randomize sync interval to avoid infinite deadlocks (#1250)

* randomize sync interval to avoid infinite deadlocks

* autoretry with a randomly increasing delay

background/sync-manager.js

@@ -11,6 +11,7 @@ const syncMan = (() => {
 
   const SYNC_DELAY = 1; // minutes
   const SYNC_INTERVAL = 30; // minutes
+  const SYNC_LOCK_RETRIES = 10; // number of retries before the error is reported for scheduled sync
   const STATES = Object.freeze({
     connected: 'connected',
     connecting: 'connecting',
@@ -26,6 +27,7 @@ const syncMan = (() => {
     currentDriveName: null,
     errorMessage: null,
     login: false,
+    lockRetries: 0,
   };
   let lastError = null;
   let ctrl;
@@ -40,9 +42,11 @@ const syncMan = (() => {
       {runNow: true});
   });
 
-  chrome.alarms.onAlarm.addListener(info => {
-    if (info.name === 'syncNow') {
-      syncMan.syncNow();
+  chrome.alarms.onAlarm.addListener(async ({name}) => {
+    if (name === 'syncNow') {
+      await syncMan.syncNow({isScheduled: true});
+      const retrying = status.lockRetries / SYNC_LOCK_RETRIES * Math.random();
+      schedule(SYNC_DELAY + SYNC_INTERVAL * (retrying || 1));
     }
   });
 
@@ -139,7 +143,7 @@ const syncMan = (() => {
       emitStatusChange();
     },
 
-    async syncNow() {
+    async syncNow({isScheduled} = {}) {
       if (ready.then) await ready;
       if (!currentDrive || !status.login) {
         console.warn('cannot sync when disconnected');
@@ -152,11 +156,16 @@ const syncMan = (() => {
       } catch (err) {
         status.errorMessage = err.message;
         lastError = err;
-
+        if (isScheduled &&
+            err.code === 409 &&
+            ++status.lockRetries <= SYNC_LOCK_RETRIES) {
+          return;
+        }
         if (isGrantError(err)) {
           status.login = false;
         }
       }
+      status.lockRetries = 0;
       emitStatusChange();
     },
   };
@@ -247,8 +256,7 @@ const syncMan = (() => {
 
   function schedule(delay = SYNC_DELAY) {
     chrome.alarms.create('syncNow', {
-      delayInMinutes: delay,
-      periodInMinutes: SYNC_INTERVAL,
+      delayInMinutes: delay, // fractional values are supported
     });
   }