CSHARP-2822: StartTransaction throws for Standalone and Unknown servers

Author: tmasternak

src/MongoDB.Driver.Core/Core/Bindings/CoreSession.cs

@@ -467,7 +467,13 @@ private void EnsureTransactionsAreSupported()
 
             foreach (var connectedDataBearingServer in connectedDataBearingServers)
             {
-                if (connectedDataBearingServer.Type == ServerType.ShardRouter)
+                var serverType = connectedDataBearingServer.Type;
+
+                if(serverType == ServerType.Standalone || serverType == ServerType.Unknown)
+                {
+                    throw new NotSupportedException("Transactions are supported only in sharded cluster of replica set deployments.");
+                }
+                else if (serverType == ServerType.ShardRouter)
                 {
                     Feature.ShardedTransactions.ThrowIfNotSupported(connectedDataBearingServer.Version);
                 }

tests/MongoDB.Driver.Core.Tests/Core/Bindings/CoreSessionTests.cs

@@ -325,6 +325,8 @@ public void EnsureTransactionsAreSupported_should_throw_when_there_are_no_connec
         }
 
         [Theory]
+        [InlineData("NT")]
+        [InlineData("UT")]
         [InlineData("PN")]
         [InlineData("PN,ST")]
         [InlineData("PT,SN")]
@@ -358,7 +360,11 @@ public void EnsureTransactionsAreSupported_should_throw_when_any_connected_data_
             var exception = Record.Exception(() => subject.EnsureTransactionsAreSupported());
 
             var e = exception.Should().BeOfType<NotSupportedException>().Subject;
-            e.Message.Should().Contain($"does not support the {unsupportedFeatureName} feature.");
+            e.Message.Should().Match<string>(
+                s => s.Contains($"does not support the {unsupportedFeatureName} feature.") || 
+                     s.Contains("Transactions are supported only in sharded cluster of replica set deployments.") ||
+                     s.Contains("StartTransaction cannot determine if transactions are supported because there are no connected servers.")
+            );
         }
 
         // private methods
@@ -446,6 +452,7 @@ private ServerType MapServerTypeCode(char code)
             switch (code)
             {
                 case 'A': return ServerType.ReplicaSetArbiter;
+                case 'N': return ServerType.Standalone;
                 case 'P': return ServerType.ReplicaSetPrimary;
                 case 'R': return ServerType.ShardRouter;
                 case 'S': return ServerType.ReplicaSetSecondary;