Avoid an error in the fallback script when a fallback doesn't have any uptime

Sometimes, the fallback generation script doesn't add attributes to the
fallbacks in the list. If this happens, log an error, and avoid selecting
that fallback.

This is a rare issue: it should not change selection behaviour.

Fixes issue #20945.

changes/fallbacks-201612

@@ -33,3 +33,5 @@
       OnionOO. When a relay operator has multiple relays, this prioritises
       relays that are up over relays that are down.
       Closes ticket #20926; bugfix on 0.2.8.3-alpha.
+    - Stop failing when a relay has no uptime data in updateFallbackDirs.py.
+      Closes ticket 20945; bugfix on tor-0.2.8.1-alpha.

scripts/maint/updateFallbackDirs.py

@@ -862,37 +862,42 @@ class Candidate(object):
       self._badexit = self._avg_generic_history(badexit) / ONIONOO_SCALE_ONE
 
   def is_candidate(self):
-    if (MUST_BE_RUNNING_NOW and not self.is_running()):
-      logging.info('%s not a candidate: not running now, unable to check ' +
-                   'DirPort consensus download', self._fpr)
-      return False
-    if (self._data['last_changed_address_or_port'] >
-        self.CUTOFF_ADDRESS_AND_PORT_STABLE):
-      logging.info('%s not a candidate: changed address/port recently (%s)',
-                   self._fpr, self._data['last_changed_address_or_port'])
-      return False
-    if self._running < CUTOFF_RUNNING:
-      logging.info('%s not a candidate: running avg too low (%lf)',
-                   self._fpr, self._running)
-      return False
-    if self._v2dir < CUTOFF_V2DIR:
-      logging.info('%s not a candidate: v2dir avg too low (%lf)',
-                   self._fpr, self._v2dir)
-      return False
-    if self._badexit is not None and self._badexit > PERMITTED_BADEXIT:
-      logging.info('%s not a candidate: badexit avg too high (%lf)',
-                   self._fpr, self._badexit)
-      return False
-    # this function logs a message depending on which check fails
-    if not self.is_valid_version():
-      return False
-    if self._guard < CUTOFF_GUARD:
-      logging.info('%s not a candidate: guard avg too low (%lf)',
-                   self._fpr, self._guard)
-      return False
-    if (not self._data.has_key('consensus_weight')
-        or self._data['consensus_weight'] < 1):
-      logging.info('%s not a candidate: consensus weight invalid', self._fpr)
+    try:
+      if (MUST_BE_RUNNING_NOW and not self.is_running()):
+        logging.info('%s not a candidate: not running now, unable to check ' +
+                     'DirPort consensus download', self._fpr)
+        return False
+      if (self._data['last_changed_address_or_port'] >
+          self.CUTOFF_ADDRESS_AND_PORT_STABLE):
+        logging.info('%s not a candidate: changed address/port recently (%s)',
+                     self._fpr, self._data['last_changed_address_or_port'])
+        return False
+      if self._running < CUTOFF_RUNNING:
+        logging.info('%s not a candidate: running avg too low (%lf)',
+                     self._fpr, self._running)
+        return False
+      if self._v2dir < CUTOFF_V2DIR:
+        logging.info('%s not a candidate: v2dir avg too low (%lf)',
+                     self._fpr, self._v2dir)
+        return False
+      if self._badexit is not None and self._badexit > PERMITTED_BADEXIT:
+        logging.info('%s not a candidate: badexit avg too high (%lf)',
+                     self._fpr, self._badexit)
+        return False
+      # this function logs a message depending on which check fails
+      if not self.is_valid_version():
+        return False
+      if self._guard < CUTOFF_GUARD:
+        logging.info('%s not a candidate: guard avg too low (%lf)',
+                     self._fpr, self._guard)
+        return False
+      if (not self._data.has_key('consensus_weight')
+          or self._data['consensus_weight'] < 1):
+        logging.info('%s not a candidate: consensus weight invalid', self._fpr)
+        return False
+    except BaseException as e:
+      logging.warning("Exception %s when checking if fallback is a candidate",
+                      str(e))
       return False
     return True