bun test: cover child-process behavior after directory scans

[lorenzozanee]

What does PR do?

Extends the existing POSIX scanner directory-fd regression test to walk a near-OPEN_MAX tree and verify that node:child_process.spawnSync still launches /bin/echo with status 0 and ok\n. It adds coverage for the directory-fd retention scenario in #42077 while leaving the production repair in #40016 unchanged.

Fixes #42077

How did you verify code works?

• git diff --check
• bun test test/cli/test/bun-test.test.ts -t 'does not retain directory fds needed by child processes' (expected failure on released Bun 1.4.0: OPEN_FDS=10420; this confirms the regression guard catches the unfixed behavior)
• bun bd could not start because this host has LLVM 21 and the repository requires Clang/LLVM 23.1.x; current-source execution remains for upstream CI.

[claude]

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[coderabbitai]

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• test/cli/test/bun-test.test.ts

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Walkthrough

The scanner directory-fd test now creates 2,600 directories, runs unfiltered bun test, and verifies that spawnSync executes /bin/echo successfully while open file descriptors remain below 256.

Changes

Scanner descriptor regression

Layer / File(s)	Summary
Large-tree spawn validation test/cli/test/bun-test.test.ts	The test creates 2,600 directories and runs unfiltered bun test. The probe logs open descriptors and /bin/echo results. Assertions verify OPEN_FDS < 256, status 0, stdout "ok\n", and error null.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Severity of issue fixed: Medium

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main change: testing child-process behavior after directory scans.
Description check	✅ Passed	The description explains the change, linked issue, verification steps, and build limitation. The first heading differs slightly from the template, but the required information is present.
Linked Issues check	✅ Passed	The change updates test/cli/test/bun-test.test.ts for #42077. It creates 2,600 nested directory paths, runs unfiltered bun test, checks that OPEN_FDS stays below 256, and verifies `spawnSync("/b…
Out of Scope Changes check	✅ Passed	The diff changes only the targeted scanner regression test in test/cli/test/bun-test.test.ts. The added directory tree, child-process probe, and assertions directly support #42077. No unrelated prod…

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[robobun]

Thanks for the PR. #42077 is now closed as already fixed: #40016 fixed it, and the fix shipped in Bun v1.4.1. So this PR is a test-only addition, and the Fixes #42077 line in the description no longer applies. The maintainers decide if they want the extra test coverage.