MPEG PS: ignore garbage at start during file type detection

Fixes #2008.

NEWS.md

@@ -31,6 +31,8 @@
   attached files. Fixes #2001.
 * mkvmerge: MP4 reader: MPEG-1/2 video read from MP4 files was written with an
   invalid codec ID (e.g. `V_MPEG7`) in certain cases. Fixes #1995.
+* mkvmerge: MPEG PS reader: made the file type detection less strict so that
+  garbage at the start of the file doesn't prevent detection. Fixes #2008.
 
 
 # Version 12.0.0 "Trust / Lust" 2017-05-20

src/input/r_mpeg_ps.cpp

@@ -47,10 +47,36 @@ mpeg_ps_reader_c::probe_file(mm_io_c *in,
     if (in->read(buf, 4) != 4)
       return 0;
 
-    if (get_uint32_be(buf) != MPEGVIDEO_PACKET_START_CODE)
+    if (get_uint32_be(buf) == MPEGVIDEO_PACKET_START_CODE)
+      return 1;
+
+    in->setFilePointer(0, seek_beginning);
+
+    auto mem      = memory_c::alloc(32 * 1024);
+    auto num_read = in->read(mem, mem->get_size());
+
+    if (num_read < 4)
       return 0;
 
-    return 1;
+    auto base                           = mem->get_buffer();
+    auto code                           = get_uint32_be(base);
+    auto offset                         = 2u;
+    auto system_header_start_code_found = false;
+    auto packet_start_code_found        = false;
+
+    while(   ((offset + 4) < num_read)
+          && (!system_header_start_code_found || !packet_start_code_found)) {
+      ++offset;
+      code = (code << 8) | base[offset];
+
+      if (code == MPEGVIDEO_SYSTEM_HEADER_START_CODE)
+        system_header_start_code_found = true;
+
+      else if (code == MPEGVIDEO_PACKET_START_CODE)
+        packet_start_code_found = true;
+    }
+
+    return system_header_start_code_found && packet_start_code_found;
 
   } catch (...) {
     return 0;

tests/results.txt

@@ -447,3 +447,4 @@ T_598aac_track_not_listed_in_pmt:444929dd4db38e68b59a3ebf833e5128-AAC:passed:201
 T_599mp4_nclx_colour_type_in_colr_atom:3639a6fdf7a0e46d158188fdd932bd2b:passed:20170514-203828:0.018287634
 T_600mpeg_ts_multiple_programs:890b456227714da673b137a941bf45b2-2a728cb7e28e2b05e8784aa8fd6f6827-d78702c82db3e49891717626ad0fb9fb-a210b7b90d61e14c7d5a5d97253f1bc2:passed:20170522-193901:1.342170107
 T_601mp4_mpeg2_via_esds:5cadaf9b8dd86a51052182ed341a4bc0:passed:20170618-150319:0.386032567
+T_602vob_with_garbage_at_start:9ea25678a9a2746f9cb9bae8e73c766d:passed:20170619-185256:0.236132323

tests/test-602vob_with_garbage_at_start.rb

@@ -0,0 +1,5 @@
+#!/usr/bin/ruby -w
+
+# T_602vob_with_garbage_at_start
+describe "mkvmerge / VOB with garbage at the beginning"
+test_identify "data/vob/starts_with_garbage.vob"