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fix: write Codex session cache atomically #295

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fix: write Codex session cache atomically #295

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38 changes: 34 additions & 4 deletions __tests__/lib/codex-sessions.test.ts
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Original file line number Diff line number Diff line change
@@ -1,8 +1,16 @@
// @vitest-environment node
import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs";
import { join } from "node:path";
import { tmpdir, homedir } from "node:os";
import {
existsSync,
mkdtempSync,
mkdirSync,
readdirSync,
readFileSync,
rmSync,
writeFileSync,
} from "node:fs";
import { dirname, join } from "node:path";
import { tmpdir } from "node:os";

const line = (obj: Record<string, unknown>): string => JSON.stringify(obj);

Expand Down Expand Up @@ -195,6 +203,7 @@ describe("lib/codex-sessions: findCodexTranscript", () => {
let originalHome: string | undefined;
let fakeHome: string;
let findCodexTranscript: typeof import("@/lib/codex-sessions").findCodexTranscript;
let getCacheFilePath: typeof import("@/lib/codex-sessions")._getCacheFilePath;

beforeEach(async () => {
originalHome = process.env.HOME;
Expand All @@ -206,7 +215,9 @@ describe("lib/codex-sessions: findCodexTranscript", () => {
const actual = await vi.importActual<typeof import("node:os")>("node:os");
return { ...actual, homedir: () => fakeHome };
});
({ findCodexTranscript } = await import("@/lib/codex-sessions"));
const mod = await import("@/lib/codex-sessions");
findCodexTranscript = mod.findCodexTranscript;
getCacheFilePath = mod._getCacheFilePath;
});

afterEach(() => {
Expand Down Expand Up @@ -236,6 +247,25 @@ describe("lib/codex-sessions: findCodexTranscript", () => {
expect(result).toBe(file);
});

it("writes discovered transcript paths through the cache file", () => {
const sid = "019dd672-cache-7a30-8671-deadbeefcafe";
const today = new Date();
const y = String(today.getUTCFullYear());
const m = String(today.getUTCMonth() + 1).padStart(2, "0");
const d = String(today.getUTCDate()).padStart(2, "0");
const dir = join(fakeHome, ".codex", "sessions", y, m, d);
mkdirSync(dir, { recursive: true });
const file = join(dir, `rollout-${sid}.jsonl`);
writeFileSync(file, "{}\n");

expect(findCodexTranscript(sid)).toBe(file);

const cachePath = getCacheFilePath();
expect(JSON.parse(readFileSync(cachePath, "utf-8"))).toEqual({ [sid]: file });
expect(existsSync(`${cachePath}.${process.pid}.tmp`)).toBe(false);
expect(readdirSync(dirname(cachePath)).filter((name) => name.endsWith(".tmp"))).toEqual([]);
});

it("locates a transcript via full tree scan when not in today/yesterday", () => {
const sid = "019dd672-bbbb-7a30-8671-deadbeefcafe";
const dir = join(fakeHome, ".codex", "sessions", "2024", "01", "15");
Expand Down
16 changes: 14 additions & 2 deletions lib/codex-sessions.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,16 @@
* parser produces (`lib/log-entries.ts`) so the existing log viewer renders
* Codex sessions without any UI-side branching.
*/
import { readFileSync, readdirSync, existsSync, writeFileSync, mkdirSync, statSync } from "node:fs";
import {
readFileSync,
readdirSync,
existsSync,
writeFileSync,
mkdirSync,
statSync,
renameSync,
unlinkSync,
} from "node:fs";
import { readFile } from "node:fs/promises";
import { dirname, join } from "node:path";
import { homedir } from "node:os";
Expand Down Expand Up @@ -48,12 +57,15 @@ function readCache(): Record<string, string> {
}

function writeCacheEntry(sessionId: string, path: string): void {
const tmpPath = `${CACHE_PATH}.${process.pid}.tmp`;
try {
mkdirSync(dirname(CACHE_PATH), { recursive: true });
const cache = readCache();
cache[sessionId] = path;
writeFileSync(CACHE_PATH, JSON.stringify(cache), "utf-8");
writeFileSync(tmpPath, JSON.stringify(cache), "utf-8");
renameSync(tmpPath, CACHE_PATH);
Comment on lines 63 to +66
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@coderabbitai coderabbitai Bot May 5, 2026

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⚠️ Potential issue | 🟠 Major | 🏗️ Heavy lift

Atomic rename here still allows concurrent lost updates

This is still a read-modify-write race: two writers can both read the same old cache, each add a different sessionId, and the last renameSync wins, dropping the other entry. Atomic replacement prevents partial JSON, but not clobbering.

Use a cross-process critical section for read+write (e.g., lock file with retry/backoff) or a merge-on-conflict retry loop after failed/changed writes so concurrent writers preserve both entries.

🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@lib/codex-sessions.ts` around lines 63 - 66, The read-modify-write around
readCache() -> modify cache[sessionId] -> writeFileSync(tmpPath) ->
renameSync(CACHE_PATH) is racy and can clobber concurrent updates; wrap that
whole sequence in a cross-process critical section (e.g., obtain an advisory
lock on CACHE_PATH or a dedicated lockfile with retry/backoff) so only one
process reads+updates+renames at a time, or implement an optimistic merge-retry:
after writing the tmp file but before rename, re-read CACHE_PATH (or
stat+contents), merge any new keys into your in-memory cache, and retry the
write/rename loop until no concurrent changes remain. Use the existing symbols
(readCache, CACHE_PATH, tmpPath, sessionId, writeFileSync, renameSync) to locate
and protect the critical region.

} catch {
try { unlinkSync(tmpPath); } catch { /* ignore */ }
// Cache is best-effort
}
}
Expand Down