test: drop PowerShell from child-process-exec.test.ts and assert exact output

[robobun]

Problem

• child-process-exec.test.ts takes 8.5s on the Windows x64 lane and under 1s elsewhere. Every exec() used PowerShell as the shell to print 262145 bytes, and PowerShell takes about a second to start.
• The tests cover exec()'s maxBuffer and encoding handling (Bun cannot handle a large stdout #5319), not the shell. The assertions only checked lengths.

Fix

• The bytes now come from a temp file read by type equals.txt or cat equals.txt through the default shell. type is a cmd.exe builtin.
• Two tests cover the shell option. One names bash (cmd.exe on Windows) and checks that its argv[0] comes back through echo $0 (%CMDCMDLINE% on Windows). The other names a missing shell and checks ENOENT.
• Assertions check the exact bytes, err null on success, and on overflow the exact length maxBuffer with err.code === "ERR_CHILD_PROCESS_STDIO_MAXBUFFER", err.message and err.cmd. The verbatim-arguments test checks child.pid > 0 and an empty stderr.
• Wall clock, one machine. Windows x64: release 2.6s to 0.10s, debug 2.8s to 1.7s. Linux x64: release 250ms to 85ms, debug with ASAN unchanged at 3.0s. 0 failures in 80 repeated runs.

Background

• exec() runs the command through /bin/sh -c, cmd.exe /d /s /c "<command>", or the shell option when it is a string. Bash sets $0 to its argv[0]. cmd.exe sets %CMDCMDLINE% to its whole command line. The default shell would print /bin/sh or the %ComSpec% path.
• maxBuffer caps the bytes exec() collects per stream. When a chunk crosses it, exec() cuts the chunk so the output is exactly maxBuffer long, kills the child and reports ERR_CHILD_PROCESS_STDIO_MAXBUFFER.

Notes

• Scale of the win: test/expected-durations.json lists the file at 8496ms on Windows (10s in build 108747) and 611ms to 912ms elsewhere. The file is one of about 5900 in an 8-shard Windows lane, so this removes 8s from one shard. The maxBuffer contract at small sizes is also covered by test/js/node/test/parallel/test-child-process-exec-maxbuf.js. This file keeps the 262145-byte case from Bun cannot handle a large stdout #5319.
• Repeated runs: 30 Windows release, 40 Linux release, 10 Linux debug, all 13 tests passed every time. Wall clock of the whole bun test process on Windows release (USE_SYSTEM_BUN=1 bun test): 2.60s, 2.62s, 2.65s before, 0.11s, 0.11s, 0.11s after. Windows debug is bun bd test: 2.80s to 2.88s before, 1.69s to 1.92s after.
• Linux debug A/B was interleaved, three runs each: old 3.06s, 2.90s, 2.97s, new 3.04s, 2.96s, 2.97s. Under ASAN bun test runs at most 5 concurrent tests, and the runner start-up dominates.
• A debug bun child as the generator (bun -e "process.stdout.write(...)") was tried first. It starts in about 1.4s under ASAN, which made the file slower than before on the debug lanes (6.3s). The file read has no such cost on any build.
• The first draft kept printf '=%.0s' {1..262145} under shell: bash as the named-shell case. That only discriminates on dash and busybox lanes: macOS /bin/sh is bash and expands braces too, and the dash output is a single =. echo $0 discriminates on every POSIX lane.
• The missing-shell test also checks err.cmd, err.path and err.spawnargs (["-c", "echo hi"]), which match Node.
• Exact truncation was probed with a 262145-byte writer in batches of 10 concurrent exec() calls, 300 runs on the debug build: every overflow returned exactly maxBuffer bytes and ERR_CHILD_PROCESS_STDIO_MAXBUFFER.
• toMatchObject on an Error does compare the non-enumerable message, checked with a negative case.
• child_process: latch exec/execFile maxBuffer overflow so truncated output never exceeds the cap #36169 adds a separate test to the end of this file for a fast-writer over-capture in src/js/node/child_process.ts. This PR does not change that behavior.

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no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/child_process/child-process-exec.test.ts

[robobun]

^{Updated 5:47 AM PT - Aug 31st, 2026}

✅ @robobun, your commit 89fd45577cc6fa2e6ea5e067e7b15d4fe5ba0a3f passed in Build #108837! 🎉

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🧪   To try this PR locally:

bunx bun-pr 41047

That installs a local version of the PR into your bun-41047 executable, so you can run:

bun-41047 --bun

[robobun]

Status: ready for review. Test-only change, no src/ edits.

How it was measured: USE_SYSTEM_BUN=1 bun test test/js/node/child_process/child-process-exec.test.ts and bun bd test on the same machines before and after.

• Windows x64, release: 2.6s before, 0.10s after. Debug: 2.8s before, 1.7s after.
• Linux x64, release: 250ms before, 85ms after. Debug with ASAN: 3.0s both.
• Repeated runs of the new file: 30 on Windows release, 40 on Linux release, 10 on Linux debug. All passed.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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Walkthrough

Changes

The child process tests now share a typed asynchronous exec wrapper. They validate exact output, encoding, max-buffer errors, shell selection, missing-shell failures, and verbatim-argument execution.

Child process exec tests

Layer / File(s)	Summary
Execution setup and output validation test/js/node/child_process/child-process-exec.test.ts	Adds shared execution setup and temporary-file commands. Tests validate output contents, encoding, max-buffer errors, and truncation.
Shell selection and failure handling test/js/node/child_process/child-process-exec.test.ts	Tests explicit shell selection and missing-shell ENOENT results, including error details and empty streams.
Verbatim arguments execution test/js/node/child_process/child-process-exec.test.ts	Uses the shared wrapper and checks the child PID, successful execution, and empty stderr.

Merge Risk: ⚪ Minimal · up to 89fd4

This localized test-only change removes unnecessary PowerShell startup overhead and strengthens assertions for exact output and error behavior; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main test changes: removing PowerShell usage and asserting exact output.
Description check	✅ Passed	The description explains the problem, implementation, test coverage, and verification results. It does not use the template headings exactly, but it provides the required information in equivalent sec…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description explains the problem, implementation, test coverage, and verification results. It does not use the template headings exactly, but it provides the required information in equivalent sections.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@test/js/node/child_process/child-process-exec.test.ts`:
- Line 93: Update the fixture path setup in the child-process test to remove the
runtime require.resolve call. Use a module-scope import and a module-relative
path to locate child-process-echo-argv.js, while preserving the existing fixture
reference used by the test.

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[claude]

LGTM — test-only refactor that strengthens assertions and cuts Windows wall time.

What was reviewed:

• Windows ENOENT test's spawnargs: ["-c", "echo hi"] — correct: no-such-shell.exe misses the cmd.exe regex in normalizeSpawnArguments, so both Bun and Node fall through to ["-c", command].
• type equals.txt 1>&2 under the default cmd.exe shell — type writes file bytes verbatim (no CRLF injection on a newline-free file) and cmd.exe supports 1>&2, so exact-content assertions hold.
• No assertions weakened: length/toContain checks became exact toBe/toEqual on content and toMatchObject on error code/message/cmd; the two maxBuffer cases are preserved via test.each.
• Harness conventions followed: tempDirWithFiles, Buffer.alloc(n, fill), bunEnv spread, helper extraction on the second repeat.

Extended reasoning...

Overview

This PR touches a single file, test/js/node/child_process/child-process-exec.test.ts. It replaces per-test PowerShell/bash byte generation with a one-time tempDirWithFiles fixture read through the platform's default shell (type on Windows, cat elsewhere), extracts a shared execAsync helper, collapses two near-duplicate maxBuffer tests into test.each, and adds two new tests covering the shell option (named shell reflected in $0/%CMDCMDLINE%, and ENOENT for a missing shell). The verbatim-arguments test is rewritten to use the helper and now resolves its fixture via path.join(import.meta.dir, ...) (the second commit on this PR).

Security risks

None. This is test code only; no production paths, no network access, no credentials. The temp directory is created via the harness helper and the commands run against a local file.

Level of scrutiny

Low-to-moderate. Test-only changes carry low blast radius, but REVIEW.md requires checking that no existing assertion was silently weakened. I traced each old assertion to its replacement: toHaveLength(SIZE) → exact buffer/string equality against the pre-built EQUALS; fuzzy toContain("maxBuffer length exceeded") + length bounds → toMatchObject with exact code, message, cmd and exact truncated output. All are strictly stronger. The two maxBuffer sizes (1024100 and 1024255-1) are both preserved in the test.each matrix. I also verified the Windows-specific claims against src/js/node/child_process.ts: the normalizeSpawnArguments cmd.exe regex does not match no-such-shell.exe, so spawnargs is ["-c", "echo hi"] on Windows too, matching Node.

Other factors

The change follows the harness conventions REVIEW.md enforces: Buffer.alloc(n, fill) over .repeat, tempDirWithFiles for temp state, { env: bunEnv, ...options } spread, helper extraction on repeated blocks, and test.each for the parameterized maxBuffer cases. The one prior third-party inline thread (coderabbitai at line 93) is marked resolved by a non-author, and a follow-up commit landed after it. The bug hunt exited on dry_streak with no findings and no ruled-out candidates. Given the scope (one test file), strengthened assertions, and no outstanding objections, this is safe to approve.