To make SniffMP3() more concrete so that we can remove false-positve responses from MPEG-PS streams.

Note: current resync logic keeps searching mp3 frames until it meets a
valid one. This resync logic works well with seek() operation, but in
SniffMP3(), this may cause false-positive classification for the
following MPEG-PS file format.
  A possible MPEG-PS video file:
      [PS header]...[mp3 frame][mp3 frame] ...
  MP3 files:
      [ID3 tag][mp3 frame][mp3 frame] ...

BUG: 5104562
Change-Id: Idd5020723f7bdb2fefc2a63fc8651580fd644399

media/libstagefright/MP3Extractor.cpp

@@ -52,7 +52,10 @@ static bool Resync(
         *post_id3_pos = 0;
     }
 
+    bool resync_from_head = false;
     if (*inout_pos == 0) {
+        resync_from_head = true;
+
         // Skip an optional ID3 header if syncing at the very beginning
         // of the datasource.
 
@@ -137,22 +140,20 @@ static bool Resync(
 
         uint32_t header = U32_AT(tmp);
 
-        if (match_header != 0 && (header & kMask) != (match_header & kMask)) {
-            ++pos;
-            ++tmp;
-            --remainingBytes;
-            continue;
-        }
-
         size_t frame_size;
-        int sample_rate, num_channels, bitrate;
-        if (!GetMPEGAudioFrameSize(
-                    header, &frame_size,
-                    &sample_rate, &num_channels, &bitrate)) {
-            ++pos;
-            ++tmp;
-            --remainingBytes;
-            continue;
+        if ((match_header != 0 && (header & kMask) != (match_header & kMask))
+                || !GetMPEGAudioFrameSize(header, &frame_size)) {
+            if (resync_from_head) {
+                // This isn't a valid mp3 file because it failed to detect
+                // a header while a valid mp3 file should have a valid
+                // header here.
+                break;
+            } else {
+                ++pos;
+                ++tmp;
+                --remainingBytes;
+                continue;
+            }
         }
 
         LOGV("found possible 1st frame at %lld (header = 0x%08x)", pos, header);