Fix race condition in WriteLinesToFile transactional mode (#13323)

[huulinhnguyen-dev]

Fixes #13323

Context

Concurrent parallel builds writing to the same file with Overwrite="true" and transactional mode could throw IOException: Cannot create a file when that file already exists. This happened because File.Move failed when another thread created the target file between the Replace check and the Move call.

Changes Made

• In WriteLinesToFile.cs: when File.Move fails with IOException, retry using File.Replace since the target file now exists
• In WriteLinesToFile_Tests.cs: rename TransactionalModePreservesAllData → TransactionalModeSucceedsWithConcurrentOverwrites to accurately reflect test behavior (Overwrite="true" means only the last writer survives, not all data)

Testing

Existing test TransactionalModeSucceedsWithConcurrentOverwrites covers the concurrent overwrite scenario with parallel MSBuild projects.

[Copilot]

Pull request overview

Fixes a race in the built-in WriteLinesToFile task’s transactional overwrite path where concurrent writers could cause File.Move to fail with “file already exists”, by falling back to File.Replace when the destination appears mid-operation.

Changes:

• Add a transactional overwrite recovery path: if File.Move fails and the destination now exists, retry via File.Replace with a small bounded retry loop.
• Rename the flaky/misleading concurrent overwrite test to reflect actual Overwrite="true" semantics.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Tasks/FileIO/WriteLinesToFile.cs	Adds a File.Replace-based fallback when Move fails due to a concurrent create, improving reliability under parallel builds.
src/Tasks.UnitTests/WriteLinesToFile_Tests.cs	Renames a test to better describe the concurrent overwrite behavior being validated.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.