table: Implement a StackWalker for StackTrace tables

The StackWalker iterator lets us call `stack_trace.walk(id)` to iterate
through the addresses in any given stack. The constructor of this
iterator takes an optional `resolver` to convert the addresses in the
iterator into symbols (or to format them according to the users' needs).

src/python/bcc/table.py

@@ -412,9 +412,37 @@ def __init__(self, *args, **kwargs):
  raise Exception("Unsupported")
 
 class StackTrace(TableBase):
+ MAX_DEPTH = 127
+
  def __init__(self, *args, **kwargs):
  super(StackTrace, self).__init__(*args, **kwargs)
 
+ class StackWalker(object):
+ def __init__(self, stack, resolve=None):
+ self.stack = stack
+ self.n = -1
+ self.resolve = resolve
+
+ def __iter__(self):
+ return self
+
+ def __next__(self):
+ return self.next()
+
+ def next(self):
+ self.n += 1
+ if self.n == StackTrace.MAX_DEPTH:
+ raise StopIteration()
+
+ addr = self.stack.ip[self.n]
+ if addr == 0 :
+ raise StopIteration()
+
+ return self.resolve(addr) if self.resolve else addr
+
+ def walk(self, stack_id, resolve=None):
+ return StackTrace.StackWalker(self[self.Key(stack_id)], resolve)
+
  def __len__(self):
  i = 0
  for k in self: i += 1