fix: detect zombie processes in isPidAlive on Linux

kill(pid, 0) succeeds for zombie processes, causing the gateway lock
to treat a zombie lock owner as alive. Read /proc/<pid>/status on
Linux to check for 'Z' (zombie) state before reporting the process
as alive. This prevents the lock from being held indefinitely by a
zombie process during gateway restart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/shared/pid-alive.test.ts

@@ -0,0 +1,47 @@
+import fsSync from "node:fs";
+import { describe, expect, it, vi } from "vitest";
+import { isPidAlive } from "./pid-alive.js";
+
+describe("isPidAlive", () => {
+  it("returns true for the current running process", () => {
+    expect(isPidAlive(process.pid)).toBe(true);
+  });
+
+  it("returns false for a non-existent PID", () => {
+    expect(isPidAlive(2 ** 30)).toBe(false);
+  });
+
+  it("returns false for invalid PIDs", () => {
+    expect(isPidAlive(0)).toBe(false);
+    expect(isPidAlive(-1)).toBe(false);
+    expect(isPidAlive(Number.NaN)).toBe(false);
+    expect(isPidAlive(Number.POSITIVE_INFINITY)).toBe(false);
+  });
+
+  it("returns false for zombie processes on Linux", async () => {
+    const zombiePid = process.pid;
+
+    // Mock readFileSync to return zombie state for /proc/<pid>/status
+    const originalReadFileSync = fsSync.readFileSync;
+    vi.spyOn(fsSync, "readFileSync").mockImplementation((filePath, encoding) => {
+      if (filePath === `/proc/${zombiePid}/status`) {
+        return `Name:\tnode\nUmask:\t0022\nState:\tZ (zombie)\nTgid:\t${zombiePid}\nPid:\t${zombiePid}\n`;
+      }
+      return originalReadFileSync(filePath as never, encoding as never) as never;
+    });
+
+    // Override platform to linux so the zombie check runs
+    const originalPlatform = process.platform;
+    Object.defineProperty(process, "platform", { value: "linux", writable: true });
+
+    try {
+      // Re-import the module so it picks up the mocked platform and fs
+      vi.resetModules();
+      const { isPidAlive: freshIsPidAlive } = await import("./pid-alive.js");
+      expect(freshIsPidAlive(zombiePid)).toBe(false);
+    } finally {
+      Object.defineProperty(process, "platform", { value: originalPlatform, writable: true });
+      vi.restoreAllMocks();
+    }
+  });
+});

src/shared/pid-alive.ts

@@ -1,11 +1,33 @@
+import fsSync from "node:fs";
+
+/**
+ * Check if a process is a zombie on Linux by reading /proc/<pid>/status.
+ * Returns false on non-Linux platforms or if the proc file can't be read.
+ */
+function isZombieProcess(pid: number): boolean {
+  if (process.platform !== "linux") {
+    return false;
+  }
+  try {
+    const status = fsSync.readFileSync(`/proc/${pid}/status`, "utf8");
+    const stateMatch = status.match(/^State:\s+(\S)/m);
+    return stateMatch?.[1] === "Z";
+  } catch {
+    return false;
+  }
+}
+
 export function isPidAlive(pid: number): boolean {
   if (!Number.isFinite(pid) || pid <= 0) {
     return false;
   }
   try {
     process.kill(pid, 0);
-    return true;
   } catch {
     return false;
   }
+  if (isZombieProcess(pid)) {
+    return false;
+  }
+  return true;
 }