* bpo-35823: subprocess: Use vfork() instead of fork() on Linux when safe
When used to run a new executable image, fork() is not a good choice
for process creation, especially if the parent has a large working set:
fork() needs to copy page tables, which is slow, and may fail on systems
where overcommit is disabled, despite that the child is not going to
touch most of its address space.
Currently, subprocess is capable of using posix_spawn() instead, which
normally provides much better performance. However, posix_spawn() does not
support many of child setup operations exposed by subprocess.Popen().
Most notably, it's not possible to express `close_fds=True`, which
happens to be the default, via posix_spawn(). As a result, most users
can't benefit from faster process creation, at least not without
changing their code.
However, Linux provides vfork() system call, which creates a new process
without copying the address space of the parent, and which is actually
used by C libraries to efficiently implement posix_spawn(). Due to sharing
of the address space and even the stack with the parent, extreme care
is required to use vfork(). At least the following restrictions must hold:
* No signal handlers must execute in the child process. Otherwise, they
might clobber memory shared with the parent, potentially confusing it.
* Any library function called after vfork() in the child must be
async-signal-safe (as for fork()), but it must also not interact with any
library state in a way that might break due to address space sharing
and/or lack of any preparations performed by libraries on normal fork().
POSIX.1 permits to call only execve() and _exit(), and later revisions
remove vfork() specification entirely. In practice, however, almost all
operations needed by subprocess.Popen() can be safely implemented on
Linux.
* Due to sharing of the stack with the parent, the child must be careful
not to clobber local variables that are alive across vfork() call.
Compilers are normally aware of this and take extra care with vfork()
(and setjmp(), which has a similar problem).
* In case the parent is privileged, special attention must be paid to vfork()
use, because sharing an address space across different privilege domains
is insecure[1].
This patch adds support for using vfork() instead of fork() on Linux
when it's possible to do safely given the above. In particular:
* vfork() is not used if credential switch is requested. The reverse case
(simple subprocess.Popen() but another application thread switches
credentials concurrently) is not possible for pure-Python apps because
subprocess.Popen() and functions like os.setuid() are mutually excluded
via GIL. We might also consider to add a way to opt-out of vfork() (and
posix_spawn() on platforms where it might be implemented via vfork()) in
a future PR.
* vfork() is not used if `preexec_fn != None`.
With this change, subprocess will still use posix_spawn() if possible, but
will fallback to vfork() on Linux in most cases, and, failing that,
to fork().
[1] https://ewontfix.com/7
Co-authored-by: Gregory P. Smith [Google LLC] <gps@google.com>
Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from v1 to v2.2.0.
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<p><em>Sourced from <a href="https://github.com/actions/upload-artifact/releases">actions/upload-artifact's releases</a>.</em></p>
<blockquote>
<h2>v2.2.0</h2>
<ul>
<li>Support for artifact retention</li>
</ul>
</blockquote>
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<li><a href="27bce4eee7"><code>27bce4e</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/actions/upload-artifact/issues/112">#112</a> from thboop/main</li>
<li><a href="f8b42f7ab4"><code>f8b42f7</code></a> update licensed files</li>
<li><a href="2106e8cf10"><code>2106e8c</code></a> update contributing.md</li>
<li><a href="db66798ebc"><code>db66798</code></a> Ignore Generated Files in Git PR's</li>
<li><a href="d359fd0772"><code>d359fd0</code></a> Manual Verification of licenses</li>
<li><a href="350822c32f"><code>350822c</code></a> Add Licensed Workflow and config</li>
<li><a href="abecf4abf4"><code>abecf4a</code></a> Updated README.md (<a href="https://github-redirect.dependabot.com/actions/upload-artifact/issues/118">#118</a>)</li>
<li><a href="604e071d21"><code>604e071</code></a> Merge pull request <a href="https://github-redirect.dependabot.com/actions/upload-artifact/issues/126">#126</a> from yacaovsnc/main</li>
<li><a href="4560c23b39"><code>4560c23</code></a> Check for invalid retention-days input</li>
<li><a href="59018c2f85"><code>59018c2</code></a> Add an option to specify retention period</li>
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Automerge-Triggered-By: GH:Mariatta
Bumps [actions/cache](https://github.com/actions/cache) from v1 to v2.1.2.
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<li><a href="68cfb2ccb7"><code>68cfb2c</code></a> Add units to description</li>
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<li><a href="4bceb75b5b"><code>4bceb75</code></a> Use parseInt instead of Number to handle empty strings</li>
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Automerge-Triggered-By: GH:Mariatta
The original tool wasn't working right and it was simpler to create a new one, partially re-using some of the old code. At this point the tool runs properly on the master. (Try: ./python Tools/c-analyzer/c-analyzer.py analyze.) It take ~40 seconds on my machine to analyze the full CPython code base.
Note that we'll need to iron out some OS-specific stuff (e.g. preprocessor). We're okay though since this tool isn't used yet in our workflow. We will also need to verify the analysis results in detail before activating the check in CI, though I'm pretty sure it's close.
https://bugs.python.org/issue36876
Add documentation to help diagnose CDLL dependent DLL loading errors
on windows for OSError with message:
"[WinError 126] The specified module could not be found"
This error is otherwise difficult to diagnose.
@ericsnowcurrently This PR will change the following:
In the library documentation importlib.rst:
- `module.__package__` can be `module.__name__` for packages;
- `spec.parent` can be `spec.__name__` for packages;
- `spec.loader` is not `None` for namespaces packages.
In the language documentation import.rst:
- `spec.loader` is not `None` for namespace packages.
Automerge-Triggered-By: GH:warsaw
This is a trivial fix to [bpo-39416](), which didn't come up until it was already committed
```
Change "Numeric" to "numeric".
I believe this is trivial enough to not need an issue or a NEWS entry, although
I'm unclear on what branches the original pull request received backports.
```
Automerge-Triggered-By: GH:merwok
[bpo-39416](): Document string representations of the Numeric classes
This is a change to the specification of the Python language.
The idea here is to put sane minimal limits on the Python language's default
representations of its Numeric classes. That way "Marty's Robotic Massage Parlor
and Python Interpreter" implementation of Python won't do anything too
crazy.
Some discussion in the email thread:
Subject: Documenting Python's float.__str__()
https://mail.python.org/archives/list/python-dev@python.org/thread/FV22TKT3S2Q3P7PNN6MCXI6IX3HRRNAL/
* Add _newline_ parameter to `pathlib.Path.write_text()`
* Update documentation of `pathlib.Path.write_text()`
* Add test case for `pathlib.Path.write_text()` calls with _newline_ parameter passed
Automerge-Triggered-By: GH:methane
Clarify in the subprocess documentation how searching for the executable to run works, noting that ``sys.executable`` is the recommended way to find the current interpreter.
Alexey Izbyshev
Jacob Neil Taylor
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