Optimise memory usage when calling /g_m_e (#1819)

* Optimise memory usage when calling /g_m_e

* cache more events

* refactor handling of device list update pokes

* Sigh

keyserver/internal/device_list_update.go

@@ -72,10 +72,8 @@ func init() {
 //     we guarantee we will get around to it. Also, more users on a given server does not increase the number of requests
 //     (as /keys/query allows multiple users to be specified) so being stuck behind matrix.org won't materially be any worse
 //     than being stuck behind foo.bar
-// In the event that the query fails, the worker spins up a short-lived goroutine whose sole purpose is to inject the server
-// name back into the channel after a certain amount of time. If in the interim the device lists have been updated, then
-// the database query will return no stale lists. Reinjection into the channel continues until success or the server terminates,
-// when it will be reloaded on startup.
+// In the event that the query fails, a lock is acquired and the server name along with the time to wait before retrying is
+// set in a map. A restarter goroutine periodically probes this map and injects servers which are ready to be retried.
 type DeviceListUpdater struct {
 	// A map from user_id to a mutex. Used when we are missing prev IDs so we don't make more than 1
 	// request to the remote server and race.
@@ -297,42 +295,38 @@ func (u *DeviceListUpdater) clearChannel(userID string) {
 }
 
 func (u *DeviceListUpdater) worker(ch chan gomatrixserverlib.ServerName) {
-	// It's possible to get many of the same server name in the channel, so in order
-	// to prevent processing the same server over and over we keep track of when we
-	// last made a request to the server. If we get the server name during the cooloff
-	// period, we'll ignore the poke.
-	lastProcessed := make(map[gomatrixserverlib.ServerName]time.Time)
-	// this can't be too long else sytest will give up trying to do a test
-	cooloffPeriod := 500 * time.Millisecond
-	shouldProcess := func(srv gomatrixserverlib.ServerName) bool {
-		// we should process requests when now is after the last process time + cooloff
-		return time.Now().After(lastProcessed[srv].Add(cooloffPeriod))
-	}
-
-	// on failure, spin up a short-lived goroutine to inject the server name again.
-	scheduledRetries := make(map[gomatrixserverlib.ServerName]time.Time)
-	inject := func(srv gomatrixserverlib.ServerName, duration time.Duration) {
-		time.Sleep(duration)
-		ch <- srv
-	}
-
-	for serverName := range ch {
-		if !shouldProcess(serverName) {
-			if time.Now().Before(scheduledRetries[serverName]) {
-				// do not inject into the channel as we know there will be a sleeping goroutine
-				// which will do it after the cooloff period expires
-				continue
-			} else {
-				scheduledRetries[serverName] = time.Now().Add(cooloffPeriod)
-				go inject(serverName, cooloffPeriod)
-				continue
+	retries := make(map[gomatrixserverlib.ServerName]time.Time)
+	retriesMu := &sync.Mutex{}
+	// restarter goroutine which will inject failed servers into ch when it is time
+	go func() {
+		for {
+			var serversToRetry []gomatrixserverlib.ServerName
+			time.Sleep(time.Second)
+			retriesMu.Lock()
+			now := time.Now()
+			for srv, retryAt := range retries {
+				if now.After(retryAt) {
+					serversToRetry = append(serversToRetry, srv)
+				}
+			}
+			for _, srv := range serversToRetry {
+				delete(retries, srv)
+			}
+			retriesMu.Unlock()
+			for _, srv := range serversToRetry {
+				ch <- srv
 			}
 		}
-		lastProcessed[serverName] = time.Now()
+	}()
+	for serverName := range ch {
 		waitTime, shouldRetry := u.processServer(serverName)
 		if shouldRetry {
-			scheduledRetries[serverName] = time.Now().Add(waitTime)
-			go inject(serverName, waitTime)
+			retriesMu.Lock()
+			_, exists := retries[serverName]
+			if !exists {
+				retries[serverName] = time.Now().Add(waitTime)
+			}
+			retriesMu.Unlock()
 		}
 	}
 }
@@ -380,7 +374,7 @@ func (u *DeviceListUpdater) processServer(serverName gomatrixserverlib.ServerNam
 		}
 	}
 	if failCount > 0 {
-		logger.WithField("total", len(userIDs)).WithField("failed", failCount).Error("failed to query device keys for some users")
+		logger.WithField("total", len(userIDs)).WithField("failed", failCount).WithField("wait", waitTime).Error("failed to query device keys for some users")
 	}
 	for _, userID := range userIDs {
 		// always clear the channel to unblock Update calls regardless of success/failure