MySqlCommand can be changed while another is executing. Fixes #867

Author: bgrainger

src/MySqlConnector/Core/ServerSession.cs

@@ -51,6 +51,7 @@ public ServerSession(ConnectionPool? pool, int poolGeneration, int id)
 
 		public string Id { get; }
 		public ServerVersion ServerVersion { get; set; }
+		public int ActiveCommandId { get; private set; }
 		public int ConnectionId { get; set; }
 		public byte[]? AuthPluginData { get; set; }
 		public uint CreatedTicks { get; }
@@ -90,7 +91,7 @@ public bool TryStartCancel(ICancellableCommand command)
 		{
 			lock (m_lock)
 			{
-				if (m_activeCommandId != command.CommandId)
+				if (ActiveCommandId != command.CommandId)
 					return false;
 				VerifyState(State.Querying, State.CancelingQuery, State.Failed);
 				if (m_state != State.Querying)
@@ -109,11 +110,11 @@ public void DoCancel(ICancellableCommand commandToCancel, MySqlCommand killComma
 			Log.Info("Session{0} canceling CommandId {1}: CommandText: {2}", m_logArguments[0], commandToCancel.CommandId, (commandToCancel as MySqlCommand)?.CommandText);
 			lock (m_lock)
 			{
-				if (m_activeCommandId != commandToCancel.CommandId)
+				if (ActiveCommandId != commandToCancel.CommandId)
 					return;
 
 				// NOTE: This command is executed while holding the lock to prevent race conditions during asynchronous cancellation.
-				// For example, if the lock weren't held, the current command could finish and the other thread could set m_activeCommandId
+				// For example, if the lock weren't held, the current command could finish and the other thread could set ActiveCommandId
 				// to zero, then start executing a new command. By the time this "KILL QUERY" command reached the server, the wrong
 				// command would be killed (because "KILL QUERY" specifies the connection whose command should be killed, not
 				// a unique identifier of the command itself). As a mitigation, we set the CommandTimeout to a low value to avoid
@@ -127,7 +128,7 @@ public void AbortCancel(ICancellableCommand command)
 		{
 			lock (m_lock)
 			{
-				if (m_activeCommandId == command.CommandId && m_state == State.CancelingQuery)
+				if (ActiveCommandId == command.CommandId && m_state == State.CancelingQuery)
 					m_state = State.Querying;
 			}
 		}
@@ -253,7 +254,7 @@ public void StartQuerying(ICancellableCommand command)
 				m_state = State.Querying;
 
 				command.CancelAttemptCount = 0;
-				m_activeCommandId = command.CommandId;
+				ActiveCommandId = command.CommandId;
 			}
 		}
 
@@ -289,7 +290,7 @@ public void FinishQuerying()
 					m_state = State.Connected;
 				else
 					VerifyState(State.Failed);
-				m_activeCommandId = 0;
+				ActiveCommandId = 0;
 			}
 		}
 
@@ -1622,7 +1623,6 @@ private enum State
 		SslStream? m_sslStream;
 		X509Certificate2? m_clientCertificate;
 		IPayloadHandler? m_payloadHandler;
-		int m_activeCommandId;
 		bool m_useCompression;
 		bool m_isSecureConnection;
 		bool m_supportsComMulti;

src/MySqlConnector/MySqlCommand.cs

@@ -157,7 +157,7 @@ private bool NeedsPrepare(out Exception? exception)
 			else if (string.IsNullOrWhiteSpace(CommandText))
 				exception = new InvalidOperationException("CommandText must be specified");
 			else if (Connection?.HasActiveReader ?? false)
-				exception = new InvalidOperationException("Cannot call Prepare when there is an open DataReader for this command; it must be closed first.");
+				exception = new InvalidOperationException("Cannot call Prepare when there is an open DataReader for this command's connection; it must be closed first.");
 
 			if (exception is not null || Connection!.IgnorePrepare)
 				return false;
@@ -179,7 +179,7 @@ public override string CommandText
 			get => m_commandText;
 			set
 			{
-				if (m_connection?.HasActiveReader ?? false)
+				if (m_connection?.ActiveCommandId == m_commandId)
 					throw new InvalidOperationException("Cannot set MySqlCommand.CommandText when there is an open DataReader for this command; it must be closed first.");
 				m_commandText = value ?? "";
 			}
@@ -194,7 +194,7 @@ public override string CommandText
 			get => m_connection;
 			set
 			{
-				if (m_connection?.HasActiveReader ?? false)
+				if (m_connection?.ActiveCommandId == m_commandId)
 					throw new InvalidOperationException("Cannot set MySqlCommand.Connection when there is an open DataReader for this command; it must be closed first.");
 				m_connection = value;
 			}

src/MySqlConnector/MySqlConnection.cs

@@ -765,6 +765,8 @@ internal void Cancel(ICancellableCommand command)
 
 		internal MySqlSslMode SslMode => GetInitializedConnectionSettings().SslMode;
 
+		internal int? ActiveCommandId => m_session?.ActiveCommandId;
+
 		internal bool HasActiveReader => m_activeReader is not null;
 
 		internal void SetActiveReader(MySqlDataReader dataReader)

tests/SideBySide/CommandTests.cs

@@ -300,6 +300,26 @@ public void CommandsAreIndependent()
 			Assert.True(reader.Read());
 		}
 
+		[Fact]
+		public void ExecutingCommandsAreIndependent()
+		{
+			using var connection = new MySqlConnection(AppConfig.ConnectionString);
+			connection.Open();
+
+			using var cmd1 = connection.CreateCommand();
+			cmd1.CommandText = "SELECT 1;";
+			using var reader1 = cmd1.ExecuteReader();
+
+			using var cmd2 = connection.CreateCommand();
+			cmd2.CommandText = "SELECT 'abc';";
+
+#if BASELINE
+			Assert.Throws<MySqlException>(() => cmd2.ExecuteReader());
+#else
+			Assert.Throws<InvalidOperationException>(() => cmd2.ExecuteReader());
+#endif
+		}
+
 		private static string GetIgnoreCommandTransactionConnectionString()
 		{
 #if BASELINE