[py3.12] Missing distutils package causes hard to debug exception.

[dwt]

Hi there,

while packaging vpn-slice for nixpkgs, we noticed a harder than necessary to debug problem, because this exception handler here eats the backtrace and amkes it way harder to find why the code failed.

I would like to propose something like:

    except Exception as e:
        if self_test:
            print('******************************************************************************************', file=stderr)
            print('*** Self-test did not pass. Double-check that you are running as root (e.g. with sudo) ***', file=stderr)
            print('******************************************************************************************', file=stderr)
        raise SystemExit(*e.args) form e

That should usually be enough to retain the original backtrace, but won't work here because SystemExit does not print a stack trace by design.

A local alternative could be:

    except Exception as e:
        import traceback
        traceback.print_exc()
        if self_test:
            print('******************************************************************************************', file=stderr)
            print('*** Self-test did not pass. Double-check that you are running as root (e.g. with sudo) ***', file=stderr)
            print('******************************************************************************************', file=stderr)
        raise SystemExit(*e.args)

More work but better would be to not catch Exception (as that is too generic) and instead catch specific expected exceptions, so that unexpected ones like ImportError can be shown with backtrace by the interpreter and help debugging.

[dwt]

Ah, forgot the reason for all this: Python 3.12 removed the deprecated distutils package. If you have setuptools around as a runtime dependency this is not a problem, as it also provides that package. This should be the case for most users, as setuptools is usually used to install vpn-slice.

However installing via uv or Nix can trigger this case.

• UV doesn't install the build tools it uses to install packages into the generated virtual environments (without being told so).
• Nix likewise separates out the build and runtime dependencies.

Thinking about that, perhaps you should declare setuptools as a runtime dependency to vpn-slice. Because it uses distutils which you can either get from the dead battery back port or from setuptools. (I recommend setuptools, as it is very likely that most current users are getting it from there.

[gmacon]

I might take a look at this to see if we can remove distutils and setuptools as runtime dependencies. Grepping the code did not immediately reveal where it's imported, but I haven't read the nixpkgs PR discussion yet. Did you already identify exactly where distutils is being imported?

[dwt]

Now that I look at it again, I think that this is already gone on the main branch. Since this patch. Not quite sure why I didn't see that before.

[dwt]

That leaves the suboptimal exception handling - my immediate problem is solved with this, but you might still want to act on not swallowing exception backtraces in the exception handling.

[gmacon]

I think if I were debugging this, I would just set a breakpoint in the exception handler. I think in most cases, the error will be that a provider for something is missing, in which case the normal error message is sufficient and a stack trace is not needed; a reasonable compromise option might be to print a stack trace if --verbose is passed and suppress it otherwise.

[dwt]

@gmacon I tried this, but the problem was that I didn't find it for quite some time - due to the missing backtraces. That is the root of my complaint here, that debugging this is needlessly hard (if you don't already know the codebase by heart) because the stack traces, and thus the location where the error happened and was caught, is discarded.

So yes, I am very much in favor of printing stack traces if --verbose is enabled.

[gmacon]

This sounds like a reasonable plan to me. I'm not actively working on it right now, but if you want to implement it and ping me on a PR, I'll be happy to review it.