Fix the sync api returning an empty sync response when reaching timeout, regardless of the since token (#595)

* Fix the sync api returning an empty sync response when reaching timeout, regardless of the since token

* Declare syncData from the start to avoid declaration shadowing

* Bump Kafka version
Since this is a huge bump, it might completely blow up - I'll revert it if that's the case

* Put the loop and the IsEmpty() check back in sync handler

* Add doc and allow returning if timeout is reached

2 files changed, 19 insertions, 10 deletions

diff --git a/scripts/install-local-kafka.sh b/scripts/install-local-kafka.sh
index d1fef38e..19ce0911 100755
--- a/scripts/install-local-kafka.sh
+++ b/scripts/install-local-kafka.sh
@@ -9,9 +9,9 @@ cd `dirname $0`/..
 mkdir -p .downloads
 
 # The mirror to download kafka from is picked from the list of mirrors at
-# https://www.apache.org/dyn/closer.cgi?path=/kafka/0.10.2.0/kafka_2.11-0.11.0.2.tgz
+# https://www.apache.org/dyn/closer.cgi?path=/kafka/2.1.0/kafka_2.11-2.1.0.tgz
 # TODO: Check the signature since we are downloading over HTTP.
-MIRROR=http://apache.mirror.anlx.net/kafka/0.11.0.2/kafka_2.11-0.11.0.2.tgz
+MIRROR=http://apache.mirror.anlx.net/kafka/2.1.0/kafka_2.11-2.1.0.tgz
 
 # Only download the kafka if it isn't already downloaded.
 test -f .downloads/kafka.tgz || wget $MIRROR -O .downloads/kafka.tgz
diff --git a/src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go b/src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go
index 5c560ff5..89137eb5 100644
--- a/src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go
+++ b/src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go
@@ -45,6 +45,8 @@ func NewRequestPool(db *storage.SyncServerDatabase, n *Notifier, adb *accounts.D
 // called in a dedicated goroutine for this request. This function will block the goroutine
 // until a response is ready, or it times out.
 func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtypes.Device) util.JSONResponse {
+	var syncData *types.Response
+
 	// Extract values from request
 	logger := util.GetLogger(req.Context())
 	userID := device.UserID
@@ -65,7 +67,7 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 
 	// If this is an initial sync or timeout=0 we return immediately
 	if syncReq.since == nil || syncReq.timeout == 0 {
-		syncData, err := rp.currentSyncForUser(*syncReq, currPos)
+		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
 			return httputil.LogThenError(req, err)
 		}
@@ -84,6 +86,12 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	userStreamListener := rp.notifier.GetListener(*syncReq)
 	defer userStreamListener.Close()
 
+	// We need the loop in case userStreamListener wakes up even if there isn't
+	// anything to send down. In this case, we'll jump out of the select but
+	// don't want to send anything back until we get some actual content to
+	// respond with, so we skip the return an go back to waiting for content to
+	// be sent down or the request timing out.
+	var hasTimedOut bool
 	for {
 		select {
 		// Wait for notifier to wake us up
@@ -91,10 +99,11 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 			currPos = userStreamListener.GetStreamPosition()
 		// Or for timeout to expire
 		case <-timer.C:
-			return util.JSONResponse{
-				Code: http.StatusOK,
-				JSON: types.NewResponse(currPos),
-			}
+			// We just need to ensure we get out of the select after reaching the
+			// timeout, but there's nothing specific we want to do in this case
+			// apart from that, so we do nothing except stating we're timing out
+			// and need to respond.
+			hasTimedOut = true
 		// Or for the request to be cancelled
 		case <-req.Context().Done():
 			return httputil.LogThenError(req, req.Context().Err())
@@ -105,17 +114,17 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 		// of calculating the sync only to get timed out before we
 		// can respond
 
-		syncData, err := rp.currentSyncForUser(*syncReq, currPos)
+		syncData, err = rp.currentSyncForUser(*syncReq, currPos)
 		if err != nil {
 			return httputil.LogThenError(req, err)
 		}
-		if !syncData.IsEmpty() {
+
+		if !syncData.IsEmpty() || hasTimedOut {
 			return util.JSONResponse{
 				Code: http.StatusOK,
 				JSON: syncData,
 			}
 		}
-
 	}
 }