[`ctypes`][`linux`] `ctypes.util.find_library()` crash instead of return None

[Jacobfaib]

Bug report

Bug description:

The problem:

ctypes.util.find_library() raises an exception on certain inputs when it cannot find it.

Expected behavior:

No exception, return None as advertised if it cannot find the library.

Reproducer:

# on python 3.10, but same deal on other versions too
>>> import ctypes.util
>>> ctypes.util.find_library('libgomp')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/path/to/lib/python3.10/ctypes/util.py", line 351, in find_library
    _get_soname(_findLib_gcc(name)) or _get_soname(_findLib_ld(name))
  File "/path/to/lib/python3.10/ctypes/util.py", line 148, in _findLib_gcc
    if not _is_elf(file):
  File "/path/to/lib/python3.10/ctypes/util.py", line 100, in _is_elf
    with open(filename, 'br') as thefile:
FileNotFoundError: [Errno 2] No such file or directory: b'liblibgomp.a'

Note that the crash does not occur if you put nonsense in:

1. find_library('asdadsas') -> OK
2. find_library('libasdasdasd') -> OK

The problem appears to be a bit more contrived. Python is attempting to parse the output from gcc, which in this case is:

/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/Scrt1.o
/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/12/crtbeginS.o
/usr/bin/ld: cannot find -llibgomp: No such file or directory
/usr/bin/ld: note to link with /usr/lib/gcc/x86_64-linux-gnu/12/libgomp.a use -l:libgomp.a or rename it to liblibgomp.a
/usr/lib/gcc/x86_64-linux-gnu/12/libgcc.a
/usr/lib/gcc/x86_64-linux-gnu/12/libgcc_s.so
/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/libgcc_s.so.1
/usr/lib/gcc/x86_64-linux-gnu/12/libgcc.a
collect2: error: ld returned 1 exit status

But then the parser is getting confused trying to parse the useful diagnostic message mentioning libgomp.a.

Machine details (from platform.uname())

uname_result(system='Linux', node='iblis', release='6.5.0-14-generic', version='#14~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Mon Nov 20 18:15:30 UTC 2', machine='x86_64')

CPython versions tested on:

3.10, 3.11, 3.12

Operating systems tested on:

Linux

Linked PRs

• gh-114257: Ignore the FileNotFound error in ctypes.util._is_elf #114394
• [3.12] gh-114257: Ignore the FileNotFound error in ctypes.util._is_elf() (GH-114394) #114444
• [3.11] gh-114257: Ignore the FileNotFound error in ctypes.util._is_elf() (GH-114394) #114445

[Jacobfaib]

cc @manopapad

[manopapad]

Full output with some prints (python pulled from conda-forge):

(test) iblis:~/jacob/legate.core> python --version
Python 3.12.1
(test) iblis:~/jacob/legate.core> cat a.py
import ctypes.util
ctypes.util.find_library('libgomp')

(test) iblis:~/jacob/legate.core> python a.py
_findLib_gcc(name=libgomp)
expr=b'[^\\(\\)\\s]*liblibgomp\\.[^\\(\\)\\s]*'
trace=b'/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/Scrt1.o\n/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/crti.o\n/usr/lib/gcc/x86_64-linux-gnu/12/crtbeginS.o\n/usr/bin/ld: cannot find -llibgomp: No such file or directory\n/usr/bin/ld: note to link with /usr/lib/gcc/x86_64-linux-gnu/12/libgomp.a use -l:libgomp.a or rename it to liblibgomp.a\n/usr/lib/gcc/x86_64-linux-gnu/12/libgcc.a\n/usr/lib/gcc/x86_64-linux-gnu/12/libgcc_s.so\n/usr/lib/gcc/x86_64-linux-gnu/12/../../../x86_64-linux-gnu/libgcc_s.so.1\n/usr/lib/gcc/x86_64-linux-gnu/12/libgcc.a\ncollect2: error: ld returned 1 exit status\n'
res=[b'liblibgomp.a']
Traceback (most recent call last):
  File "/home/mpapadakis/jacob/legate.core/a.py", line 2, in <module>
    ctypes.util.find_library('libgomp')
  File "/home/mpapadakis/mambaforge/envs/test/lib/python3.12/ctypes/util.py", line 355, in find_library
    _get_soname(_findLib_gcc(name)) or _get_soname(_findLib_ld(name))
                ^^^^^^^^^^^^^^^^^^
  File "/home/mpapadakis/mambaforge/envs/test/lib/python3.12/ctypes/util.py", line 152, in _findLib_gcc
    if not _is_elf(file):
           ^^^^^^^^^^^^^
  File "/home/mpapadakis/mambaforge/envs/test/lib/python3.12/ctypes/util.py", line 100, in _is_elf
    with open(filename, 'br') as thefile:
         ^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: b'liblibgomp.a'

I assume the double "lib" is intentional, and this call was meant to fail, and just ended up mis-parsing an unrelated part of the output diagnostic message.

[aisk]

The find_library function will invoke gcc and search for any matches to the pattern lib{name} (in this case, liblibgomp) in its output.

In the new version of gcc, it will display a recommendation tip in case you add the unnecessary lib prefix, like:

/usr/bin/ld: note to link with /usr/lib/gcc/x86_64-linux-gnu/12/libgomp.a use -l:libgomp.a or rename it to liblibgomp.a

If a matching result, such as liblibgomp.a, is found, it will be used as an argument to _is_elf. Unfortunately, if the file does not exist or for any other reason when it's opened, it will raise an error.

I'm not sure if any other OSError should be ignored, but at least FileNotFoundError should be ignored in this case.