CSHARP-1861: Fix NullReferenceException when using X509 authentication.

Author: rstam

src/MongoDB.Driver.Core/Core/Authentication/MongoDBX509Authenticator.cs

@@ -67,7 +67,7 @@ public void Authenticate(IConnection connection, ConnectionDescription descripti
         {
             Ensure.IsNotNull(connection, nameof(connection));
             Ensure.IsNotNull(description, nameof(description));
-            EnsureUsernameIsNotNullOrNullIsSupported(connection);
+            EnsureUsernameIsNotNullOrNullIsSupported(connection, description);
 
             try
             {
@@ -85,7 +85,7 @@ public async Task AuthenticateAsync(IConnection connection, ConnectionDescriptio
         {
             Ensure.IsNotNull(connection, nameof(connection));
             Ensure.IsNotNull(description, nameof(description));
-            EnsureUsernameIsNotNullOrNullIsSupported(connection);
+            EnsureUsernameIsNotNullOrNullIsSupported(connection, description);
 
             try
             {
@@ -124,9 +124,9 @@ private MongoAuthenticationException CreateException(IConnection connection, Exc
             return new MongoAuthenticationException(connection.ConnectionId, message, ex);
         }
 
-        private void EnsureUsernameIsNotNullOrNullIsSupported(IConnection connection)
+        private void EnsureUsernameIsNotNullOrNullIsSupported(IConnection connection, ConnectionDescription description)
         {
-            var serverVersion = connection.Description.ServerVersion;
+            var serverVersion = description.ServerVersion;
             if (_username == null && !Feature.ServerExtractsUsernameFromX509Certificate.IsSupported(serverVersion))
             {
                 var message = $"Username cannot be null for server version {serverVersion}.";

src/MongoDB.Driver.Core/Core/Misc/Feature.cs

@@ -50,7 +50,7 @@ public class Feature
         private static readonly Feature __partialIndexes = new Feature("PartialIndexes", new SemanticVersion(3, 2, 0));
         private static readonly ReadConcernFeature __readConcern = new ReadConcernFeature("ReadConcern", new SemanticVersion(3, 2, 0));
         private static readonly Feature __scramSha1Authentication = new Feature("ScramSha1Authentication", new SemanticVersion(3, 0, 0));
-        private static readonly Feature __serverExtractsUsernameFromX509Certificate = new Feature("ServerExtractsUsernameFromX509Certificate", new SemanticVersion(3, 4, 0));
+        private static readonly Feature __serverExtractsUsernameFromX509Certificate = new Feature("ServerExtractsUsernameFromX509Certificate", new SemanticVersion(3, 3, 12));
         private static readonly Feature __userManagementCommands = new Feature("UserManagementCommands", new SemanticVersion(2, 6, 0));
         private static readonly Feature __views = new Feature("Views", new SemanticVersion(3, 3, 11));
         private static readonly Feature __writeCommands = new Feature("WriteCommands", new SemanticVersion(2, 6, 0));

src/MongoDB.Driver/MongoIdentity.cs

@@ -130,7 +130,7 @@ public bool Equals(MongoIdentity rhs)
         public override int GetHashCode()
         {
             var hash = 17;
-            hash += 37 * _username.GetHashCode();
+            hash += 37 * (_username == null ? 0 :_username.GetHashCode());
             hash += 37 * _source.GetHashCode();
             return hash;
         }

tests/MongoDB.Driver.Core.Tests/Core/Authentication/MongoDBX509AuthenticatorTests.cs

@@ -142,17 +142,17 @@ public void Authenticate_should_not_throw_when_username_is_null_and_server_suppo
                 RawBsonDocumentHelper.FromJson("{ok: 1}"));
 
             var connection = new MockConnection(__serverId);
-            connection.Description = CreateConnectionDescription(new SemanticVersion(3, 4, 0));
             connection.EnqueueReplyMessage(reply);
+            var description = CreateConnectionDescription(new SemanticVersion(3, 4, 0));
 
             Exception exception;
             if (async)
             {
-                exception = Record.Exception(() => subject.AuthenticateAsync(connection, __description, CancellationToken.None).GetAwaiter().GetResult());
+                exception = Record.Exception(() => subject.AuthenticateAsync(connection, description, CancellationToken.None).GetAwaiter().GetResult());
             }
             else
             {
-                exception = Record.Exception(() => subject.Authenticate(connection, __description, CancellationToken.None));
+                exception = Record.Exception(() => subject.Authenticate(connection, description, CancellationToken.None));
             }
 
             exception.Should().BeNull();