FIX: better locks (rel #3)

class/Lock.js

@@ -0,0 +1,47 @@
+const crypto = require('crypto');
+
+export class Lock {
+  /**
+   *
+   * @param {Redis} redis
+   * @param {String} lock_key
+   */
+  constructor(redis, lock_key) {
+    this._redis = redis;
+    this._lock_key = lock_key;
+  }
+
+  /**
+   * Tries to obtain lock in single-threaded Redis.
+   * Returns TRUE if success.
+   *
+   * @returns {Promise<boolean>}
+   */
+  async obtainLock() {
+    if (await this._redis.get(this._lock_key)) {
+      // someone already has the lock
+      return false;
+    }
+
+    // trying to set the lock:
+    let buffer = crypto.randomBytes(10);
+    const randomValue = buffer.toString('hex');
+    await this._redis.set(this._lock_key, randomValue);
+
+    // checking if it was set:
+    let value = await this._redis.get(this._lock_key);
+    if (value !== randomValue) {
+      // someone else managed to obtain this lock
+      return false;
+    }
+
+    // success - got lock
+    await this._redis.expire(this._lock_key, 2 * 60);
+    // lock expires in 2 mins just for any case
+    return true;
+  }
+
+  async releaseLock() {
+    await this._redis.del(this._lock_key);
+  }
+}

class/index.js

@@ -1 +1,2 @@
 export * from './User';
+export * from './Lock';