Adding async method (#235)

Adding an OpenAsync method to the ReliableConnection to allow it to properly utilize the cancellation token support of the SqlConnection. It eliminates the need for a "cancellation thread" that has a high likelihood of throwing uncaught exceptions (as per Microsoft/vscode-mssql/issues/547)

src/Microsoft.SqlTools.ServiceLayer/Connection/ReliableConnection/ReliableSqlConnection.cs

@@ -31,6 +31,8 @@ using System.Data.SqlClient;
 using System.Diagnostics;
 using System.Globalization;
 using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
 using Microsoft.SqlTools.ServiceLayer.Utility;
 
 namespace Microsoft.SqlTools.ServiceLayer.Connection.ReliableConnection
@@ -260,7 +262,47 @@ SET NUMERIC_ROUNDABORT OFF;";
         /// </summary>
         public override void Open()
         {
-            OpenConnection();
+            // Check if retry policy was specified, if not, disable retries by executing the Open method using RetryPolicy.NoRetry.
+            _connectionRetryPolicy.ExecuteAction(() =>
+            {
+                if (_underlyingConnection.State != ConnectionState.Open)
+                {
+                    _underlyingConnection.Open();
+                }
+                SetLockAndCommandTimeout(_underlyingConnection);
+                SetDefaultAnsiSettings(_underlyingConnection, IsSqlDwConnection(_underlyingConnection));
+            });
+        }
+
+        /// <summary>
+        /// Opens a database connection with the settings specified by the ConnectionString
+        /// property of the provider-specific Connection object.
+        /// </summary>
+        public override Task OpenAsync(CancellationToken token)
+        {
+            // Make sure that the token isn't cancelled before we try
+            if (token.IsCancellationRequested)
+            {
+                return Task.FromCanceled(token);
+            }
+
+            // Check if retry policy was specified, if not, disable retries by executing the Open method using RetryPolicy.NoRetry.
+            try
+            {
+                return _connectionRetryPolicy.ExecuteAction(async () =>
+                {
+                    if (_underlyingConnection.State != ConnectionState.Open)
+                    {
+                        await _underlyingConnection.OpenAsync(token);
+                    }
+                    SetLockAndCommandTimeout(_underlyingConnection);
+                    SetDefaultAnsiSettings(_underlyingConnection, IsSqlDwConnection(_underlyingConnection));
+                });
+            }
+            catch (Exception e)
+            {
+                return Task.FromException(e);
+            }
         }
 
         /// <summary>
@@ -356,26 +398,6 @@ SET NUMERIC_ROUNDABORT OFF;";
             RetryPolicyUtils.RaiseSchemaAmbientRetryMessage(retryState, SqlSchemaModelErrorCodes.ServiceActions.ConnectionRetry, _azureSessionId); 
         }
 
-        /// <summary>
-        /// Opens a database connection with the settings specified by the ConnectionString and ConnectionRetryPolicy properties.
-        /// </summary>
-        /// <returns>An object representing the open connection.</returns>
-        private SqlConnection OpenConnection()
-        {
-            // Check if retry policy was specified, if not, disable retries by executing the Open method using RetryPolicy.NoRetry.
-            _connectionRetryPolicy.ExecuteAction(() =>
-            {
-                if (_underlyingConnection.State != ConnectionState.Open)
-                {
-                    _underlyingConnection.Open();
-                }
-                SetLockAndCommandTimeout(_underlyingConnection);
-                SetDefaultAnsiSettings(_underlyingConnection, IsSqlDwConnection(_underlyingConnection));
-            });
-
-            return _underlyingConnection;
-        }
-
         public void OnConnectionStateChange(object sender, StateChangeEventArgs e)
         {
             SqlConnection conn = (SqlConnection)sender;