Test | Fix Transient Fault handling and other flaky unit tests

[cheenamalhotra]

[Attempted fix with Copilot]

The Problem

When SqlConnection.Open() runs on .NET Framework (net462), the internal LoginNoFailover method enters parallel/interval-timer mode by default because TNIR (Transparent Network IP Resolution) is enabled. LoginWithFailover always uses interval timers on all platforms.

These interval timers give each login attempt only a small fraction of the total ConnectTimeout:

Path	Fraction	With 30s timeout
LoginWithFailover	8% (FailoverTimeoutStep)	2.4s per attempt
LoginNoFailover + TNIR	12.5% (FailoverTimeoutStepForTnir)	1.875s per attempt

On a busy Windows CI machine, when one of these interval timers expires mid-login, the client disconnects and retries internally inside the login loop — before the outer transient-fault retry loop ever sees the error. This produces an extra PreLogin on the TDS server that never gets a corresponding Login7 (the client abandoned the connection).

Normal flow (2 PreLogins):

1. PreLogin → Login7 → transient error 40613 → outer retry
2. PreLogin → Login7 → success

Busy CI flow (3 PreLogins):

1. PreLogin → Login7 → transient error 40613 → outer retry
2. PreLogin → interval timer expires before Login7 completes → client disconnects → internal retry
3. PreLogin → Login7 → success

The tests asserted Assert.Equal(2, server.PreLoginCount) — an exact count that doesn't account for these platform-specific internal retries. On .NET (net8.0/net9.0), TNIR doesn't exist and LoginNoFailover only uses interval timers when MultiSubnetFailover=true, so the tests pass reliably there.

The fix adds a Login7Count counter to the test TDS server so we can compute AbandonedPreLoginCount (PreLogins with no Login7), then asserts on PreLoginCount - AbandonedPreLoginCount — giving an exact, deterministic count regardless of how many internal timeout retries occurred.

Additional fixes

• Fixes the multipart identifier getting skipped due to duplicate IDs:

[xUnit.net 00:00:03.53] UnitTests: Skipping test case with duplicate ID '287fad04d43fa7e5d02c3f11743f0138e5e6db40' ('Microsoft.Data.Common.UnitTests.MultipartIdentifierTests.SinglePartIdentifierWithBracketsAndWhiteSpace_ParsesSuccessfully(partIdentifier: "word1", expected: ["word1"])' and 'Microsoft.Data.Common.UnitTests.MultipartIdentifierTests.SinglePartIdentifierWithBracketsAndWhiteSpace_ParsesSuccessfully(partIdentifier: "word1", expected: ["word1"])')

• Fixes failover port test failure, by ensuring connection pools are cleared before connecting to failover partner.

• Fixes SqlTypeWorkaroundsTests discovery by setting DisableDiscoveryEnumeration to true.

[Copilot]

Pull request overview

Updates the simulated TDS server test infrastructure and transient-fault unit tests to avoid flaky assertions on exact PreLogin counts on .NET Framework where internal TNIR/interval-timer retries can introduce extra pre-login attempts.

Changes:

• Add Login7Count tracking to GenericTdsServer and adjust TransientTdsErrorTdsServer to increment it when bypassing base.OnLogin7Request.
• Introduce AbandonedPreLoginCount (PreLogins not followed by Login7) for more deterministic test assertions.
• Remove flaky traits from transient-fault simulated server tests and update assertions to exclude “abandoned” prelogins.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/tools/TDS/TDS.Servers/TransientTdsErrorTdsServer.cs	Increments Login7 counter on error path where base handler is skipped.
src/Microsoft.Data.SqlClient/tests/tools/TDS/TDS.Servers/GenericTdsServer.cs	Adds Login7 and abandoned-prelogin counting to support deterministic login-attempt assertions.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionTests.cs	Updates transient-fault assertions to use adjusted login-attempt counting; removes flaky traits.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionRoutingTestsAzure.cs	Updates routing transient-fault assertions to use adjusted login-attempt counting; removes flaky trait.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionRoutingTests.cs	Updates routing transient-fault assertions to use adjusted login-attempt counting; removes flaky trait.
src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionFailoverTests.cs	Updates transient-fault login-attempt assertion to use adjusted counting; removes flaky trait.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.42%. Comparing base (60d4b92) to head (e7dd518).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4080      +/-   ##
==========================================
- Coverage   73.22%   68.42%   -4.81%     
==========================================
  Files         280      275       -5     
  Lines       43000    66924   +23924     
==========================================
+ Hits        31486    45791   +14305     
- Misses      11514    21133    +9619     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	68.42% <ø> (?)

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[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.