Connection::syncLoop: give a pause between syncs

As it's observed now, Synapse responds almost immediately on /sync
requests - even if there are no events to return. This downgrades
long-polling to simply polling, and since clients don't expect it,
polling loops become pretty violent. To alleviate that somehow,
syncLoop now accepts the second parameter, msecBetween (500 msecs
by default), to configure waiting between the previous sync response
and the next sync request. This is only for syncLoop();
Connection::sync() fires instantly, as before.

2 files changed, 9 insertions, 6 deletions

diff --git a/lib/connection.cpp b/lib/connection.cpp
index 50e31d52..d53c308f 100644
--- a/lib/connection.cpp
+++ b/lib/connection.cpp
@@ -57,6 +57,7 @@
 #include <QtCore/QElapsedTimer>
 #include <QtCore/QFile>
 #include <QtCore/QMimeDatabase>
+#include <QtCore/QTimer>
 #include <QtCore/QRegularExpression>
 #include <QtCore/QStandardPaths>
 #include <QtCore/QStringBuilder>
@@ -506,21 +507,23 @@ void Connection::sync(int timeout)
     });
 }
 
-void Connection::syncLoop(int timeout)
+void Connection::syncLoop(int timeout, int msecBetween)
 {
     if (d->syncLoopConnection && d->syncTimeout == timeout) {
         qCInfo(MAIN) << "Attempt to run sync loop but there's one already "
                         "running; nothing will be done";
         return;
     }
-    std::swap(d->syncTimeout, timeout);
+    std::swap(d->syncTimeout, timeout); // swap() is for the nice log below
     if (d->syncLoopConnection) {
         qCInfo(MAIN) << "Timeout for next syncs changed from"
                         << timeout << "to" << d->syncTimeout;
     } else {
-        d->syncLoopConnection = connect(this, &Connection::syncDone,
-                                        this, &Connection::syncLoopIteration,
-                                        Qt::QueuedConnection);
+        d->syncLoopConnection =
+            connect(this, &Connection::syncDone, this, [this, msecBetween] {
+                QTimer::singleShot(msecBetween, this,
+                                   &Connection::syncLoopIteration);
+            });
         syncLoopIteration(); // initial sync to start the loop
     }
 }
diff --git a/lib/connection.h b/lib/connection.h
index d5fa9eac..fb76cf3d 100644
--- a/lib/connection.h
+++ b/lib/connection.h
@@ -544,7 +544,7 @@ public slots:
     void logout();
 
     void sync(int timeout = -1);
-    void syncLoop(int timeout = -1);
+    void syncLoop(int timeout = -1, int msecBetween = 500);
 
     void stopSync();
     QString nextBatchToken() const;