Allow JPEGs with padding at image end.

Some JPEGs (e.g. Sony Alpha 6000) pad the end of the JPEG image with null bytes,
instead of the JPEG End-Of-Image marker 0xFFD9. Others apparently do the same
thing, but with 0xFF instead of 0x00.

Due to strict sanity checks, this lead to otherwise valid JPEGs returning
PARSE_EXIF_ERROR_NO_JPEG. This commit loosens the sanity check and handles
the case of 0x00/0xFF padding at the end of image, and fixes #9, fixes #7.

exif.cpp

@@ -414,12 +414,27 @@ IFEntry parseIFEntry(const unsigned char *buf, const unsigned offs,
 // Locates the EXIF segment and parses it using parseFromEXIFSegment
 //
 int easyexif::EXIFInfo::parseFrom(const unsigned char *buf, unsigned len) {
-  // Sanity check: all JPEG files start with 0xFFD8 and end with 0xFFD9
-  // This check also ensures that the user has supplied a correct value for len.
-  if (!buf || len < 4) return PARSE_EXIF_ERROR_NO_EXIF;
+  // Sanity check: all JPEG files start with 0xFFD8.
+  if (!buf || len < 4) return PARSE_EXIF_ERROR_NO_JPEG;
   if (buf[0] != 0xFF || buf[1] != 0xD8) return PARSE_EXIF_ERROR_NO_JPEG;
-  if (buf[len - 2] != 0xFF || buf[len - 1] != 0xD9)
-    return PARSE_EXIF_ERROR_NO_JPEG;
+
+  // Sanity check: some cameras pad the JPEG image with null bytes at the end.
+  // Normally, we should able to find the JPEG end marker 0xFFD9 at the end
+  // of the image, but not always. As long as there are null/0xFF bytes at the
+  // end of the image buffer, keep decrementing len until an 0xFFD9 is found,
+  // or some other bytes are. If the first non-zero/0xFF bytes from the end are
+  // not 0xFFD9, then we can be reasonably sure that the buffer is not a JPEG.
+  while (len > 2) {
+    if (buf[len - 1] == 0 || buf[len - 1] == 0xFF) {
+      len--;
+    } else {
+      if (buf[len - 1] != 0xD9 || buf[len - 2] != 0xFF) {
+        return PARSE_EXIF_ERROR_NO_JPEG;
+      } else {
+        break;
+      }
+    }
+  }
   clear();
 
   // Scan for EXIF header (bytes 0xFF 0xE1) and do a sanity check by

test-images/sony-alpha-6000.jpg.expected

@@ -0,0 +1,31 @@
+Camera make          : SONY
+Camera model         : ILCE-6000
+Software             : ILCE-6000 v1.00
+Bits per sample      : 0
+Image width          : 6000
+Image height         : 4000
+Image description    :                                
+Image orientation    : 1
+Image copyright      : 
+Image date/time      : 2014:04:18 20:33:53
+Original date/time   : 2014:04:18 20:33:53
+Digitize date/time   : 2014:04:18 20:33:53
+Subsecond time       : 
+Exposure time        : 1/0 s
+F-stop               : f/8.0
+ISO speed            : 100
+Subject distance     : 0.000000 m
+Exposure bias        : 0.000000 EV
+Flash used?          : 1
+Metering mode        : 5
+Lens focal length    : 22.000000 mm
+35mm focal length    : 33 mm
+GPS Latitude         : 0.000000 deg (0.000000 deg, 0.000000 min, 0.000000 sec ?)
+GPS Longitude        : 0.000000 deg (0.000000 deg, 0.000000 min, 0.000000 sec ?)
+GPS Altitude         : 0.000000 m
+Lens min focal length: 16.000000 mm
+Lens max focal length: 50.000000 mm
+Lens f-stop min      : f/3.5
+Lens f-stop max      : f/5.6
+Lens make            : 
+Lens model           : E PZ 16-50mm F3.5-5.6 OSS