Log an error when lsb_release cannot be executed

Right now, if lsb_release is not on the system, bindep just dumps stack
and indicates "No such file or directory". This gives no indication of
what the problem is or what to do to resolve it. This change logs an
error message about lsb_release being missing in that case.

Change-Id: I501647770478fe154d66cdb2c6adff48973dd23c

bindep/depends.py

@@ -226,9 +226,14 @@ class Depends(object):
         return sorted(profiles)
 
     def platform_profiles(self):
-        output = subprocess.check_output(
-            ["lsb_release", "-cirs"],
-            stderr=subprocess.STDOUT).decode(getpreferredencoding(False))
+        try:
+            output = subprocess.check_output(
+                ["lsb_release", "-cirs"],
+                stderr=subprocess.STDOUT).decode(getpreferredencoding(False))
+        except OSError:
+            log = logging.getLogger(__name__)
+            log.error('Unable to execute lsb_release. Is it installed?')
+            raise
         lsbinfo = output.lower().split()
         # NOTE(toabctl): distro can be more than one string (i.e. "SUSE LINUX")
         codename = lsbinfo[len(lsbinfo) - 1:len(lsbinfo)][0]

bindep/tests/test_depends.py

@@ -207,6 +207,12 @@ class TestDepends(TestCase):
                 depends.platform_profiles(), Contains("platform:pacman"))
             self.assertIsInstance(depends.platform, Pacman)
 
+    def test_missing_lsb_release(self):
+        with mock.patch('subprocess.check_output') as mock_co:
+            mock_co.side_effect = OSError
+            depends = Depends("")
+            self.assertRaises(OSError, depends.platform_profiles)
+
     def test_finds_profiles(self):
         depends = Depends(dedent("""\
             foo