fix Sound Check decode (GC-521)

Fixes the case when the gain number is negative.

beets/mediafile.py

@@ -204,7 +204,12 @@ def _sc_decode(soundcheck):
     # compared to a reference value of 1000 to get our gain in dB. We
     # play it safe by using the larger of the two values (i.e., the most
     # attenuation).
-    gain = math.log10((max(*soundcheck[:2]) or 1000) / 1000.0) * -10
+    maxgain = max(soundcheck[:2])
+    if maxgain > 0:
+        gain = math.log10(maxgain / 1000.0) * -10
+    else:
+        # Invalid gain value found.
+        gain = 0.0
 
     # SoundCheck stores peak values as the actual value of the sample,
     # and again separately for the left and right channels. We need to

docs/changelog.rst

@@ -7,6 +7,7 @@ Changelog
 * :doc:`/plugins/echonest_tempo`: API errors now issue a warning instead of
   exiting with an exception. We also avoid an error when track metadata
   contains newlines.
+* Fix a crash when iTunes Sound Check tags contained invalid data.
 
 1.1b2 (February 16, 2013)
 -------------------------

test/test_mediafile.py

@@ -205,6 +205,25 @@ class TypeTest(unittest.TestCase):
         self.mf.year = '2009'
         self.assertEqual(self.mf.year, 2009)
 
+class SoundCheckTest(unittest.TestCase):
+    def test_round_trip(self):
+        data = beets.mediafile._sc_encode(1.0, 1.0)
+        gain, peak = beets.mediafile._sc_decode(data)
+        self.assertEqual(gain, 1.0)
+        self.assertEqual(peak, 1.0)
+
+    def test_decode_zero(self):
+        data = u' 80000000 80000000 00000000 00000000 00000000 00000000 ' \
+               u'00000000 00000000 00000000 00000000'
+        gain, peak = beets.mediafile._sc_decode(data)
+        self.assertEqual(gain, 0.0)
+        self.assertEqual(peak, 0.0)
+
+    def test_malformatted(self):
+        gain, peak = beets.mediafile._sc_decode(u'foo')
+        self.assertEqual(gain, 0.0)
+        self.assertEqual(peak, 0.0)
+
 def suite():
     return unittest.TestLoader().loadTestsFromName(__name__)