Handle max_allowed_packet client-side

[bgrainger]

If a packet that is larger than max_allowed_packet (a server-defined variable) is sent, the server will reset the connection and send an error packet with the response Got a packet bigger than 'max_allowed_packet' bytes. However, once this happens the connection is destroyed and the client can't recover.

Additionally, it's a waste of time to send the query to the server when we know it'll be rejected. By running SHOW VARIABLES when the connection is established, we could get the value of max_allowed_packet, and check packets to ensure they don't exceed that length before sending them.

[bgrainger]

For efficiency in the typical use case, perhaps we could only SHOW VARIABLES once a SQL statement larger than 4MB (the MySQL Server default) has been prepared in the client?

[caleblloyd]

What about cases where the client has upped it's max_allowed_packet size before sending a large packet, but after we theoretically record max_allowed_packet? We would reject a query client-side that would have succeeded.

mysql> SELECT @@global.max_allowed_packet;
+-----------------------------+
| @@global.max_allowed_packet |
+-----------------------------+
|                     4194304 |
+-----------------------------+
1 row in set (0.00 sec)

mysql> SET @@global.max_allowed_packet = 8388608;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @@global.max_allowed_packet;
+-----------------------------+
| @@global.max_allowed_packet |
+-----------------------------+
|                     8388608 |
+-----------------------------+
1 row in set (0.00 sec)

Maybe a better approach is to record number of bytes sent in the packet, and throw an Outer Exception if packet size was > 4MB with "Exception after submitting XXMB Packet, ensure max_allowed_packet is greater than XXMB" -> Inner Exception (original exception)

[bgrainger]

MySQL Server does send a final packet with the contents #08S01, Got a packet bigger than 'max_allowed_packet' bytes but then it immediately resets the socket. As a result, our library throws a SocketException ("An existing connection was forcibly closed by the remote host" or "An established connection was aborted by the software in your host machine") when trying to read that packet in:

System.dll!System.Net.Sockets.Socket.Receive(byte[] buffer, int offset, int size, System.Net.Sockets.SocketFlags socketFlags)	Unknown
MySqlConnector.dll!MySql.Data.Protocol.Serialization.SocketByteHandler.ReadBytesAsync(System.ArraySegment<byte> buffer, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 24	C#
MySqlConnector.dll!MySql.Data.Protocol.Serialization.BufferedByteReader.ReadBytesAsync(MySql.Data.Protocol.Serialization.IByteHandler byteHandler, System.ArraySegment<byte> buffer, int count, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 34	C#
MySqlConnector.dll!MySql.Data.Protocol.Serialization.BufferedByteReader.ReadBytesAsync(MySql.Data.Protocol.Serialization.IByteHandler byteHandler, int count, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 29	C#
MySqlConnector.dll!MySql.Data.Protocol.Serialization.ProtocolUtility.ReadPacketAsync(MySql.Data.Protocol.Serialization.BufferedByteReader bufferedByteReader, MySql.Data.Protocol.Serialization.IByteHandler byteHandler, System.Func<int?> getNextSequenceNumber, MySql.Data.Protocol.Serialization.ProtocolErrorBehavior protocolErrorBehavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 13	C#
MySqlConnector.dll!MySql.Data.Protocol.Serialization.ProtocolUtility.ReadPayloadAsync(MySql.Data.Protocol.Serialization.BufferedByteReader bufferedByteReader, MySql.Data.Protocol.Serialization.IByteHandler byteHandler, System.Func<int?> getNextSequenceNumber, System.ArraySegment<byte> previousPayloads, MySql.Data.Protocol.Serialization.ProtocolErrorBehavior protocolErrorBehavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 53	C#
MySqlConnector.dll!MySql.Data.Protocol.Serialization.StandardPayloadHandler.ReadPayloadAsync(MySql.Data.Protocol.Serialization.ProtocolErrorBehavior protocolErrorBehavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior) Line 31	C#
MySqlConnector.dll!MySql.Data.Serialization.MySqlSession.TryAsync(System.Func<MySql.Data.Protocol.Serialization.ProtocolErrorBehavior, MySql.Data.Protocol.Serialization.IOBehavior, System.Threading.Tasks.ValueTask<System.ArraySegment<byte>>> func, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 471	C#
MySqlConnector.dll!MySql.Data.Serialization.MySqlSession.ReceiveReplyAsync(MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 187	C#
MySqlConnector.dll!MySql.Data.MySqlClient.Results.ResultSet.ReadResultSetHeaderAsync(MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 37	C#
MySqlConnector.dll!MySql.Data.MySqlClient.MySqlDataReader.ReadFirstResultSetAsync(MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 239	C#
MySqlConnector.dll!MySql.Data.MySqlClient.MySqlDataReader.CreateAsync(MySql.Data.MySqlClient.MySqlCommand command, System.Data.CommandBehavior behavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 227	C#
MySqlConnector.dll!MySql.Data.MySqlClient.CommandExecutors.TextCommandExecutor.ExecuteReaderAsync(string commandText, MySql.Data.MySqlClient.MySqlParameterCollection parameterCollection, System.Data.CommandBehavior behavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 59	C#
MySqlConnector.dll!MySql.Data.MySqlClient.MySqlCommand.ExecuteReaderAsync(System.Data.CommandBehavior behavior, MySql.Data.Protocol.Serialization.IOBehavior ioBehavior, System.Threading.CancellationToken cancellationToken) Line 143	C#
MySqlConnector.dll!MySql.Data.MySqlClient.MySqlCommand.ExecuteDbDataReader(System.Data.CommandBehavior behavior) Line 116	C#
System.Data.dll!System.Data.Common.DbCommand.ExecuteReader()	Unknown

[bgrainger]

Fixed in 0.16.1.