From 582ab4ac13e47d2108a43cd6eaaff9db1fb9d5fb Mon Sep 17 00:00:00 2001
From: Laurent Cozic <laurent@cozic.net>
Date: Wed, 9 Sep 2020 11:39:13 +0100
Subject: [PATCH] All: Implemented more reliable way to sync device and server
 clocks that would work with filesystem sync too

---
 ReactNativeClient/lib/file-api.js             | 42 +++++++++++++++++++
 .../lib/services/synchronizer/LockHandler.ts  |  3 +-
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/ReactNativeClient/lib/file-api.js b/ReactNativeClient/lib/file-api.js
index f53e97ff5..52200673d 100644
--- a/ReactNativeClient/lib/file-api.js
+++ b/ReactNativeClient/lib/file-api.js
@@ -6,6 +6,7 @@ const JoplinError = require('lib/JoplinError');
 const ArrayUtils = require('lib/ArrayUtils');
 const { time } = require('lib/time-utils.js');
 const { sprintf } = require('sprintf-js');
+const Mutex = require('async-mutex').Mutex;
 
 function requestCanBeRepeated(error) {
 	const errorCode = typeof error === 'object' && error.code ? error.code : null;
@@ -57,6 +58,47 @@ class FileApi {
 		this.tempDirName_ = null;
 		this.driver_.fileApi_ = this;
 		this.requestRepeatCount_ = null; // For testing purpose only - normally this value should come from the driver
+		this.remoteDateOffset_ = 0;
+		this.remoteDateNextCheckTime_ = 0;
+		this.remoteDateMutex_ = new Mutex();
+	}
+
+	// Approximates the current time on the sync target. It caches the time offset to
+	// improve performance.
+	async remoteDate() {
+		const shouldSyncTime = () => {
+			return !this.remoteDateNextCheckTime_ || Date.now() > this.remoteDateNextCheckTime_;
+		};
+
+		if (shouldSyncTime()) {
+			const release = await this.remoteDateMutex_.acquire();
+
+			try {
+				// Another call might have refreshed the time while we were waiting for the mutex,
+				// so check again if we need to refresh.
+				if (shouldSyncTime()) {
+					const tempFile = `${this.tempDirName()}/timeCheck${Math.random() * 10000}.txt`;
+					const startTime = Date.now();
+					await this.put(tempFile, 'timeCheck');
+					const stat = await this.stat(tempFile);
+					const endTime = Date.now();
+					const expectedTime = Math.round((endTime + startTime) / 2);
+					this.remoteDateOffset_ = stat.updated_time.getTime() - expectedTime;
+					// The sync target clock should rarely change but the device one might,
+					// so we need to refresh relatively frequently.
+					this.remoteDateNextCheckTime_ = Date.now() + 10 * 60 * 1000;
+					this.delete(tempFile); // No need to await for this call
+				}
+			} catch (error) {
+				this.logger().warn('Could not retrieve remote date - defaulting to device date:', error);
+				this.remoteDateOffset_ = 0;
+				this.remoteDateNextCheckTime_ = Date.now() + 60 * 1000;
+			} finally {
+				release();
+			}
+		}
+
+		return new Date(Date.now() + this.remoteDateOffset_);
 	}
 
 	// Ideally all requests repeating should be done at the FileApi level to remove duplicate code in the drivers, but
diff --git a/ReactNativeClient/lib/services/synchronizer/LockHandler.ts b/ReactNativeClient/lib/services/synchronizer/LockHandler.ts
index 62dddd7ed..17e955a56 100644
--- a/ReactNativeClient/lib/services/synchronizer/LockHandler.ts
+++ b/ReactNativeClient/lib/services/synchronizer/LockHandler.ts
@@ -1,5 +1,4 @@
 import { Dirnames } from './utils/types';
-import ntpDate from 'lib/ntpDate';
 
 const JoplinError = require('lib/JoplinError');
 const { time } = require('lib/time-utils');
@@ -114,7 +113,7 @@ export default class LockHandler {
 	// of that type instead.
 	async activeLock(lockType:LockType, clientType:string = null, clientId:string = null) {
 		const locks = await this.locks(lockType);
-		const currentDate = await ntpDate();
+		const currentDate = await this.api_.remoteDate();
 
 		if (lockType === LockType.Exclusive) {
 			const activeLocks = locks