fix(format): use communicate rathan than .stdin.write

[prousso]

to avoid dealocks due to any of the other OS pipe buffers filling
up and blocking the child process

Example that hang:

import subprocess


def apply_ruff_check_and_format(code: str) -> str:
    """Run ruff check and format in a single pipeline for better performance."""
    check_proc = subprocess.Popen(  # noqa: S603
        ["ruff", "check", "--fix", "--unsafe-fixes", "-"],
        stdin=subprocess.PIPE,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
    )
    format_proc = subprocess.Popen(  # noqa: S603
        ["ruff", "format", "-"],
        stdin=check_proc.stdout,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
    )
    if check_proc.stdout:  # pragma: no branch
        check_proc.stdout.close()
    check_proc.stdin.write(code.encode("utf-8"))  # type: ignore[union-attr]
    check_proc.stdin.close()  # type: ignore[union-attr]
    stdout, _ = format_proc.communicate()
    check_proc.wait()
    return stdout.decode("utf-8")


if __name__ == "__main__":
    subprocess.run(["dd", "if=/dev/zero", "of=cactus.py", "bs=65000", "count=1"], check=True)
    with open('cactus.py', 'r') as f:
        code = "".join([line for line in f])
        apply_ruff_check_and_format(code)
``

[coderabbitai]

📝 Walkthrough

Walkthrough

A single-line modification in the ruff code formatter that changes how input is passed to the subprocess check process, switching from direct stdin writing to using the communicate() method.

Changes

Cohort / File(s)	Summary
Subprocess Input Handling src/datamodel_code_generator/format.py	Modified CodeFormatter.apply_ruff_check_and_format to use check_proc.communicate(code.encode(self.encoding)) instead of direct stdin.write() for sending input to ruff check process

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5 minutes

Poem

🐰 A tiny hop, a change so neat,
Where stdin flows in a better beat,
Communicate now takes the lead,
For ruff to process and proceed! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title mentions the main change (using communicate instead of .stdin.write) but contains a typo: 'rathan' should be 'rather'.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing prousso:fix/pipe-buffer-deadlock (bba42bb) with main (8c60b1b)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 11 untouched benchmarks
⏩ 98 skipped benchmarks¹

Footnotes

1. 98 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[koxudaxi]

@prousso
Thank you for creating the PR.
I looked into this and found that switching to communicate() doesn’t actually solve the pipeline deadlock here, because check’s stdout is piped into format’s stdin. communicate() tries to read stdout/stderr in the parent, which conflicts with that pipe (and the parent closes the stdout pipe), so it can still hang or corrupt the stream.
Given that, I have closed this PR and proceed with a safer approach (sequential execution or a properly‑drained pipeline).
Thank you for your supports!!

[github-actions]

🎉 Released in 0.54.0

The fix/feature from PR #2967 has been included in this release. See the release notes for details.

Thank you for your contribution!