fix: treat EPERM as alive in isPidAlive (#110235)

* fix(pid): treat EPERM as alive in isPidAlive

Match isPidDefinitelyDead: process.kill(pid, 0) throwing EPERM means the
PID exists but cannot be signaled, so it should not look dead.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(pid): check Linux zombies after EPERM existence probe

EPERM means the PID exists but cannot be signaled; still run the zombie check so Linux zombies are not reported as alive.

Co-authored-by: Cursor <cursoragent@cursor.com>

* test(pid): isolate EPERM liveness probe

Co-authored-by: stantheman0128 <stanshih888@gmail.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/shared/pid-alive.test.ts

@@ -42,6 +42,45 @@ describe("isPidAlive", () => {
     expect(isPidAlive(Number.POSITIVE_INFINITY)).toBe(false);
   });
 
+  it("returns true when process probing reports EPERM", () => {
+    const error = Object.assign(new Error("permission denied"), { code: "EPERM" });
+    vi.spyOn(process, "kill").mockImplementation(() => {
+      throw error;
+    });
+    mockProcReads({
+      "/proc/42/status": "Name:\tnode\nState:\tS (sleeping)\nPid:\t42\n",
+    });
+
+    expect(isPidAlive(42)).toBe(true);
+    expect(process["kill"]).toHaveBeenCalledWith(42, 0);
+  });
+
+  it("returns false for Linux zombies even when probing reports EPERM", async () => {
+    const error = Object.assign(new Error("permission denied"), { code: "EPERM" });
+    vi.spyOn(process, "kill").mockImplementation(() => {
+      throw error;
+    });
+    mockProcReads({
+      "/proc/42/status": "Name:\tnode\nUmask:\t0022\nState:\tZ (zombie)\nTgid:\t42\nPid:\t42\n",
+    });
+
+    await withMockedPlatform("linux", async () => {
+      expect(isPidAlive(42)).toBe(false);
+    });
+
+    expect(process["kill"]).toHaveBeenCalledWith(42, 0);
+  });
+
+  it("returns false when process probing reports ESRCH", () => {
+    const error = Object.assign(new Error("missing process"), { code: "ESRCH" });
+    vi.spyOn(process, "kill").mockImplementation(() => {
+      throw error;
+    });
+
+    expect(isPidAlive(42)).toBe(false);
+    expect(process["kill"]).toHaveBeenCalledWith(42, 0);
+  });
+
   it("returns false for zombie processes on Linux", async () => {
     const zombiePid = process.pid;
 

src/shared/pid-alive.ts

@@ -32,8 +32,13 @@ export function isPidAlive(pid: number): boolean {
   }
   try {
     process.kill(pid, 0);
-  } catch {
-    return false;
+  } catch (err) {
+    // EPERM means the PID exists but we cannot signal it. Treat that as a
+    // successful existence probe, then still apply the Linux zombie check.
+    // Keep parity with isPidDefinitelyDead (EPERM is not "definitely dead").
+    if ((err as NodeJS.ErrnoException).code !== "EPERM") {
+      return false;
+    }
   }
   if (isZombieProcess(pid)) {
     return false;