Re-issue session isolation level on TransactionScope re-enlistment (fixes #146)

[priyankatiwari08]

Summary

Fix #146 — TransactionScope ambient isolation level is silently downgraded after the first pooled connection re-checkout on Azure SQL DB.

Repro

using var scope = new TransactionScope(
    TransactionScopeOption.Required,
    new TransactionOptions { IsolationLevel = IsolationLevel.Serializable },
    TransactionScopeAsyncFlowOption.Enabled);

for (int i = 0; i < 3; i++)
{
    using var c = new SqlConnection(azureConnStringWithMaxPoolSize1);
    await c.OpenAsync();
    // DBCC USEROPTIONS:
    //   i==0 -> serializable
    //   i>=1 -> read committed snapshot   <-- bug
}

On on-prem SQL Server the level survives and all three opens report serializable. On Azure SQL DB the second and subsequent opens report the database default isolation (e.g. read committed snapshot).

Root cause

1. First Open() inside the scope enlists the connection and sends SET TRANSACTION ISOLATION LEVEL <ambient>;.
2. On Close() the physical connection is returned to the transacted pool still enlisted in the same Transaction.
3. Second Open() reuses the same physical connection. SqlInternalConnectionTds.Enlist(Transaction) sees transaction.Equals(EnlistedTransaction) and short-circuits — no SET is sent.
4. A pending sp_reset_connection_keep_transaction piggybacks on the next batch. On Azure SQL DB this reset clears the session isolation level to the database default; on on-prem SQL Server it does not, which is why the bug is Azure-only in practice.

Fix

On the short-circuit path in Enlist, when reset is pending and the new behavior switch is enabled, re-issue:

SET TRANSACTION ISOLATION LEVEL <ambient>;

mapped from Transaction.IsolationLevel. The statement is queued onto the same TDS batch as the reset, so there is no extra round trip.

Gated behind a new AppContext switch for back-compat:

• Switch.Microsoft.Data.SqlClient.UseLegacyTransactionScopeIsolationBehavior (default false).

Same back-compat pattern used by the related companion PR #4330 (UseLegacyIsolationLevelBehavior).

Files changed

• src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs — new switch.
• src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/Connection/SqlConnectionInternal.cs — new re-attach branch + ReassertSessionIsolationLevel helper.
• src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/TransactionTest/TransactionScopeIsolationReassertTest.cs — new ManualTests, gated on IsAzureServer.
• src/Microsoft.Data.SqlClient/tests/ManualTests/Microsoft.Data.SqlClient.ManualTests.csproj — wire-up.

Validation

End-to-end repro against both back ends with Pooling=true; Max Pool Size=1, opening 3 connections per scope.

Scenario	Conn 1	Conn 2	Conn 3
On-prem SQL Server, default	serializable	serializable	serializable
Azure SQL DB, default (fix)	serializable	serializable	serializable
Azure SQL DB, legacy switch on	serializable	read committed snapshot	read committed snapshot

Build clean on net462, net8.0, net9.0 (0 warnings, 0 errors).

Notes

• New ManualTests are gated on DataTestUtility.IsAzureServer because the bug does not manifest against on-prem SQL Server.
• Companion PR for the related but distinct cross-user pool leak: Reset session isolation level on pool return (fixes #96) #4330.

[Copilot]

Pull request overview

Fixes an Azure SQL DB-specific TransactionScope pooling regression where session isolation level can revert to the database default after transacted-pool re-checkout by re-asserting the ambient isolation level during re-enlistment.

Changes:

• Added a new AppContext switch (Switch.Microsoft.Data.SqlClient.UseLegacyTransactionScopeIsolationBehavior) to gate the new re-assert behavior.
• Updated SqlInternalConnectionTds.Enlist(Transaction) to re-issue SET TRANSACTION ISOLATION LEVEL ... on the “same transaction” short-circuit path (intended to piggyback on the pending reset).
• Added new Azure-gated ManualTests and wired them into the ManualTests project.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/Connection/SqlConnectionInternal.cs	Adds re-attach logic to re-assert session isolation level when re-enlisting into the same ambient transaction.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs	Introduces a new AppContext switch to enable legacy behavior.
src/Microsoft.Data.SqlClient/tests/ManualTests/SQL/TransactionTest/TransactionScopeIsolationReassertTest.cs	Adds ManualTests validating isolation level stability across pooled re-opens inside a TransactionScope (Azure-only).
src/Microsoft.Data.SqlClient/tests/ManualTests/Microsoft.Data.SqlClient.ManualTests.csproj	Includes the new ManualTests source file in the build.