external-resource-lock.ts

import { fdb } from "../firestore-init";


export interface ExternalResourceLockDocument {
  isLocked: boolean;
}

export class ExternalResourceLock {

  constructor(public readonly collection: string, public readonly docId: string) {
  }

  async acquireLock() {
    const ALREADY_LOCKED = "ALREADY_LOCKED";
    const docRef = fdb.collection(this.collection).doc(this.docId);

    try {
      return await fdb.runTransaction(async t => {
        const doc = await t.get(docRef);

        const data = (doc.data() as ExternalResourceLockDocument | undefined) ?? {
          isLocked: false,
        };

        if (data.isLocked ?? false) { // we have a fallback here because the document might not
          // have had a lock before
          throw ALREADY_LOCKED;
        } else {
          data.isLocked = true;
          t.set(docRef, data);
        }
        return data;
      }, { maxAttempts: 10 });
    } catch (e) {
      if (e === ALREADY_LOCKED) {
        console.log(`ExternalResourceLock: already locked: ${this.collection}/${this.docId}`);
      } else {
        console.error(e);
        throw e;
      }
    }
    return null;
  }

  async releaseLock() {
    const docRef = fdb.collection(this.collection).doc(this.docId);
    const newDoc : ExternalResourceLockDocument = { isLocked: false };
    await docRef.set(newDoc);
  }

  async isLocked() {
    const doc = await fdb.collection(this.collection).doc(this.docId).get();
    return (doc.data() as ExternalResourceLockDocument | undefined)?.isLocked ?? false;
  }

  /**
   * Just for testing purposes, you should not need to use this. 
   * Deletes the document.
   */
  async deleteDoc() {
    await fdb.collection(this.collection).doc(this.docId).delete();
  }
}