Synchronize the background scan and closing of connections

- It was possible for the application to be torn down during
a background scan. When that happened, the timer would be disposed
before the end of the scan and would throw an ObjectDisposedException
when timer resumption happened. This change introduces a lock that avoids that
race.

Fixes #161

src/Microsoft.AspNetCore.Sockets/ConnectionManager.cs

@@ -17,6 +17,8 @@ namespace Microsoft.AspNetCore.Sockets
         private readonly ConcurrentDictionary<string, ConnectionState> _connections = new ConcurrentDictionary<string, ConnectionState>();
         private Timer _timer;
         private readonly ILogger<ConnectionManager> _logger;
+        private object _executionLock = new object();
+        private bool _disposed;
 
         public ConnectionManager(ILogger<ConnectionManager> logger)
         {
@@ -25,11 +27,19 @@ namespace Microsoft.AspNetCore.Sockets
 
         public void Start()
         {
+            lock (_executionLock)
+            {
+                if (_disposed)
+                {
+                    return;
+                }
+
                 if (_timer == null)
                 {
                     _timer = new Timer(Scan, this, TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(1));
                 }
             }
+        }
 
         public bool TryGetConnection(string id, out ConnectionState state)
         {
@@ -77,6 +87,13 @@ namespace Microsoft.AspNetCore.Sockets
 
         private void Scan()
         {
+            lock (_executionLock)
+            {
+                if (_disposed)
+                {
+                    return;
+                }
+
                 // Pause the timer while we're running
                 _timer.Change(Timeout.Infinite, Timeout.Infinite);
 
@@ -115,9 +132,19 @@ namespace Microsoft.AspNetCore.Sockets
                     _timer.Change(TimeSpan.FromSeconds(1), TimeSpan.FromSeconds(1));
                 }
             }
+        }
 
         public void CloseConnections()
         {
+            lock (_executionLock)
+            {
+                if (_disposed)
+                {
+                    return;
+                }
+
+                _disposed = true;
+
                 // Stop firing the timer
                 _timer?.Dispose();
 
@@ -130,6 +157,7 @@ namespace Microsoft.AspNetCore.Sockets
 
                 Task.WaitAll(tasks.ToArray(), TimeSpan.FromSeconds(5));
             }
+        }
 
         public async Task DisposeAndRemoveAsync(ConnectionState state)
         {