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gh-139940: Handle RuntimeError when attaching to a non-existing process in pdb.py #139941

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gh-139940: Handle RuntimeError when attaching to a non-existing process in pdb.py #139941

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frostming
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@frostming frostming commented Oct 11, 2025 •
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Before change:

OSError: Cannot open process memory map file '/proc/5555/maps' for PID 5555 section search: No such file or directory

During handling of the above exception, another exception occurred:

RuntimeError: Failed to find the PyRuntime section in process 5555 on Linux platform

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/frost/workspace/cpython/Lib/runpy.py", line 198, in _run_module_as_main
    return _run_code(code, main_globals, None,
                     "__main__", mod_spec)
  File "/home/frost/workspace/cpython/Lib/runpy.py", line 88, in _run_code
    exec(code, run_globals)
    ~~~~^^^^^^^^^^^^^^^^^^^
  File "/home/frost/workspace/cpython/Lib/pdb.py", line 3650, in <module>
    pdb.main()
    ~~~~~~~~^^
  File "/home/frost/workspace/cpython/Lib/pdb.py", line 3579, in main
    attach(opts.pid, opts.commands)
    ~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/frost/workspace/cpython/Lib/pdb.py", line 3448, in attach
    sys.remote_exec(pid, connect_script.name)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: PyRuntime address lookup failed during debug offsets initialization

After change:

Error attaching to process: Cannot open process memory map file '/proc/5555/maps' for PID 5555 section search: No such file or directory

Close gh-139940

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@frostming frostming marked this pull request as ready for review October 11, 2025 09:22
@gaogaotiantian
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I think attaching to a non-exist process is a common case where we should produce better error messages. I have a feeling that the message should be even more explicit about "you have the wrong process id". @pablogsal and @godlygeek what do you think?

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Sounds reasonable to me. Maybe we want to catch the RuntimeError on POSIX and handle it by attempting os.kill(pid, 0) and seeing if that fails as well. If it does, we could raise an exception saying "Either there is no process with pid {pid} or you do not have privileges to send signals to it". That wouldn't be reasonable to do in the sys.remote_exec implementation itself, since there's no requirement there that you be able to send signals to the process, but PDB assumes you'll be able to for Ctrl-C to work, so it'd be a reasonable thing to check at this layer.

gaogaotiantian
gaogaotiantian reviewed Oct 13, 2025
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Lib/pdb.py Outdated
except RuntimeError as e:
while e.__context__ is not None:
e = e.__context__
print(f"Error attaching to process: {e}")
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Why do you need to get e here? We have opts.pid here to inform the user. I don't think the user understands this. I think the proper way here is to figure out what are the possibilities for RuntimeError. We should just let the user know that this failed, and the highest possibility is that the process does not exist.

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@frostming frostming Oct 13, 2025 •
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Oh, this is just borrowed from

cpython/Lib/asyncio/tools.py

Lines 239 to 248 in 419441a

def _get_awaited_by_tasks(pid: int) -> list:
try:
return get_all_awaited_by(pid)
except RuntimeError as e:
while e.__context__ is not None:
e = e.__context__
print(f"Error retrieving tasks: {e}")
sys.exit(1)
except PermissionError as e:
exit_with_permission_help_text()
to make them symmetric.

The idea here is to hide the traceback as it may contain 3+ frames that users don't know how to handle, and at the same time, don't arbitrarily cover up any fatal errors. And only the top-most frame is the real cause.

But I am okay to change to a better way.

vstinner
vstinner reviewed Oct 14, 2025
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Lib/pdb.py Outdated
parser.error("argument -m: not allowed with argument --pid")
try:
attach(opts.pid, opts.commands)
except RuntimeError as e:
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Suggested change
except RuntimeError as e:
except RuntimeError as exc:

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Unfriendly traceback when using remote pdb to attach to a non-existing process

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@frostming @gaogaotiantian @godlygeek @vstinner