Avoid posting unnecessary async tasks

Former-commit-id: 08f63fdfd6c78388bba093ec2edda7d77fc9173e

src/gc.h

@@ -49,6 +49,12 @@ public:
         m_setepochOutstanding.clear();
     }
 
+    bool empty() const
+    {
+        std::unique_lock<fastlock> lock(m_lock);
+        return m_vecepochs.empty();
+    }
+
     void endEpoch(uint64_t epoch, bool fNoFree = false)
     {
         std::unique_lock<fastlock> lock(m_lock);
@@ -109,7 +115,7 @@ public:
     }
 
 private:
-    fastlock m_lock { "Garbage Collector"};
+    mutable fastlock m_lock { "Garbage Collector"};
 
     std::vector<EpochHolder> m_vecepochs;
     std::unordered_set<uint64_t> m_setepochOutstanding;

src/server.cpp

@@ -2441,13 +2441,15 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
     ProcessPendingAsyncWrites();
 
     run_with_period(10) {
-        // Server threads don't free the GC, but if we don't have a
-        //  a bgsave or some other async task then we'll hold onto the
-        //  data for too long
-        g_pserver->asyncworkqueue->AddWorkFunction([]{
-            auto epoch = g_pserver->garbageCollector.startEpoch();
-            g_pserver->garbageCollector.endEpoch(epoch);
-        });
+        if (!g_pserver->garbageCollector.empty()) {
+            // Server threads don't free the GC, but if we don't have a
+            //  a bgsave or some other async task then we'll hold onto the
+            //  data for too long
+            g_pserver->asyncworkqueue->AddWorkFunction([]{
+                auto epoch = g_pserver->garbageCollector.startEpoch();
+                g_pserver->garbageCollector.endEpoch(epoch);
+            });
+        }
     }
 
     g_pserver->cronloops++;

src/server.h

@@ -1852,6 +1852,11 @@ public:
         garbageCollectorGeneric.endEpoch(epochGeneric, fNoFree);
     }
 
+    bool empty()
+    {
+        return garbageCollectorGeneric.empty() && garbageCollectorSnapshot.empty();
+    }
+
     void shutdown()
     {
         garbageCollectorSnapshot.shutdown();