fix(gate): treat a process that exits mid-read as gone

src/cli/gate.ts

@@ -283,7 +283,7 @@ export function processStart(pid: number): string | undefined {
 		try {
 			return `${readFileSync("/proc/sys/kernel/random/boot_id", "utf8").trim()} ${startTime(pid)}`;
 		} catch (error) {
-			if ((error as NodeJS.ErrnoException).code === "ENOENT") return undefined;
+			if (["ENOENT", "ESRCH"].includes((error as NodeJS.ErrnoException).code ?? "")) return undefined;
 			throw error;
 		}
 	}

src/cli/supervise.ts

@@ -30,7 +30,10 @@ export interface Owner {
 	start?: string;
 }
 
-/** Field 22 of /proc/<pid>/stat; the command name in parentheses may hold spaces. */
+/**
+ * Field 22 of /proc/<pid>/stat; the command name in parentheses may hold spaces. Throws once the
+ * process is gone: ENOENT, or ESRCH when it exits mid-read.
+ */
 export function startTime(pid: number): string | undefined {
 	if (process.platform !== "linux") return undefined;
 	const stat = readFileSync(`/proc/${pid}/stat`, "utf8");
@@ -48,7 +51,7 @@ export function alive(owner: Owner): boolean {
 		try {
 			return startTime(owner.pid) === owner.start;
 		} catch (error) {
-			if ((error as NodeJS.ErrnoException).code === "ENOENT") return false;
+			if (["ENOENT", "ESRCH"].includes((error as NodeJS.ErrnoException).code ?? "")) return false;
 			throw error;
 		}
 	}

test/gate.test.ts

@@ -13,7 +13,7 @@ import {
 	utimesSync,
 	writeFileSync,
 } from "node:fs";
-import { constants } from "node:os";
+import { constants, hostname } from "node:os";
 import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { afterEach, describe, expect, test, vi } from "vitest";
@@ -42,9 +42,27 @@ import {
 } from "../src/cli/gate";
 import { forwards, foregroundStat, statProcess } from "../src/cli/interrupt";
 import { gateBoundary, main } from "../src/cli/main";
-import { questScope } from "../src/cli/supervise";
+import { alive as ownerAlive, questScope } from "../src/cli/supervise";
 import { created, onTerminal, run, signalled, tempDir } from "./fixture";
 
+// Linux reports a /proc read that races a process's exit as ESRCH rather than ENOENT, so fail
+// one process's stat read with either code, without timing.
+const racing = vi.hoisted(() => ({ read: undefined as { path: string; code: string } | undefined }));
+vi.mock("node:fs", async (importOriginal) => {
+	const fs = await importOriginal<typeof import("node:fs")>();
+	return {
+		...fs,
+		readFileSync: (...args: Parameters<typeof fs.readFileSync>) => {
+			if (racing.read && args[0] === racing.read.path) {
+				throw Object.assign(new Error(`${racing.read.code}: no such process, read`), {
+					code: racing.read.code,
+				});
+			}
+			return fs.readFileSync(...args);
+		},
+	};
+});
+
 const GiB = 2 ** 30;
 
 /** How this process's gate records itself in an admission. */
@@ -1146,6 +1164,24 @@ function detached(dir: string, rest: string) {
 	};
 }
 
+describe.runIf(process.platform === "linux")("a process that exits while its stat is read", () => {
+	const pid = 4242;
+	const identity = { host: hostname(), pid, start: "1" };
+	afterEach(() => (racing.read = undefined));
+
+	test.each(["ENOENT", "ESRCH"])("reads as gone when the stat read fails with %s", (code) => {
+		racing.read = { path: `/proc/${pid}/stat`, code };
+		expect(processStart(pid)).toBeUndefined();
+		expect(ownerAlive(identity)).toBe(false);
+	});
+
+	test("surfaces any other read error", () => {
+		racing.read = { path: `/proc/${pid}/stat`, code: "EACCES" };
+		expect(() => processStart(pid)).toThrow("EACCES");
+		expect(() => ownerAlive(identity)).toThrow("EACCES");
+	});
+});
+
 describe("the Windows job stub", () => {
 	test("passes the exact exit status and restores the caller's environment", async () => {
 		const script =