fix: add file-lock fallback for single-instance guard

Co-authored-by: Haze <hazeone@users.noreply.github.com>

electron/main/process-instance-lock.ts

@@ -0,0 +1,100 @@
+import { closeSync, existsSync, mkdirSync, openSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+
+export interface ProcessInstanceFileLock {
+  acquired: boolean;
+  lockPath: string;
+  ownerPid?: number;
+  release: () => void;
+}
+
+export interface ProcessInstanceFileLockOptions {
+  userDataDir: string;
+  lockName: string;
+  pid?: number;
+  isPidAlive?: (pid: number) => boolean;
+}
+
+function defaultPidAlive(pid: number): boolean {
+  try {
+    process.kill(pid, 0);
+    return true;
+  } catch (error) {
+    const errno = (error as NodeJS.ErrnoException).code;
+    return errno !== 'ESRCH';
+  }
+}
+
+function readLockOwnerPid(lockPath: string): number | undefined {
+  try {
+    const raw = readFileSync(lockPath, 'utf8').trim();
+    const parsed = Number.parseInt(raw, 10);
+    return Number.isFinite(parsed) && parsed > 0 ? parsed : undefined;
+  } catch {
+    return undefined;
+  }
+}
+
+export function acquireProcessInstanceFileLock(
+  options: ProcessInstanceFileLockOptions,
+): ProcessInstanceFileLock {
+  const pid = options.pid ?? process.pid;
+  const isPidAlive = options.isPidAlive ?? defaultPidAlive;
+
+  mkdirSync(options.userDataDir, { recursive: true });
+  const lockPath = join(options.userDataDir, `${options.lockName}.instance.lock`);
+
+  let ownerPid: number | undefined;
+
+  for (let attempt = 0; attempt < 2; attempt += 1) {
+    try {
+      const fd = openSync(lockPath, 'wx');
+      try {
+        writeFileSync(fd, String(pid), 'utf8');
+      } finally {
+        closeSync(fd);
+      }
+
+      let released = false;
+      return {
+        acquired: true,
+        lockPath,
+        release: () => {
+          if (released) return;
+          released = true;
+          try {
+            rmSync(lockPath, { force: true });
+          } catch {
+            // best-effort
+          }
+        },
+      };
+    } catch (error) {
+      const errno = (error as NodeJS.ErrnoException).code;
+      if (errno !== 'EEXIST') {
+        break;
+      }
+
+      ownerPid = readLockOwnerPid(lockPath);
+      if (ownerPid && !isPidAlive(ownerPid) && existsSync(lockPath)) {
+        try {
+          rmSync(lockPath, { force: true });
+          continue;
+        } catch {
+          // If deletion fails, treat as held lock.
+        }
+      }
+
+      break;
+    }
+  }
+
+  return {
+    acquired: false,
+    lockPath,
+    ownerPid,
+    release: () => {
+      // no-op when lock wasn't acquired
+    },
+  };
+}