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PER-11034: fix(cli-exec): stop percy exec spinning a CPU core while the child runs - #2462
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…setImmediate `percy exec` polled for the child's close with `yield new Promise(r => setImmediate(r))`, which keeps the event loop busy for the whole run of the wrapped command, so an idle test runner costs Percy roughly one CPU core (#2460, PER-11034). Race the child's error/close event against a 100ms timer instead. The command still returns the moment the child exits, and the periodic wake keeps abort signals (SIGINT, SIGTERM) interrupting the generator as before. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| Check name | Status | Explanation |
|---|---|---|
| Linked Issues check |  |
Issue #2460 requires negligible CPU while percy exec waits for an idle child, including with PERCY_ENABLE=0, while preserving interrupt behavior. packages/cli-exec/src/exec.js now waits for `err… |
| Out of Scope Changes check | |
The pull request changes only the child wait logic and focused regression tests in packages/cli-exec. These changes directly implement issue #2460. No unrelated API, configuration, or product change… |
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Check skipped - CodeRabbit’s high-level summary is enabled. |
| Title check | |
The title contains the valid Jira ID PER-11034 and clearly describes the main change: preventing percy exec from consuming excessive CPU while the child runs. |
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Inline comments:
Review comments at @packages/cli-exec/src/exec.js:
- Line 175: Update the wait loop around Promise.race in the child-process
execution flow to track whether the child has emitted close independently of its
exit code, and stop looping on either close or error. Preserve the existing
handling of non-null exit codes while ensuring signal-terminated children do not
repeatedly race an already-settled exited promise.
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📒 Files selected for processing (2)
packages/cli-exec/src/exec.jspackages/cli-exec/test/exec.test.js
🔗 Linked repositories identified
CodeRabbit considers these linked repositories for cross-repo context during reviews:
percy/percy-selenium-dotnet(auto-detected)percy/example-percy-appium-java(auto-detected)percy/percy-appium-java(auto-detected)percy/example-percy-automate-appium-python(auto-detected)percy/example-percy-automate-appium-javascript(auto-detected)percy/example-percy-appium-js(auto-detected)percy/example-percy-appium-python(auto-detected)percy/example-percy-appium-dotnet(auto-detected)percy/example-percy-appium-ruby(auto-detected)percy/example-percy-maestro-app(auto-detected)percy/puppeteer(auto-detected)percy/percy-storybook(auto-detected)percy/example-percy-automate-appium-dotnet(auto-detected)
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🧰 Additional context used
🪛 ast-grep (0.45.3)
packages/cli-exec/src/exec.js
[warning] 175-175: Avoid using the initial state variable in setState
Context: setTimeout(r, 100)
Note: [CWE-710] Improper Adherence to Coding Standards. Security best practice.
(setstate-same-var)
🔀 Multi-repo context percy/percy-selenium-dotnet, percy/example-percy-automate-appium-javascript, percy/example-percy-automate-appium-python, percy/example-percy-automate-appium-dotnet, percy/example-percy-maestro-app
Linked repositories findings
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[::percy/percy-selenium-dotnet::]
package.json:4,7runs tests throughpercy execand pins@percy/cliat^1.31.15-beta.0; its lockfile resolves@percy/cli-execto1.31.15-beta.0. The changed wait behavior affects this long-running .NET test wrapper. -
[::percy/example-percy-automate-appium-javascript::]
webdriverio/advanced/run-percy.sh:12-22captures both the child test status andpercy exec’s exit status, explicitly relying on exit-status propagation. The PR should preserve this behavior. Several package manifests pin CLI versions (wd/package.json:5,webdriverio/package.json:14,webdriverio/advanced/package.json:10). -
[::percy/example-percy-automate-appium-python::]
advanced/Makefile:23invokes tests throughpercy exec; the checked-in lockfile resolves@percy/cli-execto1.30.11. -
[::percy/example-percy-automate-appium-dotnet::]
advanced/Makefile:19runspercy execunderset -o pipefail, so preserving command failure and signal behavior is important. -
[::percy/example-percy-maestro-app::]
package.json:12depends on@percy/cliand its lockfile resolves@percy/cli-execto1.32.3, providing a newer pinned consumer for regression coverage.
Most consumers use pinned lockfile versions, so they will not exercise this fix until their lockfiles or CI setup update to a release containing it.
A child terminated by a signal emits `close` with a null code, so a loop keyed on the exit code never ended. On master that hangs `percy exec` forever; with the previous commit it would also spin, because the settled exit promise wins every race. Track completion separately from the exit code and report a signal exit the way shells do (128 + signal number, e.g. 137 for SIGKILL). Addresses CodeRabbit review on #2462. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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Claude Code Review (automated) — 5 inline finding(s). Full report in the PR comment below. Verdict: Passed.
Claude Code PR ReviewPR: #2462 • Head: a14f8db • Reviewers: stack-code-reviewer SummaryReplaces the Review Table
Findings
Verdict: PASS — no High/Critical findings. The Medium memory accumulation is worth fixing before merge. |
…tatus Address the Claude Code review on #2462: - Racing the long-lived exit promise against each 100ms timer attached a reaction to it per tick, retained until the child exited (about 62MB over 200k ticks in a standalone measurement). The exit handlers now resolve the current tick's promise directly. - Clear the wake timer in a finally block so an abort at the yield does not leave it behind. - `| 0` keeps a signal missing from os.constants.signals at status 128 instead of NaN. - Plain assignments instead of array destructuring, and a note that the setImmediate test bound is deliberately loose. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude Code PR ReviewPR: #2462 • Head: 9244cb4 • Reviewers: fallback inline checklist (built-in review; no in-scope reviewers installed) SummaryReplaces the Review Table
FindingsNo failing checks. No findings. Verdict: PASS |
2 participants
Fixes #2460 (PER-11034)
Root cause
spawn()inpackages/cli-exec/src/exec.jswaited for the child withEach iteration queues another immediate, so the event loop never goes idle for the whole lifetime of the wrapped command. An idle test runner therefore costs
percy execup to one CPU core (the issue measured 0.99 core on Linux / Node 22 and 26).Fix
The child's
error/closehandlers setdoneand resolve the current wait. Each loop iteration waits on a fresh promise that settles on that wake or after 100 ms, and the timer is cleared in afinally. The command returns as soon as the child exits. The periodic wake keeps the existing abort path:generatePromisecheckssignal.abortedbetween yields, so SIGINT/SIGTERM still reach thecatchthat kills the child, with at most 100 ms extra latency. A fresh promise per tick (rather thanPromise.raceagainst one long-lived exit promise) avoids retaining a reaction per tick on long runs.A child killed by a signal closes with a
nullcode. On master theclosed == nullloop never ended for it, sopercy exechung. It now ends the wait and reports the shell-style status128 + signal number(137 for SIGKILL), and 128 if the signal is unmapped.Behaviour note (Ctrl-C): Ctrl-C signals the whole process group. If Percy's own SIGINT is handled before the child's
close(the usual order), the abort path runs exactly as before. If it is handled after,percy execexits with the child's 130/143 instead of 0. Percy is still stopped gracefully in that case, as in a first-signal drain. This window can't be closed by a status check afterspawn(), because the parent's signal has not arrived yet at that point.Testing
forwards a shell-style status when the command is killed by a signal(hung on master).does not spin the event loop while waiting for the command: on master the loop would schedule tens of thousands of immediates over a 1s child, so it is expected to fail there. Not run against master locally; suites run in CI.PERCY_ENABLE=0, child sleeping), measuring the parent's CPU seconds per wall second between t=4s and t=10s:The exit status passes through unchanged (
exit 3→ 3). SIGTERM topercy execmid-run behaves the same before and after: drain message, status 0, no orphaned child.No public API or config change; works on every supported Node version.
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