fix: handle EAGAIN in hook scripts when stdin is non-blocking by ikbear · Pull Request #165 · openai/codex-plugin-cc

ikbear · 2026-04-07T03:15:05Z

Problem

Claude Code invokes hooks with stdin as a non-blocking pipe on macOS and Linux. This causes fs.readFileSync(0) in readHookInput() to throw EAGAIN: resource temporarily unavailable, making the hook exit with code 1. The result is a SessionStart:startup hook error banner shown on every session start.

Reported in #120.

Root cause

function readHookInput() {
  const raw = fs.readFileSync(0, "utf8").trim(); // throws EAGAIN on non-blocking stdin
  ...
}

Fix

Retry readFileSync up to 20 times (200ms total) when EAGAIN is received, using Atomics.wait for synchronous sleeping. If all retries are exhausted, returns {} so the hook exits cleanly (graceful degradation) instead of surfacing a startup error.

const MAX_RETRIES = 20;
const RETRY_DELAY_MS = 10;
const sleepBuf = new Int32Array(new SharedArrayBuffer(4));

for (let attempt = 0; attempt <= MAX_RETRIES; attempt++) {
  try {
    const raw = fs.readFileSync(0, "utf8").trim();
    return raw ? JSON.parse(raw) : {};
  } catch (err) {
    if (err.code !== "EAGAIN") throw err;
    if (attempt === MAX_RETRIES) return {};
    Atomics.wait(sleepBuf, 0, 0, RETRY_DELAY_MS);
  }
}

Applied to both session-lifecycle-hook.mjs and stop-review-gate-hook.mjs.

Relation to PR #123

PR #123 addresses the same issue with a more complex buffered approach. This PR offers a simpler implementation with the same intent.

Fixes #120

Claude Code invokes hooks with stdin as a non-blocking pipe on some platforms (macOS, Linux). This causes fs.readFileSync(0) to throw EAGAIN: resource temporarily unavailable, making the hook exit with code 1 and showing a SessionStart:startup hook error banner on every session start. Fix by retrying readFileSync up to 20 times (200ms total) when EAGAIN is received, using Atomics.wait for precise synchronous sleeping. If all retries are exhausted, returns {} so the hook succeeds gracefully rather than surfacing a startup error. Fixes openai#120

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…l reads When stdin arrives in multiple writes on a non-blocking pipe, readFileSync can consume a partial chunk internally before throwing EAGAIN, causing the already-read bytes to be silently discarded. On the next retry, readFileSync starts from an empty buffer, so the accumulated bytes are gone and JSON.parse fails on the tail chunk alone. Switch to readSync in a manual accumulation loop: each successful read appends to a chunks array, EAGAIN causes a wait-and-retry without touching the accumulated data, and EOF triggers a single Buffer.concat + JSON.parse. Fixes the review comment on openai#165.

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When the retry budget is exhausted after partial bytes have already been read, breaking out of the loop and falling through to JSON.parse risks throwing SyntaxError on truncated JSON, which fails the hook instead of degrading gracefully. Return {} directly on max retries to guarantee clean hook exit regardless of how much data was buffered.

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…penai#255, openai#165, openai#253, openai#209, openai#239, openai#307, openai#315) - Raise adversarial review inline diff limits to 50 files / 1MB (openai#327) - Add wall-clock timeouts to JSON-RPC requests (openai#302) - Use codex-namespaced clientInfo.name (openai#200, openai#255) - Handle EAGAIN in hook scripts for non-blocking stdin (openai#165) - Replace Node.js SessionStart hook with faster bash script (openai#253) - Add --fast flag for service_tier support (openai#209) - Set timeout: 600000 on foreground review/adversarial-review flows (openai#239) - Rename gpt-5-4-prompting skill to codex-prompting (openai#307) Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

…eview prompt via stdin New lib/hook-input.mjs readHookInput(): streams stdin with a deadline (2 s default, CODEX_HOOK_STDIN_TIMEOUT_MS for tests), a 1 MiB byte limit and a StringDecoder across chunk boundaries. EOF parses the whole input; at the deadline a buffer that is already a complete JSON object is accepted (only the EOF was late), anything else is `timeout`; over the limit is `overflow` without parsing; bad JSON is `invalid-json`. Listeners and the timer are removed and stdin destroyed, so the hook exits right after deciding. Stop hook: any read error blocks as before (fail-closed), except a timeout with nothing received while the workspace gate (CLAUDE_PROJECT_DIR / cwd) is off, which allows (openai#530/openai#544). With the gate on, a timeout blocks naming "hook input did not arrive". The review prompt now reaches the companion via `task --prompt-stdin`, not argv, lifting the Windows/Linux argv limits. SessionStart/SessionEnd hook: reads with a 1 s deadline before the 12 s SessionEnd budget starts; on any read error it prints one stderr line and returns without cleanup. The hooks.json timeout test now asserts 15 s > budget + read. readStdinIfPiped (companion): readSync loop that keeps accumulated bytes across EAGAIN on a non-blocking stdin (up to 50 x 20 ms consecutive) and throws on exhaustion instead of returning a partial prompt (openai#120/openai#247/openai#123/openai#150/openai#165). Co-authored-by: stantheman0128 <[email protected]> Co-authored-by: tmchow <[email protected]> Co-Authored-By: Claude Fable 5.1 <[email protected]>

ikbear requested a review from a team April 7, 2026 03:15

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Comment thread plugins/codex/scripts/session-lifecycle-hook.mjs Resolved

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Comment thread plugins/codex/scripts/session-lifecycle-hook.mjs Resolved

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Comment thread plugins/codex/scripts/session-lifecycle-hook.mjs Resolved

stantheman0128 mentioned this pull request Jul 23, 2026

fix: bound Stop hook stdin read so disabled gate cannot hang on Windows (#530) #544

Open

This was referenced Sep 28, 2026

release: v1.4.0 CBEPX/codex-plugin-cc#8

Merged

codex-companion crashes with EAGAIN on concurrent sessions (readStdinIfPiped sync read) #247

Open

nicoache1 mentioned this pull request Oct 9, 2026

EAGAIN crash in hook scripts: readFileSync(0) fails when stdin is non-blocking #120

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fix: handle EAGAIN in hook scripts when stdin is non-blocking - #165

fix: handle EAGAIN in hook scripts when stdin is non-blocking#165
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ikbear commented Apr 7, 2026

Problem

Root cause

Fix

Relation to PR #123

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