dahlia-concurrency/src/primitives/rw-lock.js

High-level concurrency primitives and patterns for Node.js using worker_threads (queues, semaphore, mutex, worker pool, scheduler, and more)

const { Mutex } = require("./mutex.js");

class RwLock {
  constructor(readMutexBuffer, writeMutexBuffer, politics = true) {
    this.politics = politics;
    // true is fair
    // false is greedy
    this.reader = 0;
    this.writer = false;

    this.readMutex = new Mutex(readMutexBuffer);
    this.writeMutex = new Mutex(writeMutexBuffer);

    if (politics) {
      this.waitingWriters = 0;
      const readerGateBuffer = new SharedArrayBuffer(4);
      this.readerGate = new Mutex(readerGateBuffer);
    }
  }

  readLockGreedy() {
    this.readMutex.lock();
    this.reader++;
    if (this.reader === 1) {
      this.writeMutex.lock();
    }
    this.readMutex.unlock();
  }

  readLockFair() {
    this.readerGate.lock();
    this.readMutex.lock();
    this.reader++;
    if (this.reader === 1) {
      this.writeMutex.lock();
    }
    this.readMutex.unlock();
    this.readerGate.unlock();
  }

  writeLockGreedy() {
    this.readMutex.lock();
    if (this.reader > 0) {
      this.readMutex.unlock();
      throw new Error("Cannot acquire write lock while read lock is active");
    }
    this.readMutex.unlock();

    this.writeMutex.lock();
    this.writer = true;
  }

  writeLockFair() {
    this.readMutex.lock();
    if (this.reader > 0) {
      this.readMutex.unlock();
      throw new Error("Cannot acquire write lock while read lock is active");
    }
    this.readMutex.unlock();

    this.waitingWriters++;

    this.readerGate.lock();
    this.writeMutex.lock();
    this.writer = true;
    this.waitingWriters--;
  }

  readLock() {
    return this.politics ? this.readLockFair() : this.readLockGreedy();
  }

  writeLock() {
    return this.politics ? this.writeLockFair() : this.writeLockGreedy();
  }

  readUnlock() {
    this.readMutex.lock();
    this.reader--;
    if (this.reader === 0) {
      this.writeMutex.unlock();

      if (this.politics && this.waitingWriters > 0) {
        this.readerGate.unlock();
      }
    }
    this.readMutex.unlock();
  }

  writeUnlock() {
    this.writer = false;
    this.writeMutex.unlock();
    if (this.politics) {
      this.readerGate.unlock();
    }
  }
}

module.exports = { RwLock };