Avoid infinite loops in BPF.Table destructor

In the case that python process tries to clear() a table object while
the bpf program is still running and doing lookup_or_init, the python
clean() routine will race with the bpf program (and bpf will win in the
case of frequent kprobe hits) causing a hang. Instead, first get the
full list of keys and then delete them rather than rely on the default
popitem implementation.

Fixes: #233
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

src/python/bcc/__init__.py

@@ -223,14 +223,9 @@ class BPF(object):
                     raise KeyError
 
         def clear(self):
-            if self.ttype in (BPF.ARRAY, BPF.PROG_ARRAY):
-                # Special case clear, since this class is currently behaving
-                # like a dict but popitem on an array causes an infinite loop.
-                # TODO: derive Table from array.array instead
-                for k in self.keys():
-                    self.__delitem__(k)
-            else:
-                super(BPF.Table, self).clear()
+            # default clear uses popitem, which can race with the bpf prog
+            for k in self.keys():
+                self.__delitem__(k)
 
         @staticmethod
         def _stars(val, val_max, width):