This is one line of well-thought code :)

We basically solved the debugger overhead in a big loop by disabling LINE events, the one (major) thing left was recursive calls - or rather, calls in general. We won't prevent CALL events from happening for every unrelated call because that would require a much larger change, but we can disable CALL events after we hit it once.

Notice that CALL event has a two effects which made it a bit more complicated:

1. It could be the event that the user actually wants (only happens instep)
2. It returns the local trace function that enables all the other events on thiscode object (not function/frame).

No 2. makes it a bit challenging and much more rewarding.

We need to make sure we don't need anynew events on this code object, before we can disable the CALL event.

First of all, if there's any breakpoint on the code object, we can't do that. But we already have check for that where the code was added.

Then we'll realize, if it's anext,until orreturn - it will just work fine, because they all rely on events onexisting frame and code objects. It's impossible for those commands to stop at a new frame.

The only command that could stop at a new frame, isstep - but if it's a step, it would either

• hitstop_here() already on this specificCALL event so we can't reach to this point
• or skip this function because of specifiedskip argument, which means it won't stop in this function in the future

So really, we don't need to care about anything. If we don't need to trace this function now, we won't need anynew events for this code object in the future (until the user interaction), so we can simply disable the CALL event.

This basically gives us a zero overhead debugger. There's no observable overhead for the recursive fib implementation.

• Issue:Make it possible to usesys.monitoring for pdb/bdb #120144