Backport fix by tismer for #210682

fixed an old buglet that caused bdb to be unable to
continue in the botframe, after a breakpoint was set.
the key idea is not to set botframe to the bottom level frame,
but its f_back, which actually might be None.
Additional changes: migrated old exception trick to use
sys._getframe(), which exists both in 2.1 and 2.2 .

Note: I believe Mark Hammond needs to look over his code now.
F5 correctly starts up in the debugger, but later on doesn't stop at a given
breakpoint any longer.

kind regards - chris

Lib/bdb.py

@@ -64,7 +64,7 @@ class Bdb:
         # XXX 'arg' is no longer used
         if self.botframe is None:
             # First call of dispatch since reset()
-            self.botframe = frame
+            self.botframe = frame.f_back # (CT) Note that this may also be None!
             return self.trace_dispatch
         if not (self.stop_here(frame) or self.break_anywhere(frame)):
             # No need to trace this function
@@ -90,8 +90,8 @@ class Bdb:
     # definition of stopping and breakpoints.
 
     def stop_here(self, frame):
-        if self.stopframe is None:
-            return 1
+        # (CT) stopframe may now also be None, see dispatch_call.
+        # (CT) the former test for None is therefore removed from here.
         if frame is self.stopframe:
             return 1
         while frame is not None and frame is not self.stopframe:
@@ -168,10 +168,7 @@ class Bdb:
 
     def set_trace(self):
         """Start debugging from here."""
-        try:
-            1 + ''
-        except:
-            frame = sys.exc_info()[2].tb_frame.f_back
+        frame = sys._getframe().f_back
         self.reset()
         while frame:
             frame.f_trace = self.trace_dispatch
@@ -188,10 +185,7 @@ class Bdb:
         if not self.breaks:
             # no breakpoints; run without debugger overhead
             sys.settrace(None)
-            try:
-                1 + ''  # raise an exception
-            except:
-                frame = sys.exc_info()[2].tb_frame.f_back
+            frame = sys._getframe().f_back
             while frame and frame is not self.botframe:
                 del frame.f_trace
                 frame = frame.f_back