Fix RequestCores/ReleaseCores fallback in OOP TaskHost: throw NotImplementedException instead of logging error

[JanProvaznik]

Context

Fixes #13333

PR #13306 added RequestCores/ReleaseCores IPC callback support to OutOfProcTaskHostNode, correctly gating the new callback path behind CallbacksSupported. However, it also changed the fallback behavior (when callbacks are not supported) from hrow NotImplementedException() to LogErrorFromResource() + return 0.

This broke all callers. All known consumers (MonoAOTCompiler, EmccCompile, ILStrip, EmitBundleBase in dotnet/runtime) use this pattern:

try {
    allowedParallelism = be9.RequestCores(N);
} catch (NotImplementedException) {
    be9 = null; // fall back to own parallelism estimate
}
// ...
finally { be9?.ReleaseCores(allowedParallelism); }

After #13306, no exception is thrown, so the \catch\ block never fires. Instead:

1. *\LogErrorFromResource* silently pushes a \BuildErrorEventArgs\ — the build is doomed to fail and the task can't prevent it
2. *
   eturn 0* violates the API contract (\RequestCores\ guarantees return ∈ [1, requestedCores])

Changes Made

**\OutOfProcTaskHostNode.cs**: Restore \ hrow NotImplementedException()\ in both \RequestCores\ and \ReleaseCores\ fallback paths (CLR2 and !CallbacksSupported). This preserves the pre-#13306 behavior until the packet version is bumped to enable callbacks.

**\TaskHostTask.cs**: Default \grantedCores\ to \1\ (not \

[Copilot]

Pull request overview

Restores the historical fallback semantics for IBuildEngine9.RequestCores/ReleaseCores when OOP TaskHost callbacks are unavailable, ensuring real-world tasks that rely on catching NotImplementedException continue to work in cross-version scenarios.

Changes:

• Reverted OOP TaskHost RequestCores/ReleaseCores fallback from “log error + return 0” back to throwing NotImplementedException.
• Adjusted worker-side callback handling to default grantedCores to 1 for RequestCores (and 0 for ReleaseCores acknowledgments).
• Updated/added unit tests to cover the regression scenario and the common “catch + skip release” caller pattern.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/MSBuild/OutOfProcTaskHostNode.cs	Restores NotImplementedException fallback behavior for unsupported callback scenarios.
src/Build/Instance/TaskFactories/TaskHostTask.cs	Ensures a contract-friendly default for RequestCores responses when handling callback packets.
src/Build.UnitTests/BackEnd/TaskHostCallback_Tests.cs	Adds regression coverage for the cross-version callback-disabled behavior and the common caller pattern.
src/Build.UnitTests/BackEnd/RequestCoresWithFallbackTask.cs	Introduces a test task that mirrors real consumer fallback logic to validate behavior end-to-end.

---

You can also share your feedback on Copilot code review. Take the survey.

[rainersigwald]

fyi @lewing, this should be the durable fix for p3.