coverage uses open() which means it is impossible to run tests under conditions where opening files is not possible #2091
Description
Describe the bug
Twisted has some tests which intentionally exhaust the limit of open file descriptors in order to verify the real-world robustness of error handling behaviors in that scenario, of various APIs (particularly those which involve receiving EMFILE when calling .accept() on sockets, which is challenging to mock comprehensively).
Coverage then fails like this:
Traceback (most recent call last):
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/twisted/internet/tcp.py", line 1414, in doRead
for accepted, (skt, addr) in enumerate(clients, 1):
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/twisted/internet/tcp.py", line 1210, in _accept
with reservedFD:
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/twisted/internet/tcp.py", line 1016, in __enter__
if self._fileDescriptor is None:
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/coverage/sysmon.py", line 433, in sysmon_branch_either
multiline_map = get_multiline_map(code.co_filename)
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/coverage/sysmon.py", line 473, in get_multiline_map
parser = PythonParser(filename=filename)
File "/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/coverage/parser.py", line 61, in __init__
raise NoSource(f"No source for code: '{self.filename}': {err}") from err
coverage.exceptions.NoSource: No source for code: '/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/twisted/internet/tcp.py': [Errno 24] Too many open files: '/home/runner/work/twisted/twisted/.tox/nodeps-withcov-posix/lib/python3.14/site-packages/twisted/internet/tcp.py'
Which you can see here: twisted/twisted#12535
To Reproduce
- Call
open()(or similar) until you start gettingOSErrorexceptions - Leave all those files open
- Run coverage until it needs to open a Python source file for some reason.
Answer the questions below:
- What version of Python are you using? 3.14
- What version of coverage.py shows the problem? The output of
coverage debug sysis helpful.-- sys ------------------------------------------------------- coverage_version: 7.11.3 coverage_module: /Users/glyph/.virtualenvs/Twisted/lib/python3.13/site-packages/coverage/__init__.py core: -none- CTracer: available from /Users/glyph/.virtualenvs/Twisted/lib/python3.13/site-packages/coverage/tracer.cpython-313-darwin.so plugins.file_tracers: -none- plugins.configurers: -none- plugins.context_switchers: -none- configs_attempted: /Users/glyph/Projects/Twisted/.coveragerc configs_read: /Users/glyph/Projects/Twisted/.coveragerc config_file: /Users/glyph/Projects/Twisted/.coveragerc config_contents: b'[run]\nbranch = True\nparallel = True\nsource = twisted\nomit =\n .tox/*/tmp/_trial_temp/*\n */twisted/internet/gtk2reactor.py\n\n[paths]\nsource=\n src/twisted\n */src/twisted\n */site-packages/twisted\n benchmarks\n\n[report]\nprecision = 2\nignore_errors = True\nexclude_lines =\n pragma: no cover\n if TYPE_CHECKING\n \\s*\\.\\.\\.$\n raise NotImplementedError\n' data_file: -none- python: 3.13.8 (v3.13.8:a15ae614deb, Oct 7 2025, 08:28:01) [Clang 16.0.0 (clang-1600.0.26.6)] platform: macOS-26.1-arm64-arm-64bit-Mach-O implementation: CPython build: v3.13.8:a15ae614deb Oct 7 2025 08:28:01 gil_enabled: True executable: /Users/glyph/.virtualenvs/Twisted/bin/python3 def_encoding: utf-8 fs_encoding: utf-8 pid: 98077 cwd: /Users/glyph/Projects/Twisted path: /Users/glyph/.virtualenvs/Twisted/bin /Library/Frameworks/Python.framework/Versions/3.13/lib/python313.zip /Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13 /Library/Frameworks/Python.framework/Versions/3.13/lib/python3.13/lib-dynload /Users/glyph/.virtualenvs/Twisted/lib/python3.13/site-packages /Users/glyph/Projects/Twisted/src environment: HOME = /Users/glyph PYTHONSTARTUP = /Users/glyph/Settings/Python/prompt.py VIRTUALENVWRAPPER_PYTHON = /Users/glyph/.local/pipx/venvs/virtualenvwrapper/bin/python command_line: /Users/glyph/.virtualenvs/Twisted/bin/coverage debug sys time: 2025-11-17 12:20:06 - What versions of what packages do you have installed? The output of
pip freezeis helpful.alabaster==0.7.16 anyio==4.11.0 appdirs==1.4.4 appnope==0.1.4 astroid==4.0.2 asttokens==3.0.1 attrs==25.4.0 Automat==25.4.16 babel==2.17.0 bcrypt==5.0.0 blessed==1.24.0 bpython==0.26 CacheControl==0.14.4 certifi==2025.11.12 cffi==2.0.0 charset-normalizer==3.4.4 click==8.3.1 comm==0.2.3 ConfigArgParse==1.7.1 constantly==23.10.4 coverage==7.11.3 cryptography==46.0.3 curtsies==0.4.3 cwcwidth==0.1.12 cython-test-exception-raiser==1.0.2 debugpy==1.8.17 decorator==5.2.1 dill==0.4.0 docutils==0.18.1 epc==0.0.5 executing==2.2.1 filelock==3.20.0 greenlet==3.2.4 h11==0.16.0 h2==4.3.0 hpack==4.1.0 httpcore==1.0.9 httpx==0.28.1 hyperframe==6.1.0 hyperlink==21.0.0 hypothesis==6.148.1 idna==3.11 imagesize==1.4.1 importmagic==0.1.7 incremental==24.7.2 ipykernel==7.1.0 ipython==9.7.0 ipython_pygments_lexers==1.1.1 iso8601==2.1.0 isort==7.0.0 jedi==0.19.2 Jinja2==3.1.6 jupyter_client==8.6.3 jupyter_core==5.9.1 lunr==0.8.0 MarkupSafe==3.0.3 matplotlib-inline==0.2.1 mccabe==0.7.0 msgpack==1.1.2 mypy==1.10.1 mypy-zope==1.0.6 mypy_extensions==1.1.0 nest-asyncio==1.6.0 packaging==25.0 parso==0.8.5 pexpect==4.9.0 platformdirs==4.5.0 priority==1.3.0 prompt_toolkit==3.0.52 psutil==7.1.3 ptyprocess==0.7.0 pudb==2025.1.3 pure_eval==0.2.3 pyasn1==0.6.1 pyasn1_modules==0.4.2 pycparser==2.23 pydoctor==25.4.0 pyflakes==2.5.0 Pygments==2.19.2 PyHamcrest==2.1.0 pylint==4.0.3 pyobjc-core==12.1 pyobjc-framework-CFNetwork==12.1 pyobjc-framework-Cocoa==12.1 pyOpenSSL==25.3.0 pyserial==3.5 python-dateutil==2.9.0.post0 python-subunit==1.4.5 pyxdg==0.28 pyzmq==27.1.0 requests==2.32.5 service-identity==24.2.0 setuptools==80.9.0 sexpdata==1.0.2 six==1.17.0 sniffio==1.3.1 snowballstemmer==3.0.1 sortedcontainers==2.4.0 Sphinx==6.2.1 sphinx-rtd-theme==1.3.0 sphinxcontrib-applehelp==2.0.0 sphinxcontrib-devhelp==2.0.0 sphinxcontrib-htmlhelp==2.1.0 sphinxcontrib-jquery==4.1 sphinxcontrib-jsmath==1.0.1 sphinxcontrib-qthelp==2.0.0 sphinxcontrib-serializinghtml==2.0.0 stack-data==0.6.3 testtools==2.8.0 tomlkit==0.13.3 tornado==6.5.2 towncrier==23.11.0 traitlets==5.14.3 -e git+https://github.com/twisted/twisted@cb828df6884ba0ceda00631533c57767537a9c56#egg=Twisted twistedchecker==0.7.4 types-cffi==1.17.0.20250915 types-pyOpenSSL==24.1.0.20240722 types-setuptools==80.9.0.20250822 typing_extensions==4.15.0 urllib3==2.5.0 urwid==2.6.16 urwid_readline==0.15.1 wcwidth==0.2.14 wsproto==1.3.1 zope.event==6.1 zope.interface==8.1.1 zope.schema==8.1 - What code shows the problem? Give us a specific commit of a specific repo that we can check out. If you've already worked around the problem, please provide a commit before that fix.
https://github.com/twisted/twisted/blob/1af796ae654acc2f947c3420d9950d3f4a3113b8/src/twisted/internet/test/test_tcp.py#L966-L1038 but you probably don't actually want to read all of that - What commands should we run to reproduce the problem? Be specific. Include everything, even
git clone,pip install, and so on. Explain like we're five!
tox -e withcovshould do it, but only on some systems. I'm not sure why this only started happening recently and only in certain environments
Expected behavior
It's understandable that coverage normally expects to run under conditions where opening files is possible. However, under precise integration test conditions, both EMFILE exhaustion and system-under-test sensitivity to exact file descriptor numbers may cause this behavior to perturb the code being tested. My preference would be for a public API to allow for eager allocation of source code maps or any other external resource which may otherwise require external I/O at unpredictable points in execution.