Fix "Can't replace active reader". Fixes #1469

If an exception happens during MySqlConnection.Close(), the connection may be left in an invalid state, thinking it still has an open MySqlDataReader. Clear this field during cleanup to remove this invalid state.

src/MySqlConnector/MySqlConnection.cs

@@ -1248,6 +1248,9 @@ private async Task DoCloseAsync(bool changeState, IOBehavior ioBehavior)
 		{
 			if (m_session is not null)
 			{
+				// under exceptional circumstances, we may enter this code without having called FinishQuerying, which clears this field
+				m_activeReader = null;
+
 				if (GetInitializedConnectionSettings().Pooling)
 				{
 					await m_session.ReturnToPoolAsync(ioBehavior, this).ConfigureAwait(false);

tests/MySqlConnector.Tests/ConnectionTests.cs

@@ -234,6 +234,49 @@ public void ReadInfinity()
 		Assert.False(reader.Read());
 	}
 
+	[Fact]
+	public void ReplaceActiveReader()
+	{
+		var connection = new MySqlConnection(m_csb.ConnectionString);
+		connection.Open();
+		using (var command = connection.CreateCommand())
+		{
+			command.CommandText = "select disconnect";
+			command.CommandTimeout = 600;
+			var reader = command.ExecuteReader();
+			Assert.True(reader.Read());
+			Assert.Equal(1, reader.GetInt32(0));
+		}
+
+		try
+		{
+			connection.Close();
+		}
+		catch (MySqlEndOfStreamException)
+		{
+		}
+
+		connection.Open();
+		using (var command = connection.CreateCommand())
+		{
+			command.CommandText = "SELECT 1;";
+			var reader = command.ExecuteReader();
+			Assert.True(reader.Read());
+			Assert.Equal(1, reader.GetInt32(0));
+		}
+		connection.Close();
+
+		connection.Open();
+		using (var command = connection.CreateCommand())
+		{
+			command.CommandText = "SELECT 2;";
+			var reader = command.ExecuteReader();
+			Assert.True(reader.Read());
+			Assert.Equal(2, reader.GetInt32(0));
+		}
+		connection.Close();
+	}
+
 	private static async Task WaitForConditionAsync<T>(T expected, Func<T> getValue)
 	{
 		var sw = Stopwatch.StartNew();

tests/MySqlConnector.Tests/FakeMySqlServerConnection.cs

@@ -179,6 +179,20 @@ public async Task RunAsync(TcpClient client, CancellationToken token)
 								for (var packetIndex = 0; packetIndex < packets.Length; packetIndex++)
 									await SendAsync(stream, packetIndex + 1, x => x.Write(packets[packetIndex]));
 							}
+							else if (query == "select disconnect")
+							{
+								var packets = new[]
+								{
+								new byte[] { 1 }, // one column
+								[3, 0x64, 0x65, 0x66, 0, 0, 0, 1, 0x41, 0, 0x0c, 0x3f, 0, 1, 0, 0, 0, 3, 0x01, 0, 0x1F, 0, 0], // column definition (int)
+								[0xFE, 0, 0, 2, 0], // EOF
+								[1, 0x31], // 1
+							};
+								for (var packetIndex = 0; packetIndex < packets.Length; packetIndex++)
+									await SendAsync(stream, packetIndex + 1, x => x.Write(packets[packetIndex]));
+								stream.Close();
+								client.Close();
+							}
 							else
 							{
 								await SendAsync(stream, 1, x => WriteError(x, "Unhandled query: " + query));