Fix the null reference exception

LinuxCommunicator/HostsFile/HostsFileManager.cs

@@ -110,13 +110,13 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.HostsFile
                 FileInfo fileInfo = new FileInfo(this.filepath);
                 if (!fileInfo.Exists)
                 {
-                    LinuxCommunicator.Instance.Tracer.TraceInfo("[HostsFileManager] The hosts file doesn't exists: {0}", this.filepath);
+                    LinuxCommunicator.Instance?.Tracer?.TraceInfo("[HostsFileManager] The hosts file doesn't exists: {0}", this.filepath);
                     return;
                 }
 
                 if (fileInfo.LastWriteTimeUtc <= this.lastModified)
                 {
-                    LinuxCommunicator.Instance.Tracer.TraceInfo("[HostsFileManager] The hosts file isn't changed since last load");
+                    LinuxCommunicator.Instance?.Tracer?.TraceInfo("[HostsFileManager] The hosts file isn't changed since last load");
                     return;
                 }
 
@@ -160,16 +160,16 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.HostsFile
                         newHostList.AddRange(newEntries.Values.Where(entry => entry.Name.Contains('.')));
                         this.ManagedEntries = newHostList;
                         this.UpdateId = Guid.NewGuid();
-                        LinuxCommunicator.Instance.Tracer.TraceInfo("[HostsFileManager] The managed host entries updated, current update Id is {0}", this.UpdateId);
+                        LinuxCommunicator.Instance?.Tracer?.TraceInfo("[HostsFileManager] The managed host entries updated, current update Id is {0}", this.UpdateId);
                     }
                     else
                     {
-                        LinuxCommunicator.Instance.Tracer.TraceInfo("[HostsFileManager] No update to HPC managed host entries, current update Id is {0}", this.UpdateId);
+                        LinuxCommunicator.Instance?.Tracer?.TraceInfo("[HostsFileManager] No update to HPC managed host entries, current update Id is {0}", this.UpdateId);
                     }
                 }
                 else
                 {
-                    LinuxCommunicator.Instance.Tracer.TraceWarning("[HostsFileManager] Hosts file was not managed by HPC");
+                    LinuxCommunicator.Instance?.Tracer?.TraceWarning("[HostsFileManager] Hosts file was not managed by HPC");
                     this.ManagedEntries.Clear();
                 }
 
@@ -178,7 +178,7 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.HostsFile
             }
             catch (Exception e)
             {
-                LinuxCommunicator.Instance.Tracer.TraceWarning("[HostsFileManager] Failed to reload host file: {0}", e);
+                LinuxCommunicator.Instance?.Tracer?.TraceWarning("[HostsFileManager] Failed to reload host file: {0}", e);
             }
             finally
             {
@@ -188,7 +188,7 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.HostsFile
                 }
                 catch (Exception te)
                 {
-                    LinuxCommunicator.Instance.Tracer.TraceWarning("[HostsFileManager] Failed to restart reload timer: {0}", te);
+                    LinuxCommunicator.Instance?.Tracer?.TraceWarning("[HostsFileManager] Failed to restart reload timer: {0}", te);
                 }
             }
         }

LinuxCommunicator/LinuxCommunicator.cs

@@ -69,9 +69,9 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator
 
         public void Dispose()
         {
-            this.server.Dispose();
-            this.MonitoringConfigManager.Dispose();
-            this.HostsManager.Dispose();
+            this.server?.Dispose();
+            this.MonitoringConfigManager?.Dispose();
+            this.HostsManager?.Dispose();
             GC.SuppressFinalize(this);
         }
 
@@ -127,8 +127,8 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator
         {
             this.Tracer.TraceInfo("Initializing LinuxCommunicator.");
 
-            this.MonitoringConfigManager = new MonitoringConfigManager();
-            Task.Run(() => this.MonitoringConfigManager.Initialize(this.headNodeFqdn.Value));
+            this.MonitoringConfigManager = new MonitoringConfigManager(this.headNodeFqdn.Value);
+            Task.Run(() => this.MonitoringConfigManager.Initialize());
             this.HostsManager = new HostsFileManager();
 
             ServicePointManager.ServerCertificateValidationCallback += (s, cert, chain, sslPolicyErrors) =>
@@ -162,7 +162,7 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator
 
         public bool Start()
         {
-            this.MonitoringConfigManager.Start();
+            this.MonitoringConfigManager?.Start();
             return this.Start(0);
         }
 
@@ -177,12 +177,12 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator
                 return false;
             }
 
-            if (this.cancellationTokenSource != null) { this.cancellationTokenSource.Dispose(); }
+            this.cancellationTokenSource?.Dispose();
             this.cancellationTokenSource = new CancellationTokenSource();
 
             try
             {
-                this.server.Start().Wait();
+                this.server?.Start().Wait();
             }
             catch (AggregateException aggrEx)
             {
@@ -205,11 +205,11 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator
 
         public bool Stop()
         {
-            this.Tracer.TraceInfo("Stopping LinuxCommunicator.");
-            this.server.Stop();
-            this.MonitoringConfigManager.Stop();
-            this.cancellationTokenSource.Cancel();
-            this.cancellationTokenSource.Dispose();
+            this.Tracer?.TraceInfo("Stopping LinuxCommunicator.");
+            this.server?.Stop();
+            this.MonitoringConfigManager?.Stop();
+            this.cancellationTokenSource?.Cancel();
+            this.cancellationTokenSource?.Dispose();
             this.cancellationTokenSource = null;
             return true;
         }

LinuxCommunicator/Monitoring/MonitoringConfigManager.cs

@@ -7,6 +7,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using System.Timers;
 using Microsoft.Hpc.Monitoring;
+using System.Net.Sockets;
 
 namespace Microsoft.Hpc.Communicators.LinuxCommunicator.Monitoring
 {
@@ -64,21 +65,27 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.Monitoring
         };
 
         private MetricCountersConfig metricCountersConfig = new MetricCountersConfig();
+        private string server;
 
-        public MonitoringConfigManager() { }
+        public MonitoringConfigManager(string server)
+        {
+            this.server = server;
+            this.checkConfigTimer.AutoReset = true;
+            this.checkConfigTimer.Interval = 5 * 60 * 1000;
+            this.checkConfigTimer.Enabled = false;
+            this.checkConfigTimer.Elapsed += this.checkConfigTimer_Elapsed;
+        }
 
-        public void Initialize(string server)
+
+        public void Initialize()
         {
             RetryManager rm = new RetryManager(new PeriodicRetryTimer(30 * 1000));
             while (true)
             {
                 try
                 {
-                    this.Store = MonitoringStoreConnection.Connect(server, "LinuxCommunicator");
-                    this.checkConfigTimer_Elapsed(this, null);
-                    this.checkConfigTimer.AutoReset = true;
-                    this.checkConfigTimer.Interval = 5 * 60 * 1000;
-                    this.checkConfigTimer.Elapsed += checkConfigTimer_Elapsed;
+                    this.Store = MonitoringStoreConnection.Connect(this.server, "LinuxCommunicator");
+                    this.CheckConfig();
                     break;
                 }
                 catch (Exception e)
@@ -123,6 +130,21 @@ namespace Microsoft.Hpc.Communicators.LinuxCommunicator.Monitoring
         }
 
         private void checkConfigTimer_Elapsed(object sender, ElapsedEventArgs e)
+        {
+            try
+            {
+                this.CheckConfig();
+            }
+            catch (SocketException ex)
+            {
+                this.Stop();
+                LinuxCommunicator.Instance?.Tracer?.TraceException(ex);
+                this.Initialize();
+                this.Start();
+            }
+        }
+
+        private void CheckConfig()
         {
             var metrics = this.Store.GetMetrics(MetricTarget.ComputeNode);
             if (this.currentConfig.UpdateWhenChanged(metrics))