Enlighten CallTarget task for multithreaded mode

[jankratochvilcz]

Enlighten CallTarget task for multithreaded mode

Parent: #11834

Context

CallTarget is listed in the migration epic (#11834) under "No Modifications Required (added attribute only)" but remains unchecked. It invokes other targets within the same project file by delegating to MSBuild.ExecuteTargets and MSBuild.CreateTargetLists.

The task has zero cwd-dependent operations:

• No file system access of any kind
• No Environment.CurrentDirectory, Path.GetFullPath, or environment variable usage
• No ProcessStartInfo usage
• All work is delegated to the MSBuild engine via BuildEngine3
• Instance state is a single List<ITaskItem> for collecting target outputs
• Already decorated with [RunInMTA], indicating awareness of threading concerns

This is the simplest possible migration — attribute-only with no code changes.

Approach

Apply the PR #13045 (attribute-only) pattern: add [MSBuildMultiThreadableTask] to the class declaration. No IMultiThreadableTask implementation is needed.

1. Decorate CallTarget with [MSBuildMultiThreadableTask].
2. No other code changes required.

ChangeWave consideration

No behavioral change. The attribute declares the task safe for multithreaded scheduling. Observable behavior is byte-identical. No ChangeWave needed.

Test coverage assessment

Existing unit tests

CallTarget is tested via integration tests that exercise target invocation, RunEachTargetSeparately, and output collection.

Gaps to fill in this PR

• G1 — Concurrency test. Two CallTarget instances targeting different targets execute concurrently, asserting each collects correct, independent TargetOutputs. Verifies no shared mutable state interference.
• G2 — Baseline parity. Verify all existing tests continue to pass. [Output] TargetOutputs items are identical before and after.

Acceptance criteria

• CallTarget decorated with [MSBuildMultiThreadableTask].
• No IMultiThreadableTask implementation (not needed).
• All existing tests pass on net472 and net10.0.
• [Output] TargetOutputs items have byte-identical ItemSpec and metadata.
• Concurrency test (G1) added and passes.
• No new compiler warnings (warnings-as-errors in official build).
• multithreaded-task-migration SKILL sign-off checklist walked and passes.

Implementation order

1. Apply [MSBuildMultiThreadableTask] to CallTarget.
2. Add G1 (concurrency) test.
3. Run Tasks.UnitTests; verify clean.
4. Run repo build with -v quiet to ensure no new warnings.

Risks / open questions

• CallTarget delegates to MSBuild.ExecuteTargets, which itself should be enlightened separately (tracked in Migrate Tasks in the MSbuild repo to the new Task type #11834 under "Tasks with simple transition"). The CallTarget migration is independent — it only declares that CallTarget itself has no thread-unsafe operations. The engine handles scheduling of the called targets.
• The [RunInMTA] attribute is already present and compatible with [MSBuildMultiThreadableTask].

References

• Parent epic: Migrate Tasks in the MSbuild repo to the new Task type #11834
• Pattern PRs:
  • Enlighten more tasks that require no change #13045 — Enlighten more tasks that require no change (attribute-only baseline)
  • Enlighten public versions of intrinsic tasks. #13267 — Enlighten public versions of intrinsic tasks
• SKILL: .github/skills/multithreaded-task-migration/SKILL.md