fix(update): launch installers after the current app exits

tests/update-launch-order.test.ts

@@ -0,0 +1,58 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { spawn } from "node:child_process";
+import { afterEach, describe, expect, it } from "vitest";
+import { buildDeferredInstallerLaunch } from "../src/main/update";
+
+const tempDirs: string[] = [];
+
+afterEach(() => {
+  for (const dir of tempDirs.splice(0)) {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
+async function waitForFile(filePath: string, timeoutMs: number): Promise<void> {
+  const deadline = Date.now() + timeoutMs;
+  while (!fs.existsSync(filePath)) {
+    if (Date.now() >= deadline) {
+      throw new Error("marker timeout");
+    }
+    await new Promise((resolve) => setTimeout(resolve, 50));
+  }
+}
+
+describe.runIf(process.platform === "win32")("deferred update launch", () => {
+  it("starts the installer only after the previous application process exits", async () => {
+    const root = fs.mkdtempSync(path.join(os.tmpdir(), "mdd-update-launch-"));
+    tempDirs.push(root);
+    const installerPath = path.join(root, "installer.cmd");
+    const markerPath = path.join(root, "started.txt");
+    fs.writeFileSync(installerPath, `@echo off\r\n> "${markerPath}" echo started\r\n`, "utf8");
+    const previous = spawn(process.env.ComSpec || "cmd.exe", ["/d", "/s", "/c", "ping 127.0.0.1 -n 3 > nul"], {
+      stdio: "ignore",
+      windowsHide: true
+    });
+    if (!previous.pid) {
+      throw new Error("previous process missing pid");
+    }
+    const deferred = buildDeferredInstallerLaunch(installerPath, previous.pid);
+    const launcher = spawn(deferred.command, deferred.args, {
+      stdio: "ignore",
+      windowsHide: true
+    });
+
+    await new Promise((resolve) => setTimeout(resolve, 500));
+    expect(fs.existsSync(markerPath)).toBe(false);
+
+    await new Promise<void>((resolve, reject) => {
+      previous.once("exit", () => resolve());
+      previous.once("error", reject);
+    });
+    await waitForFile(markerPath, 5_000);
+    expect(fs.readFileSync(markerPath, "utf8").trim()).toBe("started");
+
+    launcher.unref();
+  });
+});