cordis/client/webapp/auto-verifier.ts

import * as electronRemote from '@electron/remote';
const electronConsole = electronRemote.getGlobal('console') as Console;
import Logger from '../../logger/logger';
import { Changes, WithEquals } from './data-types';
const LOG = Logger.create(__filename, electronConsole);

/**
 * This is probably complex piece of code in this entire project.
 * Some complexity comes because the result of the trusted function
 * can be invalidated while we are awaiting the trusted function.
 * That complexity stacks on top of the already complex cache-verification
 * to make this one fiesta of a class.
 */
export class AutoVerifier<T> {
	private trustedPromise: Promise<T | null> | null = null;
	private trustedStatus: 'none' | 'fetching' | 'verifying' | 'verified' = 'none';

	/**
	 * Allows a trusted function to verify the primary function
	 * @param primaryFunc The primary function
	 * @param trustedFunc The trusted function that will verify the results of the primary function (one time)
	 * @param verifyFunc The verification function that is called when the primary function's results need to be verified against the trusted function's results
	 */
	constructor(
		private primaryFunc: () => Promise<T | null>,
		private trustedFunc: () => Promise<T | null>,
		private verifyFunc: (primaryResult: T | null, trustedResult: T | null) => Promise<void>
	) {}

	static getChanges<T extends WithEquals<T> & { id: string }>(primaryResult: T[] | null, trustedResult: T[] | null): Changes<T> {
		let changes: Changes<T> = { added: [], updated: [], deleted: [] };

		if (primaryResult === null && trustedResult === null) {
			return changes;
		} else if (primaryResult === null) {
			changes.added = trustedResult as T[];
			return changes;
		} else if (trustedResult === null) {
			changes.deleted = primaryResult as T[];
			return changes;
		}

		primaryResult = primaryResult as T[];
		trustedResult = trustedResult as T[];

		for (let trustedElement of trustedResult) {
			let primaryElement = primaryResult.find(primaryElement => primaryElement.id === trustedElement.id);
			if (primaryElement) {
				changes.updated.push({ oldDataPoint: primaryElement, newDataPoint: trustedElement });
			} else {
				changes.added.push(trustedElement);
			}
		}
		for (let primaryElement of primaryResult) {
			let trustedElement = trustedResult.find(trustedElement => trustedElement.id === primaryElement.id);
			if (!trustedElement) {
				changes.deleted.push(primaryElement);
			}
		}

		return changes;
	}

	static createStandardListAutoVerifier<T extends WithEquals<T> & { id: string }>(
		primaryFunc: () => Promise<T[] | null>,
		trustedFunc: () => Promise<T[] | null>,
		changesFunc: (changes: Changes<T>) => Promise<void>
	) {
		return new AutoVerifier<T[]>(
			primaryFunc,
			trustedFunc,
			async (primaryResult: T[] | null, trustedResult: T[] | null) => {
				await changesFunc(AutoVerifier.getChanges(primaryResult, trustedResult));
			}
		);
	}

	// You CAN safely call this while another fetch is going on! How convenient
	unverify(): void {
		this.trustedPromise = null;
		this.trustedStatus = 'none';
	}

	// Fetches the result of the primary fetchable
	// If the primary fetchable returns null but has not been verified yet, this will return the result of the trusted fetchable
	// If the trusted fetchable has not been used to verify the primary fetchable yet, this queries the trusted fetchable and calls verify
	async fetchAndVerifyIfNeeded(): Promise<T | null> {
		return await new Promise<T | null>(async (resolve, reject) => {
			let resolved = false;
			try {
				let primaryPromise = this.primaryFunc();

				if (this.trustedStatus === 'none') {
					this.trustedStatus = 'fetching';
					this.trustedPromise = this.trustedFunc();
				}

				let primaryResult = await primaryPromise;

				if (primaryResult) {
					resolve(primaryResult);
					resolved = true;
				}

				//@ts-ignore (could be changed by an unverify during primaryPromise)
				if (this.trustedStatus === 'none') {
					this.trustedStatus = 'fetching';
					this.trustedPromise = this.trustedFunc();
				}

				let tryResolveTrustedPromise = async () => {
					if (this.trustedPromise) {
						// There is a trusted promise that we can check
	
						if (this.trustedStatus === 'fetching') {
							// No one has started verifying the trusted yet
	
							this.trustedStatus = 'verifying';
							
							// Note: Promises that have already resolved will return the same value when awaited again :)
							let origTrustedPromise: Promise<T | null> | null = this.trustedPromise;
							let trustedResult = await origTrustedPromise;
	
							if (this.trustedPromise !== origTrustedPromise) {
								// we've been invalidated while we were waiting for the trusted result!
								LOG.warn('RARE ALERT: we got unverified while trying to fetch a trusted promise for verification!');
								await tryResolveTrustedPromise();
								return;
							}

							if (!resolved) {
								resolve(trustedResult);
								resolved = true;
							}

							await this.verifyFunc(primaryResult, trustedResult);
							if (this.trustedPromise === origTrustedPromise) {
								this.trustedStatus = 'verified';
							} else {
								LOG.warn('RARE ALERT: we got unverified during verification!');
								// We don't have to re-resolve since we already would have resolved with the correct trusted result
							}
						} else {
							// Some code is already dealing with (or dealt with) verifying the trusted result
							// Await the same trusted promise and return its result if we didn't get a result
							// from the primary source.
	
							// Note: Promises that have already resolved will return the same value when awaited again :)
							let origTrustedPromise: Promise<T | null> | null = this.trustedPromise;
							let trustedResult = await origTrustedPromise;
	
							if (this.trustedPromise !== origTrustedPromise) {
								// we've been invalidated while we were waiting for the trusted result!
								LOG.warn('ULTRA RARE ALERT: we got unverified while awaiting a trusted promise another path was verifying!');
								await tryResolveTrustedPromise();
								return;
							}

							if (!resolved) {
								resolve(trustedResult);
								resolved = true;
							}
						}
					} else {
						// we are all up to date, make sure to resolve if primaryResult is null
						if (!resolved) {
							resolve(null);
							resolved = true;
						}
					}
				}
				await tryResolveTrustedPromise();
			} catch (e) {
				if (!resolved) {
					reject(e);
				} else {
					LOG.warn('server request failed after returning cache value', e);
				}
			}
		});
	}
}