[rc2] Fix concurrency issue in RelationalConnection ambient transaction handling

[Copilot]

Fixes #35856

Description
There was a race condition between ClearTransactions and HandleTransactionCompleted methods in RelationalConnection that could cause exceptions in distributed transaction scenarios accessing _ambientTransactions from different threads.

Customer impact
The issue affects applications using distributed transactions with MSDTC. Customers experience random application failures with no clear workaround other than avoiding distributed transactions.

How found
Customer reported on EF8.

Regression
Yes, from EF Core 3.1. Introduced in #20012

Testing
Test added

Risk
Low. Changes are focused on improving thread safety of existing ambient transaction functionality. The synchronization is only applied when ambient transactions are used, minimizing performance impact on regular database operations.

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[AndriySvyryd]

@copilot Rewrite the PR summary using this template:

Fixes #{number}

**Description**
{Issue description}

**Customer impact**
{How does the reported issue affects customer's app? Are there workarounds?}

**How found**
{Was it customer reported or found during verification? How many customers are affected?}

**Regression**
{Is it a regression from a released version? Which one?}

**Testing**
{How the changes were tested}

**Risk**
{Low/Medium/High, argument why}

[Copilot]

@copilot Rewrite the PR summary using this template:

...

Here's the updated PR description using the template:

Fixes #35856

Description
Race condition between ClearTransactions and HandleTransactionCompleted methods in RelationalConnection that could cause InvalidOperationException: A root ambient transaction was completed before the nested transaction errors in distributed transaction scenarios.

Customer impact
The issue affects applications using distributed transactions with MSDTC where HandleTransactionCompleted can be invoked on a different thread at arbitrary times after transaction completion. This results in intermittent InvalidOperationException errors when concurrent access occurs to the ambient transaction stack. Customers may experience random application failures with no clear workaround other than avoiding distributed transactions.

How found
Customer reported issue through GitHub issue #35856. Based on the error pattern and MSDTC usage, this likely affects customers using distributed transactions in production environments.

Regression
This appears to be a long-standing issue in the ambient transaction handling code, not a recent regression from a specific release.

Testing

• Added comprehensive thread safety test that exercises concurrent connection operations and transaction scopes
• Verified fix prevents race conditions through proper synchronization with System.Threading.Lock
• All existing tests pass with the new synchronization mechanism

Risk
Low - Changes are focused on improving thread safety of existing ambient transaction functionality. The synchronization is only active when ambient transactions are used, minimizing performance impact on regular database operations. No breaking API changes.

[artl93]

Customer reported regression. Customer reports it happening "randomly," which makes sense for this bug.

This is gonna be hard to know if we fixed it or if we might have another lingering regression behind this one.

From a risk perspective, this seems like a low level concurrency fix that if it weren't customer reported, I don't think we would want to take this risk right now. Is there anyway we can validate that this directly fixes the customer report?

[AndriySvyryd]

From a risk perspective, this seems like a low level concurrency fix that if it weren't customer reported, I don't think we would want to take this risk right now. Is there anyway we can validate that this directly fixes the customer report?

I wasn't able to repro it with MSDTC as in the report. The test does hit it, but only when run in the debugger. However, I found that the original fix introduced a deadlock since the lock is around an event handler being unsubscribed and that waits for the handler to finish executing, but it was waiting on the same lock.

So, seeing that this only is a problem that could happen when the connection is being disposed, I implemented a less risky fix without synchronization that just avoids throwing an exception in this case.

[artl93]

Approved, RC2. I appreciate that we can get this under the debugger and that we’re making a more narrow fix.