[Flaky Test] Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeSucceedsWithConcurrentOverwrites

**Flaky-test key** (automated de-duplication — do not edit):
```text
flaky-test-id: Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeSucceedsWithConcurrentOverwrites
```

## Evidence

- **Distinct sources:** 3 (0 PRs + 3 rolling main builds)
- **Rolling builds:** [1496519](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1496519), [1506834](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1506834), [1514373](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1514373)
- **Legs:** FullOnWindows, FullOnWindows Release
- **TFM:** net472
- **Assembly:** Microsoft.Build.Tasks.UnitTests
- **First seen:** 2026-07-07 | **Last seen:** 2026-07-17
- **Error hashes:** 2 distinct (24ca1695, 321cf66e) — multiple distinct failure signatures confirm non-determinism
- **Sample build:** [1496519](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1496519)

**Sample error:**
```
Microsoft.Build.Exceptions.BuildAbortedException: Build was canceled. Failed to successfully launch or connect to a child MSBuild.exe process.
```

## Notes

This test will be quarantined via `[ActiveIssue("https://github.com/dotnet/msbuild/issues/")]` (from `Microsoft.DotNet.XUnitV3Extensions`, namespace `Xunit`) until the underlying nondeterminism is fixed.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [Flaky Test Triage](https://github.com/dotnet/msbuild/actions/runs/29643670790) · 114.5 AIC · ⌖ 4.91 AIC · ⊞ 13.5K · [◷](https://github.com/search?q=repo%3Adotnet%2Fmsbuild+is%3Aissue+%22gh-aw-workflow-call-id%3A+dotnet%2Fmsbuild%2Fflaky-test-detector.agent%22&type=issues)

Locate Microsoft.Build.Tasks.UnitTests.WriteLinesToFile_Tests.TransactionalModeSucceedsWithConcurrentOverwrites in the Microsoft.Build.Tasks.UnitTests assembly and reproduce the net472 failure using the linked rolling builds. Read the test's transactional concurrent-overwrite path and investigate both reported BuildAbortedException signatures. Done means the test is deterministic across the affected Windows legs and no longer requires ActiveIssue quarantine.

Written by the indexing model from the issue text.