Write to File Object Created by `argparse` Without Context Manager Fails on Windows

[Doekin]

Describe the Bug

I encountered an issue when building p11-kit on Windows. The build process fails due to an empty header file being generated.

Upon investigation, it seems that writing to a file object without using a with outfile: context manager does not work with meson runpython on Windows.

To Reproduce

This issue can be reproduced by building p11-kit on Windows, or by running the following example header generator:

# file: writefile.py
import argparse

def write(outfile):
    outfile.write("Done")

if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--outfile", type=argparse.FileType("w"), default="foo.txt")
    args = parser.parse_args()
    write(args.outfile)

• On Windows:

  • Running python writefile.py correctly writes "Done" to foo.txt.
  • Running meson runpython writefile.py results in an empty foo.txt.

• On Linux:

  • Both methods work as expected, writing "Done" to foo.txt.

Expected Behavior

Aligning the behavior with direct Python execution would be beneficial. This would allow scripts to function as expected on Windows.

System Parameters

• Native build
• Windows 10
• Python 3.13.2
• Meson 1.7.1
• Ninja 1.12.1

[eli-schwartz]

Running meson runpython writefile.py results in an empty foo.txt.

When does this happen? Is this when using the Meson MSI installer or when using an ordinary python and pip install meson?

[Doekin]

When does this happen?

Both my Python and Meson installations were done using Scoop with the command scoop install python meson.

[eli-schwartz]

I have never used scoop so I wouldn't really know how to tell what it's doing.

When does this happen? Is this when another program installs the Meson MSI installer on your behalf or when another program runs pip install meson on your behalf?

[Doekin]

Scoop essentially downloads the MSI installer for Meson and handles the installation automatically. It requires Python to be installed separately.

I encountered this issue while building p11-kit, where the header generator script produced empty files during the build process.

[eli-schwartz]

Okay.

Does it also reproduce when cloning this git repository, and running:

python /path/to/meson/clone/meson.py runpython writefile.py

[Doekin]

Yes, running python D:\projects\meson\meson.py runpython writefile.py results in an empty foo.txt.

Additionally, executing python D:\projects\meson\meson.py --version outputs 1.7.99.

[eli-schwartz]

Okay. So that seems to imply the problem occurs somewhere in the layer that PyInstaller introduces.

PyInstaller is the name of the software we use to create native standalone executables. It bundles Python together with our source code as a custom executable file. In theory that could have some unintended effect that prevents python from closing the file at shutdown, thus it never actually writes to disk.

I'm not sure how to debug this. Maybe the PyInstaller developers know what the problem is?

[Doekin]

Thank you for your generous help. I hope this issue can be resolved in the future.

[Doekin]

I tried packaging writefile.py with pyinstaller .\writefile.py, and when I ran writefile.exe in dist\writefile, it successfully output Done to foo.txt.

[eli-schwartz]

Meson's runpython subcommand works a bit different than simply packaging up a script. See:

meson/mesonbuild/mesonmain.py

Lines 141 to 148 in 5d9b973

	def run_runpython_command(self, options: argparse.Namespace) -> int:
	sys.argv[1:] = options.script_args
	if options.eval_arg:
	exec(options.script_file)
	else:
	import runpy
	sys.path.insert(0, os.path.dirname(options.script_file))
	runpy.run_path(options.script_file, run_name='__main__')

To be a good comparison, you'd want to use PyInstaller to package up a "runner program" that takes writefile.py as an argument and dispatches it using the runpy module.

[Doekin]

Appreciate the insights you provided

I created a runpy.py file alongside writefile.py with the following content:

# file: runpy.py
import runpy
runpy.run_path("writefile.py", run_name='__main__')

When I ran it, foo.txt was empty. However, with this small change, it worked:

- runpy.run_path("writefile.py", run_name='__main__')
+ d = runpy.run_path("writefile.py", run_name='__main__')

[bonzini]

That's weird... What about:

1. adding the fake assignment to mesonbuild/mesonmain.py inside your clone (around line 150)

2. something like

try:
    runpy.run_path(...)
finally:
    pass

[eli-schwartz]

Thanks for analyzing and testing, @Doekin

Given the issue reproduces without using meson at all, just a tiny python script, I think that this probably merits an upstream bug report.

We'll keep this bug report open while we wait to hear back about the best course of action here.

[Doekin]

Unfortunately, neither approach worked. It's quite puzzling.