fix: respect placement hints in local-first directors

src/Orleans.Runtime/Placement/PreferLocalPlacementDirector.cs

@@ -19,8 +19,16 @@ internal class PreferLocalPlacementDirector : RandomPlacementDirector, IPlacemen
         public override Task<SiloAddress> 
             OnAddActivation(PlacementStrategy strategy, PlacementTarget target, IPlacementContext context)
         {
+            var compatibleSilos = context.GetCompatibleSilos(target);
+
+            // If a valid placement hint was specified, use it.
+            if (IPlacementDirector.GetPlacementHint(target.RequestContextData, compatibleSilos) is { } placementHint)
+            {
+                return Task.FromResult(placementHint);
+            }
+
             // if local silo is not active or does not support this type of grain, revert to random placement
-            if (context.LocalSiloStatus != SiloStatus.Active || !context.GetCompatibleSilos(target).Contains(context.LocalSilo))
+            if (context.LocalSiloStatus != SiloStatus.Active || !compatibleSilos.Contains(context.LocalSilo))
             {
                 return base.OnAddActivation(strategy, target, context);
             }

src/Orleans.Runtime/Placement/StatelessWorkerDirector.cs

@@ -9,6 +9,12 @@ public Task<SiloAddress> OnAddActivation(PlacementStrategy strategy, PlacementTa
         {
             var compatibleSilos = context.GetCompatibleSilos(target);
 
+            // If a valid placement hint was specified, use it.
+            if (IPlacementDirector.GetPlacementHint(target.RequestContextData, compatibleSilos) is { } placementHint)
+            {
+                return Task.FromResult(placementHint);
+            }
+
             // If the current silo is not shutting down, place locally if we are compatible
             if (!context.LocalSiloStatus.IsTerminating())
             {

test/Orleans.Core.Tests/Runtime/LocalPlacementDirectorTests.cs

@@ -0,0 +1,58 @@
+using System.Collections.Generic;
+using NSubstitute;
+using Orleans.Runtime;
+using Orleans.Runtime.Placement;
+using Xunit;
+
+namespace UnitTests.Runtime
+{
+    [TestCategory("BVT"), TestCategory("Placement")]
+    public class LocalPlacementDirectorTests
+    {
+        [Fact]
+        public async Task PreferLocalPlacementDirector_UsesPlacementHintWhenLocalSiloIsCompatible()
+        {
+            var localSilo = Silo("127.0.0.1:100@1");
+            var hintedSilo = Silo("127.0.0.1:101@1");
+            var director = new PreferLocalPlacementDirector();
+            var placementContext = CreatePlacementContext(localSilo, SiloStatus.Active, localSilo, hintedSilo);
+            var target = CreateTarget(hintedSilo);
+
+            var result = await director.OnAddActivation(strategy: null!, target, placementContext);
+
+            Assert.Equal(hintedSilo, result);
+        }
+
+        [Fact]
+        public async Task StatelessWorkerDirector_UsesPlacementHintWhenLocalSiloIsCompatible()
+        {
+            var localSilo = Silo("127.0.0.1:100@1");
+            var hintedSilo = Silo("127.0.0.1:101@1");
+            var director = new StatelessWorkerDirector();
+            var placementContext = CreatePlacementContext(localSilo, SiloStatus.Active, localSilo, hintedSilo);
+            var target = CreateTarget(hintedSilo);
+
+            var result = await director.OnAddActivation(strategy: null!, target, placementContext);
+
+            Assert.Equal(hintedSilo, result);
+        }
+
+        private static IPlacementContext CreatePlacementContext(SiloAddress localSilo, SiloStatus localSiloStatus, params SiloAddress[] compatibleSilos)
+        {
+            var placementContext = Substitute.For<IPlacementContext>();
+            placementContext.LocalSilo.Returns(localSilo);
+            placementContext.LocalSiloStatus.Returns(localSiloStatus);
+            placementContext.GetCompatibleSilos(Arg.Any<PlacementTarget>()).Returns(compatibleSilos);
+            return placementContext;
+        }
+
+        private static PlacementTarget CreateTarget(SiloAddress placementHint) =>
+            new(
+                GrainId.Create("test", "grain-1"),
+                new Dictionary<string, object> { [IPlacementDirector.PlacementHintKey] = placementHint },
+                default,
+                0);
+
+        private static SiloAddress Silo(string value) => SiloAddress.FromParsableString(value);
+    }
+}