Enlighten AssignProjectConfiguration task for multithreaded mode

[jankratochvilcz]

Enlighten AssignProjectConfiguration task for multithreaded mode

Parent: #11834

Context

AssignProjectConfiguration is the highest-execution unmigrated task (22.9B total executions) listed in the migration epic (#11834) under "Other (either simple or with unknown issues)". It maps solution configuration/platform pairs onto project references, optionally adding synthetic project references for solution-level dependencies.

The task has no cwd-dependent operations:

• No Environment.CurrentDirectory, Path.GetFullPath, or File.*/Directory.* usage
• All path resolution uses MSBuild's GetMetadata("FullPath") which returns absolute paths resolved by the engine
• Platform mapping dictionaries are instance-scoped lazy-init — no cross-thread sharing
• The only static method (SetBuildInProjectAndReferenceOutputAssemblyMetadata) is pure logic with no mutable state
• Base class ResolveProjectBase uses reference.GetMetadata("FullPath") — engine-resolved absolute paths

This is an attribute-only migration: the task needs [MSBuildMultiThreadableTask] but does NOT need IMultiThreadableTask or TaskEnvironment because it performs no file system operations, environment variable access, or process launches.

Approach

Apply the PR #13045 (attribute-only) pattern: add [MSBuildMultiThreadableTask] to the class declaration. No IMultiThreadableTask implementation is needed since the task has no cwd-dependent operations.

1. Decorate AssignProjectConfiguration with [MSBuildMultiThreadableTask].
2. No other code changes required — the task is already thread-safe by design.

ChangeWave consideration

No behavioral change — the attribute simply declares the task safe for multithreaded scheduling. Observable behavior (return value, [Output] properties, error/warning messages) is byte-identical. No ChangeWave needed.

Test coverage assessment

Existing unit tests (src/Tasks.UnitTests/AssignProjectConfiguration_Tests.cs)

Existing tests cover configuration assignment, platform mapping, synthetic project references, and edge cases. Coverage is good for the functional behavior.

Gaps to fill in this PR

• G1 — Concurrency test. Two AssignProjectConfiguration instances with different SolutionConfigurationContents and ProjectReferences execute concurrently, asserting each produces correct, independent output. Verifies no shared mutable state interference.
• G2 — Baseline parity. Verify all existing tests continue to pass with no behavioral changes. [Output] AssignedProjects items have identical metadata (Configuration, Platform, SetConfiguration, SetPlatform, etc.) before and after the attribute addition.

Acceptance criteria

• AssignProjectConfiguration decorated with [MSBuildMultiThreadableTask].
• No IMultiThreadableTask implementation (not needed — no cwd-dependent operations).
• All existing AssignProjectConfiguration_Tests pass on net472 and net10.0.
• [Output] AssignedProjects 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 AssignProjectConfiguration.
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

• Base class ResolveProjectBase is also used by ResolveNonMSBuildProjectOutput. The attribute has Inherited = false, so each concrete class must be independently decorated — no implicit inheritance of the attribute from the base class. This is correct and intentional.
• AddSyntheticProjectReferences mutates the _projectReferences array via ResolveProjectBase. This is instance state, not static — safe under the per-instance-per-thread model.

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
  • Enlighten FindUnderPath and AssignTargetPath tasks #13069 — Enlighten FindUnderPath and AssignTargetPath
• SKILL: .github/skills/multithreaded-task-migration/SKILL.md