gh-111051: Check if file is modifed during debugging in pdb

[gaogaotiantian]

This added a sanity check for file modification. If the file is modified during debugging, a warning will be generated.

You'll probably be tempted to use os.path.getmtime() in combination with time.time() on pdb initialization, but mtime's resolution is not perfect and there can be issues (for example, the test has to explicitly wait for a while to work). On the other hand, this implementation generates 0 false positive and 0 false negative if the file is seen at least once by pdb. It does not work if the pdb never executes a frame in the file, but the file is somehow modifed after pdb started.

The behavior is clear and independent of the resolution of system clock/file system.

We can make a heuristic check for the files, which has it own issue - what if the module is modified between pdb started and the module is loaded?

Overall I believe this implementation is absolutely better than what we have now.

• Issue: pdb should check if the executing file is modified and warn users #111051

[gaogaotiantian]

Hi @iritkatriel , do you have some time to take a look at this PR? Thanks!

[gaogaotiantian]

Friendly ping @iritkatriel , this is brought up in #112952 again so users are having confusions about it. We basically only add a warning message to users when the files were changed.

[iritkatriel]

Maybe "until program is rerun" actually. I think this could look like "until restart of pdb".

[iritkatriel]

And also, "after pdb restarted" is not quite accurate, right?

[gaogaotiantian]

It's not the full truth but it's accurate. If the user sees this warning, then the file must be edited after pdb started. However, it's possible that the file was edited after the pdb started, but the warning was not generated - because of how we detect this kind of issue.