Require global opt-in for distributed transactions

src/libraries/System.Transactions.Local/ref/System.Transactions.Local.cs

@@ -4,6 +4,8 @@
 // Changes to this file must follow the https://aka.ms/api-review process.
 // ------------------------------------------------------------------------------
 
+using System.Runtime.Versioning;
+
 namespace System.Transactions
 {
     [System.Runtime.Versioning.UnsupportedOSPlatform("browser")]
@@ -191,6 +193,7 @@ public static partial class TransactionManager
         [System.Diagnostics.CodeAnalysis.DisallowNullAttribute]
         public static System.Transactions.HostCurrentTransactionCallback? HostCurrentCallback { get { throw null; } set { } }
         public static System.TimeSpan MaximumTimeout { get { throw null; } set { } }
+        public static bool ImplicitDistributedTransactions { get; [System.Runtime.Versioning.SupportedOSPlatform("windows")] [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCode("Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.")] set; }
         public static event System.Transactions.TransactionStartedEventHandler? DistributedTransactionStarted { add { } remove { } }
         public static void RecoveryComplete(System.Guid resourceManagerIdentifier) { }
         public static System.Transactions.Enlistment Reenlist(System.Guid resourceManagerIdentifier, byte[] recoveryInformation, System.Transactions.IEnlistmentNotification enlistmentNotification) { throw null; }

src/libraries/System.Transactions.Local/src/Resources/Strings.resx

@@ -423,4 +423,10 @@
   <data name="DistributedNotSupportedOn32Bits" xml:space="preserve">
     <value>Distributed transactions are currently unsupported in 32-bit processes.</value>
   </data>
-</root>
+  <data name="ImplicitDistributedTransactionsDisabled" xml:space="preserve">
+    <value>Implicit distributed transactions have not been enabled. If you're intentionally starting a distributed transaction, set TransactionManager.ImplicitDistributedTransactions to true.</value>
+  </data>
+  <data name="ImplicitDistributedTransactionsCannotBeChanged" xml:space="preserve">
+    <value>TransactionManager.ImplicitDistributedTransaction cannot be changed once set, or once System.Transactions distributed transactions have been initialized. Set this flag once at the start of your program.</value>
+  </data>
+</root>

src/libraries/System.Transactions.Local/src/System/Transactions/DtcProxyShim/DtcProxyShimFactory.cs

@@ -21,6 +21,11 @@ internal sealed class DtcProxyShimFactory
     // at the same time.
     private static readonly object _proxyInitLock = new();
 
+    // This object will perform the actual distributed transaction connection.
+    // It will be set only if TransactionManager.ImplicitDefaultTransactions
+    // is set to true, allowing the relevant code to be trimmed otherwise.
+    internal static ITransactionConnector? s_transactionConnector;
+
     // Lock to protect access to listOfNotifications.
     private readonly object _notificationLock = new();
 
@@ -41,6 +46,7 @@ internal DtcProxyShimFactory(EventWaitHandle notificationEventHandle)
 
     // https://docs.microsoft.com/previous-versions/windows/desktop/ms678898(v=vs.85)
     [DllImport(Interop.Libraries.Xolehlp, CharSet = CharSet.Unicode, ExactSpelling = true, PreserveSig = false)]
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private static extern void DtcGetTransactionManagerExW(
         [MarshalAs(UnmanagedType.LPWStr)] string? pszHost,
         [MarshalAs(UnmanagedType.LPWStr)] string? pszTmName,
@@ -49,7 +55,7 @@ private static extern void DtcGetTransactionManagerExW(
         object? pvConfigPararms,
         [MarshalAs(UnmanagedType.Interface)] out ITransactionDispenser ppvObject);
 
-    [RequiresUnreferencedCode("Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.")]
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private static void DtcGetTransactionManager(string? nodeName, out ITransactionDispenser localDispenser) =>
         DtcGetTransactionManagerExW(nodeName, null, Guids.IID_ITransactionDispenser_Guid, 0, null, out localDispenser);
 
@@ -61,15 +67,27 @@ public void ConnectToProxy(
         out byte[] whereabouts,
         out ResourceManagerShim resourceManagerShim)
     {
-        switch (RuntimeInformation.ProcessArchitecture)
+        if (RuntimeInformation.ProcessArchitecture == Architecture.X86)
+        {
+            throw new PlatformNotSupportedException(SR.DistributedNotSupportedOn32Bits);
+        }
+
+        lock (TransactionManager.s_implicitDistributedTransactionsLock)
         {
-            case Architecture.X86:
-                throw new PlatformNotSupportedException(SR.DistributedNotSupportedOn32Bits);
+            if (s_transactionConnector is null)
+            {
+                // We set TransactionManager.ImplicitDistributedTransactionsInternal, so that any attempt to change it
+                // later will cause an exception.
+                TransactionManager.s_implicitDistributedTransactions = false;
+
+                throw new NotSupportedException(SR.ImplicitDistributedTransactionsDisabled);
+            }
         }
 
-        ConnectToProxyCore(nodeName, resourceManagerIdentifier, managedIdentifier, out nodeNameMatches, out whereabouts, out resourceManagerShim);
+        s_transactionConnector.ConnectToProxyCore(this, nodeName, resourceManagerIdentifier, managedIdentifier, out nodeNameMatches, out whereabouts, out resourceManagerShim);
     }
 
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
     private void ConnectToProxyCore(
         string? nodeName,
         Guid resourceManagerIdentifier,
@@ -80,9 +98,7 @@ private void ConnectToProxyCore(
     {
         lock (_proxyInitLock)
         {
-#pragma warning disable IL2026 // This warning is left in the product so developers get an ILLink warning when trimming an app using this transaction support
             DtcGetTransactionManager(nodeName, out ITransactionDispenser? localDispenser);
-#pragma warning restore IL2026
 
             // Check to make sure the node name matches.
             if (nodeName is not null)
@@ -353,6 +369,8 @@ internal ITransactionTransmitter GetCachedTransmitter(ITransaction transaction)
 
     internal void ReturnCachedTransmitter(ITransactionTransmitter transmitter)
     {
+        // Note that due to race conditions, we may end up enqueuing above s_maxCachedInterfaces.
+        // This is benign, as this is only a best-effort cache, and there are no negative consequences.
         if (_cachedTransmitters.Count < s_maxCachedInterfaces)
         {
             transmitter.Reset();
@@ -375,10 +393,46 @@ internal ITransactionReceiver GetCachedReceiver()
 
     internal void ReturnCachedReceiver(ITransactionReceiver receiver)
     {
+        // Note that due to race conditions, we may end up enqueuing above s_maxCachedInterfaces.
+        // This is benign, as this is only a best-effort cache, and there are no negative consequences.
         if (_cachedReceivers.Count < s_maxCachedInterfaces)
         {
             receiver.Reset();
             _cachedReceivers.Enqueue(receiver);
         }
     }
+
+    internal interface ITransactionConnector
+    {
+        void ConnectToProxyCore(
+            DtcProxyShimFactory proxyShimFactory,
+            string? nodeName,
+            Guid resourceManagerIdentifier,
+            object managedIdentifier,
+            out bool nodeNameMatches,
+            out byte[] whereabouts,
+            out ResourceManagerShim resourceManagerShim);
+    }
+
+    [RequiresUnreferencedCode(TransactionManager.DistributedTransactionTrimmingWarning)]
+    internal sealed class DtcTransactionConnector : ITransactionConnector
+    {
+        public void ConnectToProxyCore(
+            DtcProxyShimFactory proxyShimFactory,
+            string? nodeName,
+            Guid resourceManagerIdentifier,
+            object managedIdentifier,
+            out bool nodeNameMatches,
+            out byte[] whereabouts,
+            out ResourceManagerShim resourceManagerShim)
+        {
+            proxyShimFactory.ConnectToProxyCore(
+                nodeName,
+                resourceManagerIdentifier,
+                managedIdentifier,
+                out nodeNameMatches,
+                out whereabouts,
+                out resourceManagerShim);
+        }
+    }
 }

src/libraries/System.Transactions.Local/src/System/Transactions/TransactionManager.cs

@@ -4,8 +4,12 @@
 using System.Collections;
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
+using System.Runtime.Versioning;
 using System.Threading;
 using System.Transactions.Configuration;
+#if WINDOWS
+using System.Transactions.DtcProxyShim;
+#endif
 using System.Transactions.Oletx;
 
 namespace System.Transactions
@@ -29,6 +33,10 @@ public static class TransactionManager
         private static TransactionTable? s_transactionTable;
 
         private static TransactionStartedEventHandler? s_distributedTransactionStartedDelegate;
+
+        internal const string DistributedTransactionTrimmingWarning =
+            "Distributed transactions support may not be compatible with trimming. If your program creates a distributed transaction via System.Transactions, the correctness of the application cannot be guaranteed after trimming.";
+
         public static event TransactionStartedEventHandler? DistributedTransactionStarted
         {
             add
@@ -391,6 +399,60 @@ public static TimeSpan MaximumTimeout
             }
         }
 
+        /// <summary>
+        /// Controls whether usage of System.Transactions APIs that require escalation to a distributed transaction will do so;
+        /// if your application requires distributed transaction, opt into using them by setting this to <see langword="true" />.
+        /// If set to <see langword="false" /> (the default), escalation to a distributed transaction will throw a <see cref="NotSupportedException" />.
+        /// </summary>
+#if WINDOWS
+        public static bool ImplicitDistributedTransactions
+        {
+            get => DtcProxyShimFactory.s_transactionConnector is not null;
+
+            [SupportedOSPlatform("windows")]
+            [RequiresUnreferencedCode(DistributedTransactionTrimmingWarning)]
+            set
+            {
+                lock (s_implicitDistributedTransactionsLock)
+                {
+                    // Make sure this flag can only be set once, and that once distributed transactions have been initialized,
+                    // it's frozen.
+                    if (s_implicitDistributedTransactions is null)
+                    {
+                        s_implicitDistributedTransactions = value;
+
+                        if (value)
+                        {
+                            DtcProxyShimFactory.s_transactionConnector ??= new DtcProxyShimFactory.DtcTransactionConnector();
+                        }
+                    }
+                    else if (value != s_implicitDistributedTransactions)
+                    {
+                        throw new InvalidOperationException(SR.ImplicitDistributedTransactionsCannotBeChanged);
+                    }
+                }
+            }
+        }
+
+        internal static bool? s_implicitDistributedTransactions;
+        internal static object s_implicitDistributedTransactionsLock = new();
+#else
+        public static bool ImplicitDistributedTransactions
+        {
+            get => false;
+
+            [SupportedOSPlatform("windows")]
+            [RequiresUnreferencedCode(DistributedTransactionTrimmingWarning)]
+            set
+            {
+                if (value)
+                {
+                    throw new PlatformNotSupportedException(SR.DistributedNotSupported);
+                }
+            }
+        }
+#endif
+
         // This routine writes the "header" for the recovery information, based on the
         // type of the calling object and its provided parameter collection.  This information
         // we be read back by the static Reenlist method to create the necessary transaction

src/libraries/System.Transactions.Local/tests/OleTxTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.IO;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Threading;
 using Microsoft.DotNet.RemoteExecutor;
@@ -492,6 +493,80 @@ public void GetDtcTransaction()
             Retry(() => Assert.Equal(TransactionStatus.Committed, tx.TransactionInformation.Status));
         });
 
+    [ConditionalFact(nameof(IsRemoteExecutorSupportedAndNotNano))]
+    public void Distributed_transactions_require_ImplicitDistributedTransactions_true()
+    {
+        // Temporarily skip on 32-bit where we have an issue.
+        if (!Environment.Is64BitProcess)
+        {
+            return;
+        }
+
+        using var _ = RemoteExecutor.Invoke(() =>
+        {
+            Assert.False(TransactionManager.ImplicitDistributedTransactions);
+
+            using var tx = new CommittableTransaction();
+
+            Assert.Throws<NotSupportedException>(MinimalOleTxScenario);
+        });
+    }
+
+    [ConditionalFact(nameof(IsRemoteExecutorSupportedAndNotNano))]
+    public void ImplicitDistributedTransactions_cannot_be_changed_after_being_set()
+    {
+        // Temporarily skip on 32-bit where we have an issue.
+        if (!Environment.Is64BitProcess)
+        {
+            return;
+        }
+
+        using var _ = RemoteExecutor.Invoke(() =>
+        {
+            TransactionManager.ImplicitDistributedTransactions = true;
+
+            Assert.Throws<InvalidOperationException>(() => TransactionManager.ImplicitDistributedTransactions = false);
+        });
+    }
+
+    [ConditionalFact(nameof(IsRemoteExecutorSupportedAndNotNano))]
+    public void ImplicitDistributedTransactions_cannot_be_changed_after_being_read_as_true()
+    {
+        // Temporarily skip on 32-bit where we have an issue.
+        if (!Environment.Is64BitProcess)
+        {
+            return;
+        }
+
+        using var _ = RemoteExecutor.Invoke(() =>
+        {
+            TransactionManager.ImplicitDistributedTransactions = true;
+
+            MinimalOleTxScenario();
+
+            Assert.Throws<InvalidOperationException>(() => TransactionManager.ImplicitDistributedTransactions = false);
+            TransactionManager.ImplicitDistributedTransactions = true;
+        });
+    }
+
+    [ConditionalFact(nameof(IsRemoteExecutorSupportedAndNotNano))]
+    public void ImplicitDistributedTransactions_cannot_be_changed_after_being_read_as_false()
+    {
+        // Temporarily skip on 32-bit where we have an issue.
+        if (!Environment.Is64BitProcess)
+        {
+            return;
+        }
+
+        using var _ = RemoteExecutor.Invoke(() =>
+        {
+            Assert.Throws<NotSupportedException>(MinimalOleTxScenario);
+
+            Assert.Throws<InvalidOperationException>(() => TransactionManager.ImplicitDistributedTransactions = true);
+            TransactionManager.ImplicitDistributedTransactions = false;
+        });
+    }
+
     private static void Test(Action action)
     {
         // Temporarily skip on 32-bit where we have an issue.
@@ -500,6 +575,8 @@ private static void Test(Action action)
             return;
         }
 
+        TransactionManager.ImplicitDistributedTransactions = true;
+
         // In CI, we sometimes get XACT_E_TMNOTAVAILABLE; when it happens, it's typically on the very first
         // attempt to connect to MSDTC (flaky/slow on-demand startup of MSDTC), though not only.
         // This catches that error and retries.
@@ -549,23 +626,25 @@ private static void Retry(Action action)
         }
     }
 
+    static void MinimalOleTxScenario()
+    {
+        using var tx = new CommittableTransaction();
+
+        var enlistment1 = new TestEnlistment(Phase1Vote.Prepared, EnlistmentOutcome.Committed);
+        var enlistment2 = new TestEnlistment(Phase1Vote.Prepared, EnlistmentOutcome.Committed);
+
+        tx.EnlistDurable(Guid.NewGuid(), enlistment1, EnlistmentOptions.None);
+        tx.EnlistDurable(Guid.NewGuid(), enlistment2, EnlistmentOptions.None);
+
+        tx.Commit();
+    }
+
     public class OleTxFixture
     {
         // In CI, we sometimes get XACT_E_TMNOTAVAILABLE on the very first attempt to connect to MSDTC;
         // this is likely due to on-demand slow startup of MSDTC. Perform pre-test connecting with retry
         // to ensure that MSDTC is properly up when the first test runs.
         public OleTxFixture()
-            => Test(() =>
-            {
-                using var tx = new CommittableTransaction();
-
-                var enlistment1 = new TestEnlistment(Phase1Vote.Prepared, EnlistmentOutcome.Committed);
-                var enlistment2 = new TestEnlistment(Phase1Vote.Prepared, EnlistmentOutcome.Committed);
-
-                tx.EnlistDurable(Guid.NewGuid(), enlistment1, EnlistmentOptions.None);
-                tx.EnlistDurable(Guid.NewGuid(), enlistment2, EnlistmentOptions.None);
-
-                tx.Commit();
-            });
+            => Test(MinimalOleTxScenario);
     }
 }