python: Grab all keys before zeroing

To avoid a potential race with the key zeroing modifying
the next hash key retrieved by the loop in `Table.zero()`,
retrieve all the keys in user space first before starting
the zeroing loop. See discussion on #780.

Tested with `funccount 'SyS_*' -i 1` while running a heavy
read/write test application (`dd`) in the background for
several minutes with no visible issues.

src/python/bcc/table.py

@@ -200,7 +200,11 @@ class TableBase(MutableMapping):
             self.__delitem__(k)
 
     def zero(self):
-        for k in self.keys():
+        # Even though this is not very efficient, we grab the entire list of
+        # keys before enumerating it. This helps avoid a potential race where
+        # the leaf assignment changes a hash table bucket that is being
+        # enumerated by the same loop, and may lead to a hang.
+        for k in list(self.keys()):
             self[k] = self.Leaf()
 
     def __iter__(self):