MySQL Bugs: #90845: NullReferenceException on connection close

Bug #90845	NullReferenceException on connection close
Submitted:	12 May 2018 10:46	Modified:	20 Feb 2021 23:32
Reporter:	Francesco Cattoni	Email Updates:
Status:	No Feedback	Impact on me:	None
Category:	Connector / NET	Severity:	S3 (Non-critical)
Version:	8.0.11	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	exception

Description:
A NullReferenceException is thrown on MySqlConnection Dispose

The NullReference came from the setter of the property Reader.
In the getter the driver has a null check but in the setter it is treated as a NotNull value

internal MySqlDataReader Reader
{
  get
  {
    return this.driver?.reader;
  }
  set
  {
    this.driver.reader = value; // here the NullReferenceException appear
    this.IsInUse = this.driver.reader != null;
  }
}

How to repeat:
Open a MySqlConnection
Execute a MySqlCommand with the CloseConnection flag

Suggested fix:
Execute a null check on the driver field

Sorry
I forgot to write that the exception is thrown while Disposing of the MySqlCommand

This sounds similar to bug #89159. (But without a self-contained repro or full call stack, it's hard to say for sure.)

Do you need a call stack?

Hello Francesco,

Thank you for the bug report.
Could you please provide repeatable steps (exact steps/sample project, full stack trace etc. - please make it as private if you prefer) to confirm this issue at our end?

Thanks,
Chiranjeevi.

Here it is.

- MySql server version:

5.7.18-log
Hosted on my local computer.

- packages.config MySql.Data line:

  <package id="MySql.Data" version="8.0.11" targetFramework="net461" />

- Framework:

.NET Framework v4.6.1 (tested as library in .NET 4.5.2 too)
C# 7.0

- CPU architecture:

My computer x64
Built as AnyCPU with "32 bit preferred" disabled

- Code to reproduce [as NUnit test fixture]:

using System;
using System.Data;
using MySql.Data.MySqlClient;
using NUnit.Framework;

namespace Tests {
	[TestFixture]
	public class MySqlBug {
		[Test]
		public void Diy() {
			MySqlConnectionStringBuilder connectionStringBuilder = new MySqlConnectionStringBuilder() {
				Server = "localhost",
				UserID = "root",
				Password = "foobar",
				Database = "tests",
				PersistSecurityInfo = true,
				Pooling = true,
				AllowUserVariables = true,
				ConvertZeroDateTime = true,
				SslMode = MySqlSslMode.None
			};
			var conn = new MySqlConnection(connectionStringBuilder.GetConnectionString(true));
			conn.Open();
			Console.WriteLine(conn.ServerVersion);
			var command = new MySqlCommand("SELECT 1", conn);
			using (command) {
				var reader = command.ExecuteReader(CommandBehavior.CloseConnection);
			}
		}
	}
}

- Console output:

Tests.MySqlBug.Diy

System.NullReferenceException : Riferimento a un oggetto non impostato su un'istanza di oggetto.
   in MySql.Data.MySqlClient.MySqlConnection.set_Reader(MySqlDataReader value)
   in MySql.Data.MySqlClient.MySqlCommand.Dispose(Boolean disposing)
   in MySql.Data.MySqlClient.MySqlCommand.Dispose()
   in Tests.MySqlBug.Diy() in C:\_INGAGE\_Plugins\DataManager\Tests\MySqlBug.cs:riga 30

5.7.18-log

Just ask if you need anything else.

Hello Francesco Cattoni,

Thank you for the bug report.
Verified this behavior on Visual Studio 2017 and Connector/NET 8.0.11 version..

Thanks,
Chiranjeevi.

Screenshot

Attachment: Bug_90845.PNG (image/png, text), 109.35 KiB.

http://bugs.mysql.com/bug.php?id=91106 marked as duplicate of this one.

This NullReferenceException can alo get thrown on Finalize(), i.e. when the MySqlDataReader is garbage collected. See https://bugs.mysql.com/bug.php?id=91106 (marked as duplicate of this bug) for an example.

In this case, the entire client application is forced to terminate; there is no possibility whatsoever for the client to handle it gracefully.

For that reason, could the Severity of this bug please be raised? For reference, the duplicate bug had S1 (Critical).

I encounter the exact same bug. Same stack trace.

Environment: 
Database server MySQL 8.0.19 running on Ubuntu
Windows client target on Microsoft .Net 4.8 with latest Connector/NET 8.0.19.

I have the same problem with Connector/NET 6.10.9

My current work around is to stay on Connector/NET 6.9.9

I am encountering the same issue.  Will this be fixed?  Should I fall back to a different version?

Hi,

Can you please verify using the latest C/NET release, v8.0.23? Should be
fixed.

Daniel

I can confirm for my scenario that this bug #90845 has been fixed in version 8.0.23 of NuGet package MySql.Data.

Unfortunaly now having another issue.

DataReader.Read throwing System.ArgumentOutOfRangeException "Non-negative number required." So I can't still update to latest package. I will research this new issue a bit further and comment or create new ticket for this if needed.

Given that this issue is no longer happening, I will proceed to close it and if there's another bug, please create a new report.

// There is actually one more issue. Probably related.
        // The null reference happens in reader.Dispoase after the EndOfStreamException is reader.Read()
        // here below is the test code with details

        static void test1()
        {
            // create a table and add 15-20 records with some big data
            // and then try to read it with the a reader and sleep (to simulate long processing)

            using (MySqlConnection c = new MySqlConnection(conn))
            {
                c.Open();

                using (MySqlCommand cmd = new MySqlCommand("some stored procedure or a simple sql", c))
                {
                    using (MySqlDataReader r = cmd.ExecuteReader())
                    {
                        bool read = r.Read();
                        while (read)
                        {
                            var bigdata = (String)r["some big data"];
                            if (sleep > 0)
                            {
                                // let's imaging that it takes time to handle the received data.
                                // so simulate it with a sleep.
                                // better to use sleep more than 60 seconds. 
                                // 60 seconds is default parameter in mysql config for net_write_timeout
                                Console.WriteLine($"sleeping {sleep} seconds...");
                                System.Threading.Thread.Sleep(sleep * 1000);
                            }

                            try
                            {
                                read = r.Read();
                            }
                            catch (Exception ex)
                            {
                                // here we got the exception when the net_write_timeout exceeded
                                // the exception can happen not at the first read but at some later - looks like depends on the amount of data 
                                // MySql.Data.MySqlClient.MySqlException (0x80004005): Fatal error encountered during data read.
                                // --->MySql.Data.MySqlClient.MySqlException(0x80004005): Reading from the stream has failed.
                                // --->System.IO.EndOfStreamException: Attempted to read past the end of the stream.
                                // the exception happens because of the timeout configured on mysql side in parameter net_write_timeout
                                // and it is OK that we got that exception - no issue here.
                                // The issue is the next NullRefernceException which happens in reader.Dispose after that(see below)
                                Console.WriteLine($"Failed to read from the reader: {ex.ToString()}");
                                read = false;
                            }
                        }
                    } // HERE r.Dispose is called and it throws null reference exception. The dispose should not throw it.

                    // the exception is the below. Happens on version 8.0.19 and 8.0.23 as well.
                    // --->System.NullReferenceException: Object reference not set to an instance of an object.
                    // at MySql.Data.MySqlClient.NativeDriver.GetResult(Int32 & affectedRow, Int64 & insertedId) in MySQL.Data\src\NativeDriver.cs:line 487
                    // at MySql.Data.MySqlClient.Driver.GetResult(Int32 statementId, Int32 & affectedRows, Int64 & insertedId) in MySQL.Data\src\Driver.cs:line 400
                    // at MySql.Data.MySqlClient.Driver.NextResult(Int32 statementId, Boolean force) in MySQL.Data\src\Driver.cs:line 389
                    // at MySql.Data.MySqlClient.MySqlDataReader.NextResult() in MySQL.Data\src\datareader.cs:line 849
                    // at MySql.Data.MySqlClient.MySqlDataReader.Close() in MySQL.Data\src\datareader.cs:line 163
                    // at MySql.Data.MySqlClient.MySqlDataReader.Dispose(Boolean disposing) in MySQL.Data\src\datareader.cs:line 1075
                    // at MySql.Data.MySqlClient.MySqlDataReader.Dispose() in MySQL.Data\src\datareader.cs:line 1067
                }
            }
        }

No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".

This bug is occurring again in the latest build 8.0.24.

Stack trace:
   at MySql.Data.MySqlClient.MySqlConnection.set_Reader(MySqlDataReader value)
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataTable[] dataTables, Int32 startRecord, Int32 maxRecords, IDbCommand command, CommandBehavior behavior)
   at System.Data.Common.DbDataAdapter.Fill(DataTable dataTable)
   at XPlatformWindowsShared.Data.MySqlBase.ExecuteQuery(String sqlcmd) in ...

This bug is still occuring. Looking at the code, it is still possible for Dispose to throw a null reference exception because the driver is null in the DataReader or this line that sets the Reader to null could fail because the driver in the Connection is null.

 // always ensure internal reader is null (Bug #55558)
        _connection.Reader = null;