ioc: ensure db_cancel_event() before ~MonitorControlOp

previously, ~SingleSourceSubscriptionCtx would ~MonitorControlOp
before the base class ~SubscriptionCtx would db_cancel_event()
leading to a use-after-free.

ioc/groupsrcsubscriptionctx.h

@@ -37,6 +37,7 @@ public:
     // This is so that we can merge this with the subsequent values that come in before all initial events are in
     Value currentValue;
 
+    // must db_cancel_event() before ~MonitorControlOp
     std::vector<FieldSubscriptionCtx> fieldSubscriptionContexts{};
     explicit GroupSourceSubscriptionCtx(Group& subscribedGroup)
             :group(subscribedGroup), currentValue(subscribedGroup.valueTemplate.cloneEmpty()) {

ioc/singlesrcsubscriptionctx.h

@@ -51,6 +51,8 @@ public:
 
     ~SingleSourceSubscriptionCtx() {
         assert(!eventsEnabled);
+        // must db_cancel_event() before ~MonitorControlOp
+        cancel();
     }
 };
 

ioc/subscriptionctx.h

@@ -45,6 +45,9 @@ public:
         if(!sub)
             throw std::runtime_error("Failed to create db subscription");
     }
+    void cancel() {
+        sub.reset();
+    }
     void enable() {
         if(sub) {
             db_event_enable(sub.get());
@@ -67,6 +70,10 @@ public:
     Subscription pPropertiesEventSubscription;
     bool hadValueEvent = false;
     bool hadPropertyEvent = false;
+    void cancel() {
+        pValueEventSubscription.cancel();
+        pPropertiesEventSubscription.cancel();
+    }
 };
 
 } // ioc