budibase/packages/core/src/common/lock.js

import { split } from 'lodash/fp';
import { $ } from './index';

const lockOverlapMilliseconds = 10;

export const getLock = async (app, lockFile, timeoutMilliseconds, maxLockRetries, retryCount = 0) => {
  try {
    const timeout = (await app.getEpochTime())
            + timeoutMilliseconds;

    const lock = {
      timeout,
      key: lockFile,
      totalTimeout: timeoutMilliseconds,
    };

    await app.datastore.createFile(
      lockFile,
      getLockFileContent(
        lock.totalTimeout,
        lock.timeout,
      ),
    );

    return lock;
  } catch (e) {
    if (retryCount == maxLockRetries) { return NO_LOCK; }

    const lock = parseLockFileContent(
      lockFile,
      await app.datastore.loadFile(lockFile),
    );

    const currentEpochTime = await app.getEpochTime();

    if (currentEpochTime < lock.timeout) {
      return NO_LOCK;
    }

    try {
      await app.datastore.deleteFile(lockFile);
    } catch (_) {
      //empty
    }

    await sleepForRetry();

    return await getLock(
      app, lockFile, timeoutMilliseconds,
      maxLockRetries, retryCount + 1,
    );
  }
};

export const getLockFileContent = (totalTimeout, epochTime) => `${totalTimeout}:${epochTime.toString()}`;

const parseLockFileContent = (key, content) => $(content, [
  split(':'),
  parts => ({
    totalTimeout: new Number(parts[0]),
    timeout: new Number(parts[1]),
    key,
  }),
]);

export const releaseLock = async (app, lock) => {
  const currentEpochTime = await app.getEpochTime();
  // only release if not timedout
  if (currentEpochTime < (lock.timeout - lockOverlapMilliseconds)) {
    try {
      await app.datastore.deleteFile(lock.key);
    } catch (_) {
      //empty
    }
  }
};

export const extendLock = async (app, lock) => {
  const currentEpochTime = await app.getEpochTime();
  // only release if not timedout
  if (currentEpochTime < (lock.timeout - lockOverlapMilliseconds)) {
    try {
      lock.timeout = currentEpochTime + lock.timeoutMilliseconds;
      await app.datastore.updateFile(
        lock.key,
        getLockFileContent(lock.totalTimeout, lock.timeout),
      );
      return lock;
    } catch (_) {
      //empty
    }
  }
  return NO_LOCK;
};

export const NO_LOCK = 'no lock';
export const isNolock = id => id === NO_LOCK;

const sleepForRetry = () => new Promise(resolve => setTimeout(resolve, lockOverlapMilliseconds));