2015 +0200| [96b87d5cfaa553b01da60b6982a7ca292a04ea8e] | committer: Carl Eugen Hoyos

avformat/mp3dec: improve junk skipping heuristic

Commit 2b3e9bbfb529e6bde238aeb511b55ebe461664c8 caused problems for a
certain API user:

https://code.google.com/p/chromium/issues/detail?id=537725
https://code.google.com/p/chromium/issues/detail?id=542032

The problem seems rather arbitrary, because if there's junk, anything
can happen. In this case, the imperfect junk skipping just caused it to
read different junk, from what I can see.

We can improve the accuracy of junk detection by a lot by checking if 2
consecutive frames use the same configuration. While in theory it might
be completely fine for the 1st frame to have a different format than the
2nd frame, it's exceedingly unlikely, and I can't think of a legitimate
use-case.

This is approximately the same mpg123 does for junk skipping. The
set of compared header bits is the same as the libavcodec mp3 parser
uses for similar purposes.
(cherry picked from commit de1b1a7da9e6ddf42447271e519099a88b389e4a)

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=96b87d5cfaa553b01da60b6982a7ca292a04ea8e
---

 libavformat/mp3dec.c |   28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/libavformat/mp3dec.c b/libavformat/mp3dec.c
index 007c6ea..42f97db 100644
--- a/libavformat/mp3dec.c
+++ b/libavformat/mp3dec.c
@@ -42,6 +42,9 @@
 
 #define XING_TOC_COUNT 100
 
+#define SAME_HEADER_MASK \
+   (0xffe00000 | (3 << 17) | (3 << 10) | (3 << 19))
+
 typedef struct {
     AVClass *class;
     int64_t filesize;
@@ -54,7 +57,7 @@ typedef struct {
     int is_cbr;
 } MP3DecContext;
 
-static int check(AVIOContext *pb, int64_t pos);
+static int check(AVIOContext *pb, int64_t pos, uint32_t *header);
 
 /* mp3 read */
 
@@ -374,12 +377,21 @@ static int mp3_read_header(AVFormatContext *s)
 
     off = avio_tell(s->pb);
     for (i = 0; i < 64 * 1024; i++) {
+        uint32_t header, header2;
+        int frame_size;
         if (!(i&1023))
             ffio_ensure_seekback(s->pb, i + 1024 + 4);
-        if (check(s->pb, off + i) >= 0) {
-            av_log(s, AV_LOG_INFO, "Skipping %d bytes of junk at %"PRId64".\n", i, off);
-            avio_seek(s->pb, off + i, SEEK_SET);
-            break;
+        frame_size = check(s->pb, off + i, &header);
+        if (frame_size > 0) {
+            avio_seek(s->pb, off, SEEK_SET);
+            ffio_ensure_seekback(s->pb, i + 1024 + frame_size + 4);
+            if (check(s->pb, off + i + frame_size, &header2) >= 0 &&
+                (header & SAME_HEADER_MASK) == (header2 & SAME_HEADER_MASK))
+            {
+                av_log(s, AV_LOG_INFO, "Skipping %d bytes of junk at %"PRId64".\n", i, off);
+                avio_seek(s->pb, off + i, SEEK_SET);
+                break;
+            }
         }
         avio_seek(s->pb, off, SEEK_SET);
     }
@@ -420,7 +432,7 @@ static int mp3_read_packet(AVFormatContext *s, AVPacket *pkt)
 
 #define SEEK_WINDOW 4096
 
-static int check(AVIOContext *pb, int64_t pos)
+static int check(AVIOContext *pb, int64_t pos, uint32_t *ret_header)
 {
     int64_t ret = avio_seek(pb, pos, SEEK_SET);
     unsigned header;
@@ -434,6 +446,8 @@ static int check(AVIOContext *pb, int64_t pos)
     if (avpriv_mpegaudio_decode_header(&sd, header) == 1)
         return -1;
 
+    if (ret_header)
+        *ret_header = header;
     return sd.frame_size;
 }
 
@@ -461,7 +475,7 @@ static int64_t mp3_sync(AVFormatContext *s, int64_t target_pos, int flags)
             continue;
 
         for(j=0; j<MIN_VALID; j++) {
-            ret = check(s->pb, pos);
+            ret = check(s->pb, pos, NULL);
             if(ret < 0)
                 break;
             if ((target_pos - pos)*dir <= 0 && abs(MIN_VALID/2-j) < score) {