gh-142315: Don't pass the "real path" of Pdb script target to system functions

[johnslavik]

Oh the irony that this was never a real path.

I need to figure out how to test this -- it's a regression, after all, so no excuses this time.

CC @ZeroIntensity needs backport to 3.13, needs backport to 3.14

• Issue: Pdb doesn't handle anonymous pipe symlinks #142315

[johnslavik]

I need to figure out how to test this -- it's a regression, after all, so no excuses this time.

Or... maybe I just mention the issue in a comment above the affected lines -- this will be my escape hatch to not write tests 💡

[johnslavik]

Ok, this one is clean and works!

[jaraco]

Thanks for the contrib!

I need to figure out how to test this -- it's a regression, after all, so no excuses this time.

Or... maybe I just mention the issue in a comment above the affected lines -- this will be my escape hatch to not write tests 💡

My preference would be to create a regression test. It seems it shouldn't be too hard to create or identify a pseudofs object to pass in and catch the SystemExit. I'll work on one.

I saw in email that @johnslavik did some analysis on the root cause in readpath/readlink. It'd be nice to get that published somewhere. Even if there's nothing wrong with Python's wrapper around realpath, it may still be worth filing an issue to capture the investigation and consider updating the docs to flag this potential issue.

[jaraco]

I'm thinking we should have a one-line change in the constructor, something like:

Suggested change

	if not os.path.exists(target):
	self._target = self._safe_realpath(target)
	if not os.path.exists(self._target):

Then, _safe_realpath would have the logic and a docstring instead of a comment describing why it exists and how it protects against non-existent readlink targets (the code below).

[johnslavik]

And maybe it will help with the test, yeah :)

[johnslavik]

I'm worried safe_realpath could be confusing near the current safe_path(-P) comment. I'll fixup that comment to reduce confusion.

[johnslavik]

#142371 (comment)

@jaraco To not change the behavior out of scope of the issue, I'd have to keep the file existence validation invoked prior to calling _safe_realpath anyway. os.path.realpath does suppress OSError, but I'd not mess with that if we can avoid double-checking.

[johnslavik]

Change done.

[jaraco]

I've created #142383 with a test. Assuming that test is failing as expected, I'll try merging it with this fix.

[johnslavik]

I think I like it the way it is now. It's a problem of maintaining the equivalence invariant -- easy to follow.