test(server): reproduce shared daemon lock loss across PID namespaces

Retain acquisition and stop regressions for #4307. These tests intentionally fail on main; they do not choose a namespace ownership or crash-recovery policy.

Co-authored-by: Cong Xu <3174585+xu-c@users.noreply.github.com>

packages/server/src/server/pid-lock-namespace.test.ts

@@ -0,0 +1,43 @@
+import { mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { expect, test } from "vitest";
+import { stopDaemonInstance } from "./daemon-instance.js";
+import { acquirePidLock } from "./pid-lock.js";
+
+// A shared home exposes a live supervisor's lock to a PID namespace in which its PID
+// does not exist. The Docker reproduction in #4307 establishes that condition; these
+// filesystem tests retain it without requiring a container runtime in the unit suite.
+async function withUnseenOwner(run: (home: string, contents: string) => Promise<void>) {
+  const home = await mkdtemp(join(tmpdir(), "paseo-unseen-owner-"));
+  const contents = JSON.stringify({
+    pid: 2_147_483_647,
+    startedAt: new Date().toISOString(),
+    hostname: "shared-host",
+    uid: process.getuid?.() ?? 0,
+    listen: null,
+    heartbeat: true,
+  });
+  try {
+    await writeFile(join(home, "paseo.pid"), contents);
+    await run(home, contents);
+  } finally {
+    await rm(home, { recursive: true, force: true });
+  }
+}
+
+test("acquiring a lock preserves a fresh heartbeat from an unseen owner", async () => {
+  await withUnseenOwner(async (home, contents) => {
+    await expect(acquirePidLock(home, null)).rejects.toThrow();
+    expect(await readFile(join(home, "paseo.pid"), "utf8")).toBe(contents);
+  });
+});
+
+test("stopping an unseen owner preserves its fresh shared lock", async () => {
+  await withUnseenOwner(async (home, contents) => {
+    // Either refusing the operation or returning without modifying the foreign lock is safe.
+    // The failure being locked down is deleting the live supervisor's lock.
+    await stopDaemonInstance(home).catch(() => undefined);
+    expect(await readFile(join(home, "paseo.pid"), "utf8")).toBe(contents);
+  });
+});