CLOUDSTACK-10136: Fix RemoteHostEndPoint thread growth

This fixes the following:
- Unchecked thread growth in RemoteEndHostEndPoint
- Potential NPE while finding EP for a storage/scope

Unbounded thread growth can be reproduced with following findings:
- Every unreachable template would produce 6 new threads (in a single
ScheduledExecutorService instance) spaced by 10 seconds
- Every reachable template url without the template would produce 1 new
thread (and one ScheduledExecutorService instance), it errors out quickly without
causing more thread growth.
- Every valid url will produce upto 10 threads as the same ep (endpoint
instance) will be reused to query upload/download (async callback)
progresses.

Every RemoteHostEndPoint instances creates its own
ScheduledExecutorService instance which is why in the jstack dump, we
see several threads that share the prefix RemoteHostEndPoint-{1..10}
(given poolsize is defined as 10, it uses suffixes 1-10).

This fixes the discovered thread leakage with following notes:
- Instead of ScheduledExecutorService instance, a cached pool could be
used instead and was implemented, and with `static` scope to be reused
among other future RemoteHostEndPoint instances.
- It was not clear why we would want to wait when we've Answers returned
from the remote EP, and therefore a scheduled/delayed Runnable was
not required at all for processing answers. ScheduledExecutorService
was therefore not really required, moved to ExecutorService instead.
- Another benefit of using a cached pool is that it will shutdown
threads if they are not used in 60 seconds, and they get re-used for
future runnable submissions.
- Caveat: the executor service is still unbounded, however, the use-case
that this method is used for short jobs to check upload/download
progresses fits the case here.
- Refactored CmdRunner to not use/reference objects from parent class.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

engine/storage/src/org/apache/cloudstack/storage/RemoteHostEndPoint.java

@@ -18,17 +18,15 @@
  */
 package org.apache.cloudstack.storage;
 
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
-import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.TimeUnit;
 
 import javax.inject.Inject;
 
-import org.apache.log4j.Logger;
-
 import org.apache.cloudstack.engine.subsystem.api.storage.EndPoint;
 import org.apache.cloudstack.framework.async.AsyncCompletionCallback;
 import org.apache.cloudstack.managed.context.ManagedContextRunnable;
+import org.apache.log4j.Logger;
 
 import com.cloud.agent.AgentManager;
 import com.cloud.agent.Listener;
@@ -54,9 +52,11 @@ import com.cloud.vm.dao.SecondaryStorageVmDao;
 
 public class RemoteHostEndPoint implements EndPoint {
     private static final Logger s_logger = Logger.getLogger(RemoteHostEndPoint.class);
+
     private long hostId;
     private String hostAddress;
     private String publicAddress;
+
     @Inject
     AgentManager agentMgr;
     @Inject
@@ -65,10 +65,10 @@ public class RemoteHostEndPoint implements EndPoint {
     protected SecondaryStorageVmDao vmDao;
     @Inject
     protected HostDao _hostDao;
-    private ScheduledExecutorService executor;
+
+    private static ExecutorService executorService = Executors.newCachedThreadPool(new NamedThreadFactory("RemoteHostEndPoint"));
 
     public RemoteHostEndPoint() {
-        executor = Executors.newScheduledThreadPool(10, new NamedThreadFactory("RemoteHostEndPoint"));
     }
 
     private void configure(Host host) {
@@ -134,17 +134,17 @@ public class RemoteHostEndPoint implements EndPoint {
     }
 
     private class CmdRunner extends ManagedContextRunnable implements Listener {
-        final AsyncCompletionCallback<Answer> callback;
-        Answer answer;
+        private final AsyncCompletionCallback<Answer> callback;
+        private Answer answer;
 
-        public CmdRunner(AsyncCompletionCallback<Answer> callback) {
+        CmdRunner(final AsyncCompletionCallback<Answer> callback) {
             this.callback = callback;
         }
 
         @Override
         public boolean processAnswers(long agentId, long seq, Answer[] answers) {
-            answer = answers[0];
-            executor.schedule(this, 10, TimeUnit.SECONDS);
+            this.answer = answers[0];
+            RemoteHostEndPoint.executorService.submit(this);
             return true;
         }
 
@@ -204,7 +204,7 @@ public class RemoteHostEndPoint implements EndPoint {
 
         @Override
         protected void runInContext() {
-            callback.complete(answer);
+            this.callback.complete(this.answer);
         }
     }
 

engine/storage/src/org/apache/cloudstack/storage/endpoint/DefaultEndPointSelector.java

@@ -106,16 +106,19 @@ public class DefaultEndPointSelector implements EndPointSelector {
         StringBuilder sbuilder = new StringBuilder();
         sbuilder.append(sqlBase);
 
-        if (scope.getScopeType() == ScopeType.HOST) {
-            sbuilder.append(" and h.id = ");
-            sbuilder.append(scope.getScopeId());
-        } else if (scope.getScopeType() == ScopeType.CLUSTER) {
-            sbuilder.append(" and h.cluster_id = ");
-            sbuilder.append(scope.getScopeId());
-        } else if (scope.getScopeType() == ScopeType.ZONE) {
-            sbuilder.append(" and h.data_center_id = ");
-            sbuilder.append(scope.getScopeId());
+        if (scope != null) {
+            if (scope.getScopeType() == ScopeType.HOST) {
+                sbuilder.append(" and h.id = ");
+                sbuilder.append(scope.getScopeId());
+            } else if (scope.getScopeType() == ScopeType.CLUSTER) {
+                sbuilder.append(" and h.cluster_id = ");
+                sbuilder.append(scope.getScopeId());
+            } else if (scope.getScopeType() == ScopeType.ZONE) {
+                sbuilder.append(" and h.data_center_id = ");
+                sbuilder.append(scope.getScopeId());
+            }
         }
+
         // TODO: order by rand() is slow if there are lot of hosts
         sbuilder.append(") t where t.value<>'true' or t.value is null");    //Added for exclude cluster's subquery
         sbuilder.append(" ORDER by rand() limit 1");