Guard ActivelyBuildingTargets removal against stale request cleanup after cancellation timeout

[Copilot]

Context

BuildManager.Reset() clears BuildRequestConfiguration.ActivelyBuildingTargets at end-of-build, but TargetBuilder removed entries from that table by target name only, ignoring the owning request ID. If a request's cancellation times out while the task keeps running, it can resume later and remove an entry that a newer request (reusing the same retained configuration) has since recorded for the same target name — making the configuration falsely appear inactive. This can let another request build the same target concurrently, corrupt circular-dependency detection, or let CacheIfPossible() cache the ProjectInstance while it's still in use.

Changes Made

• BuildRequestConfiguration: added RemoveActivelyBuildingTargetIfOwnedBy(targetName, globalRequestId), which only removes the entry if it is still recorded as owned by the given request ID.
• TargetBuilder: replaced all 4 unconditional ActivelyBuildingTargets.Remove(name) call sites (end-of-build stack cleanup, error-execution catch, target-completed cleanup, dependency-pop-on-failure) with the new guarded removal.

// Before: any request could clear a target owned by someone else
_requestEntry.RequestConfiguration.ActivelyBuildingTargets.Remove(currentTargetEntry.Name);

// After: only removes if still owned by the calling request
_requestEntry.RequestConfiguration.RemoveActivelyBuildingTargetIfOwnedBy(
    currentTargetEntry.Name, _requestEntry.Request.GlobalRequestId);

Testing

Added a unit test in BuildRequestConfiguration_Tests verifying a stale owner's removal is a no-op while the current owner's removal succeeds. Existing TargetBuilder_Tests and RequestBuilder_Tests continue to pass.

Notes

No behavioral change for the common case where the current request still owns the entry; this only prevents cross-request interference during the stale-cleanup race described in the issue.

• Fixes An active ProjectInstance can be cached after stale  /mt  request cleanup #14694

[dotnet-policy-service]

Hello @copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.