Allow configuring auto-join transaction globally

doc/reference/modules/configuration.xml

@@ -905,6 +905,25 @@ var session = sessions.OpenSession(conn);
                             </para>
                         </entry>
                     </row>
+                    <row>
+                        <entry>
+                            <literal>transaction.auto_join</literal>
+                        </entry>
+                        <entry>
+                            Should sessions check on every operation whether there is an ongoing system transaction or not, and enlist
+                            into it if any?
+                            <para>
+                                Default is <literal>true</literal>. It can also be controlled at session opening, with
+                                <literal>ISessionFactory.WithOptions</literal>. A session can also be instructed to explicitly join the current
+                                transaction by calling <literal>ISession.JoinTransaction</literal>. This setting has no effect when using a
+                                transaction factory that is not system transactions aware.
+                            </para>
+                            <para>
+                                <emphasis role="strong">eg.</emphasis>
+                                <literal>false</literal>
+                            </para>
+                        </entry>
+                    </row>
                     <row>
                         <entry>
                             <literal>default_flush_mode</literal>

doc/reference/modules/transactions.xml

@@ -633,9 +633,11 @@ finally
                     will then fail to use it.
                 </para>
                 <para>
-                    As of NHibernate v5.0, session auto-enlistment can be disabled from the session builder
+                    Session auto-enlistment can be controlled from the session builder
                     obtained with <literal>ISessionFactory.WithOptions()</literal>, using the
-                    <literal>AutoJoinTransaction</literal> option. The connection may still enlist itself
+                    <literal>AutoJoinTransaction</literal> option. It can also be controlled at the configuration level,
+                    see <literal>transaction.auto_join</literal> in <xref linkend="configuration-optional"/>.
+                    When auto-join is disabled, the connection may still enlist itself
                     if connection string <literal>Enlist</literal> setting is not <literal>false</literal>.
                     A session can explicitly join the current system transaction by calling
                     <literal>ISession.JoinTransaction()</literal>.

src/NHibernate.Test/SystemTransactions/AutoJoinSettingFixture.cs

@@ -0,0 +1,55 @@
+using System.Transactions;
+using NHibernate.Cfg;
+using NUnit.Framework;
+
+namespace NHibernate.Test.SystemTransactions
+{
+	[TestFixture(true)]
+	[TestFixture(false)]
+	[TestFixture(new object[] { null })]
+	public class AutoJoinSettingFixture : TestCase
+	{
+		private readonly bool? _autoJoinTransaction;
+
+		public AutoJoinSettingFixture(bool? autoJoinTransaction)
+		{
+			_autoJoinTransaction = autoJoinTransaction;
+		}
+
+		protected override string[] Mappings => new[] { "TransactionTest.Person.hbm.xml" };
+
+		protected override string MappingsAssembly => "NHibernate.Test";
+
+		protected override void Configure(Configuration configuration)
+		{
+			if (_autoJoinTransaction.HasValue)
+				configuration.SetProperty(Environment.AutoJoinTransaction, _autoJoinTransaction.ToString());
+			else
+				configuration.Properties.Remove(Environment.AutoJoinTransaction);
+		}
+
+		[Test]
+		public void CheckTransactionJoined()
+		{
+			using (new TransactionScope())
+			using (var s = OpenSession())
+			{
+				Assert.That(
+					s.GetSessionImplementation().TransactionContext,
+					_autoJoinTransaction == false ? Is.Null : Is.Not.Null);
+			}
+		}
+
+		[Theory]
+		public void CanOverrideAutoJoin(bool autoJoin)
+		{
+			using (new TransactionScope())
+			using (var s = Sfi.WithOptions().AutoJoinTransaction(autoJoin).OpenSession())
+			{
+				Assert.That(
+					s.GetSessionImplementation().TransactionContext,
+					autoJoin ? Is.Not.Null : Is.Null);
+			}
+		}
+	}
+}

src/NHibernate/Cfg/Environment.cs

@@ -123,6 +123,10 @@ public static string Version
 		[Obsolete("This setting has no usages and will be removed in a future version")]
 		public const string OutputStylesheet = "xml.output_stylesheet";
 
+		/// <summary>
+		/// The class name of a custom <see cref="Transaction.ITransactionFactory"/> implementation. Defaults to the
+		/// built-in <see cref="Transaction.AdoNetWithSystemTransactionFactory" />.
+		/// </summary>
 		public const string TransactionStrategy = "transaction.factory_class";
 		/// <summary>
 		/// <para>Timeout duration in milliseconds for the system transaction completion lock.</para>
@@ -144,6 +148,14 @@ public static string Version
 		/// transaction preparation, while still benefiting from <see cref="FlushMode.Auto"/> on querying.
 		/// </summary>
 		public const string UseConnectionOnSystemTransactionPrepare = "transaction.use_connection_on_system_prepare";
+		/// <summary>
+		/// Should sessions check on every operation whether there is an ongoing system transaction or not, and enlist
+		/// into it if any? Default is <see langword="true"/>. It can also be controlled at session opening, see
+		/// <see cref="ISessionFactory.WithOptions" />. A session can also be instructed to explicitly join the current
+		/// transaction by calling <see cref="ISession.JoinTransaction" />. This setting has no effect when using a
+		/// transaction factory that is not system transactions aware.
+		/// </summary>
+		public const string AutoJoinTransaction = "transaction.auto_join";
 
 		// Since v5.0.1
 		[Obsolete("This setting has no usages and will be removed in a future version")]

src/NHibernate/Cfg/Settings.cs

@@ -53,6 +53,15 @@ public Settings()
 
 		public string SessionFactoryName { get; internal set; }
 
+		/// <summary>
+		/// Should sessions check on every operation whether there is an ongoing system transaction or not, and enlist
+		/// into it if any? Default is <see langword="true"/>. It can also be controlled at session opening, see
+		/// <see cref="ISessionFactory.WithOptions" />. A session can also be instructed to explicitly join the current
+		/// transaction by calling <see cref="ISession.JoinTransaction" />. This setting has no effect if using a
+		/// transaction factory that is not system transactions aware.
+		/// </summary>
+		public bool AutoJoinTransaction { get; internal set; }
+
 		public bool IsAutoCreateSchema { get; internal set; }
 
 		public bool IsAutoDropSchema { get; internal set; }
@@ -146,4 +155,4 @@ internal string GetFullCacheRegionName(string name)
 			return name;
 		}
 	}
-}
+}

src/NHibernate/Cfg/SettingsFactory.cs

@@ -291,6 +291,7 @@ public Settings BuildSettings(IDictionary<string, string> properties)
 			settings.TransactionFactory = transactionFactory;
 			// Not ported - TransactionManagerLookup
 			settings.SessionFactoryName = sessionFactoryName;
+			settings.AutoJoinTransaction = PropertiesHelper.GetBoolean(Environment.AutoJoinTransaction, properties, true);
 			settings.MaximumFetchDepth = maxFetchDepth;
 			settings.IsQueryCacheEnabled = useQueryCache;
 			settings.IsSecondLevelCacheEnabled = useSecondLevelCache;

src/NHibernate/ISession.cs

@@ -771,7 +771,8 @@ public partial interface ISession : IDisposable
 		/// <para>
 		/// Sessions auto-join current transaction by default on their first usage within a scope.
 		/// This can be disabled with <see cref="ISessionBuilder{T}.AutoJoinTransaction(bool)"/> from
-		/// a session builder obtained with <see cref="ISessionFactory.WithOptions()"/>.
+		/// a session builder obtained with <see cref="ISessionFactory.WithOptions()"/>, or with the
+		/// auto-join transaction configuration setting.
 		/// </para>
 		/// <para>
 		/// This method allows to explicitly join the current transaction. It does nothing if it is already

src/NHibernate/Impl/SessionFactoryImpl.cs

@@ -1415,7 +1415,7 @@ internal class SessionBuilderImpl<T> : ISessionBuilder<T>, ISessionCreationOptio
 			private ConnectionReleaseMode _connectionReleaseMode;
 			private FlushMode _flushMode;
 			private bool _autoClose;
-			private bool _autoJoinTransaction = true;
+			private bool _autoJoinTransaction;
 
 			public SessionBuilderImpl(SessionFactoryImpl sessionFactory)
 			{
@@ -1424,6 +1424,7 @@ public SessionBuilderImpl(SessionFactoryImpl sessionFactory)
 				// set up default builder values...
 				_connectionReleaseMode = sessionFactory.Settings.ConnectionReleaseMode;
 				_autoClose = sessionFactory.Settings.IsAutoCloseSessionEnabled;
+				_autoJoinTransaction = sessionFactory.Settings.AutoJoinTransaction;
 				// NH different implementation: not using Settings.IsFlushBeforeCompletionEnabled
 				_flushMode = sessionFactory.Settings.DefaultFlushMode;
 			}

src/NHibernate/nhibernate-configuration.xsd

@@ -92,7 +92,14 @@
 										<xs:enumeration value="show_sql" />
 										<xs:enumeration value="max_fetch_depth" />
 										<xs:enumeration value="current_session_context_class" />
-										<xs:enumeration value="transaction.factory_class" />
+										<xs:enumeration value="transaction.factory_class">
+											<xs:annotation>
+												<xs:documentation>
+													The class name of a custom ITransactionFactory implementation.
+													Defaults to the built-in AdoNetWithSystemTransactionFactory.
+												</xs:documentation>
+											</xs:annotation>
+										</xs:enumeration>
 										<xs:enumeration value="cache.provider_class" />
 										<xs:enumeration value="cache.use_query_cache" />
 										<xs:enumeration value="cache.query_cache_factory" />
@@ -174,6 +181,17 @@
 												</xs:documentation>
 											</xs:annotation>
 										</xs:enumeration>
+										<xs:enumeration value="transaction.auto_join">
+											<xs:annotation>
+												<xs:documentation>
+													Should sessions check on every operation whether there is an ongoing system transaction or not, and enlist
+													into it if any? Default is true. It can also be controlled at session opening, with
+													ISessionFactory.WithOptions. A session can also be instructed to explicitly join the current
+													transaction by calling ISession.JoinTransaction. This setting has no effect when using a
+													transaction factory that is not system transactions aware.
+												</xs:documentation>
+											</xs:annotation>
+										</xs:enumeration>
 										<xs:enumeration value="oracle.use_n_prefixed_types_for_unicode">
 											<xs:annotation>
 												<xs:documentation>