python: Expose active probes from USDT object

Allow callers to obtain the list of active (enabled) USDT
probes from the USDT object, if they need to enable the
uprobes by hand without using the bcc attach support. This
is required when the same uprobe needs to be enabled in
more than one process.

src/python/bcc/usdt.py

@@ -145,13 +145,15 @@ class USDT(object):
     # This is called by the BPF module's __init__ when it realizes that there
     # is a USDT context and probes need to be attached.
     def attach_uprobes(self, bpf):
+        probes = self.enumerate_active_probes()
+        for (binpath, fn_name, addr, pid) in probes:
+            bpf.attach_uprobe(name=binpath, fn_name=fn_name,
+                              addr=addr, pid=pid)
+
+    def enumerate_active_probes(self):
         probes = []
         def _add_probe(binpath, fn_name, addr, pid):
             probes.append((binpath, fn_name, addr, pid))
 
         lib.bcc_usdt_foreach_uprobe(self.context, _USDT_PROBE_CB(_add_probe))
-
-        for (binpath, fn_name, addr, pid) in probes:
-            bpf.attach_uprobe(name=binpath, fn_name=fn_name,
-                              addr=addr, pid=pid)
-
+        return probes