Exceptions raised while tracing certain statements cannot be caught

Bug report

So I found a really odd bug today. except blocks don't catch exceptions raised when tracing certain statements that themselves "cannot raise". For example:

def f():
    breakpoint()
    try:
        pass  # If you raise while tracing this line, the exception isn't caught.
    except:
        ...
    try:
        42  # Ditto.
    except:
        ...
    try:
        return  # Ditto.
    except:
        ...

A couple of observations:

• Exceptions raised while tracing other "normal" statements are handled the normal way.
• The code for the exception handling is still present in these examples, even though it's statically unreachable.
• If two or more of these "non-raising" statements occur together in the same block (like several pass statements in a row), only the last one will have the bug:

def f():
    breakpoint()
    try:
        pass  # No bug
        pass  # No bug
        pass  # Bug
    except:
        ...

I haven't invested any time into figuring out what the root cause is, I've just been poking at it with code examples. It's definitely a bug in the bytecode compiler though, not the interpreter (dis shows missing exception table entries). Probably something to do with how we emit exception tables for NOP or anything that unwinds the block stack, if I had to guess?

(This also affects 3.12, and maybe even earlier versions, but only tagging for 3.13 and newer since I don't think this is a "security" issue.)

Linked PRs

• gh-148333

复现 issue 中的示例并检查其 dis 输出，重点关注 pass、42 和 return 缺失的异常表条目。将这些情况与普通语句的跟踪进行比较，并检查编译器对 NOP 的处理以及块栈展开。完成的标准是，except 块能够捕获跟踪这些语句时引发的异常，并且为所述用例提供回归覆盖。