Add Effect.FlushlessCreateNextChild for appending numbered child effects

Introduces a flushless primitive that finds the next free direct-child slot
under a given parent EffectId (highest existing child index + 1) and records
the provided content there as a completed effect, returning the new child's
EffectId. Index-selection and reservation happen under a single lock so
concurrent callers cannot collide on the same index.

Generalizes the hand-rolled next-child bookkeeping in IdempotencyKeys.

Author: stidsborg

Core/Cleipnir.ResilientFunctions.Tests/InMemoryTests/CreateNextChildTests.cs

@@ -0,0 +1,108 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using Cleipnir.ResilientFunctions.CoreRuntime;
+using Cleipnir.ResilientFunctions.CoreRuntime.Serialization;
+using Cleipnir.ResilientFunctions.Domain;
+using Cleipnir.ResilientFunctions.Helpers;
+using Cleipnir.ResilientFunctions.Storage;
+using Cleipnir.ResilientFunctions.Tests.Utils;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Cleipnir.ResilientFunctions.Tests.InMemoryTests;
+
+[TestClass]
+public class CreateNextChildTests
+{
+    private static Effect CreateEffect(IReadOnlyList<StoredEffect> existingEffects)
+    {
+        var storedId = TestStoredId.Create();
+        var effectResults = new EffectResults(
+            TestFlowId.Create(),
+            storedId,
+            existingEffects,
+            new InMemoryFunctionStore().EffectsStore,
+            DefaultSerializer.Instance,
+            storageSession: null,
+            clearChildren: true
+        );
+
+        return new Effect(
+            effectResults,
+            utcNow: () => DateTime.UtcNow,
+            new FlowTimeouts(),
+            new FlowExecutionState(storedId, subflows: 1, waitingSubflows: 0, new FlowTimeouts(), completed: ForeverTask.Instance)
+        );
+    }
+
+    [TestMethod]
+    public void CreateNextChildAppendsSequentiallyFromZero()
+    {
+        var effect = CreateEffect(existingEffects: new List<StoredEffect>());
+        var parent = new EffectId([1]);
+
+        var first = effect.FlushlessCreateNextChild(parent, content: "a");
+        var second = effect.FlushlessCreateNextChild(parent, content: "b");
+        var third = effect.FlushlessCreateNextChild(parent, content: "c");
+
+        first.ShouldBe(parent.CreateChild(0));
+        second.ShouldBe(parent.CreateChild(1));
+        third.ShouldBe(parent.CreateChild(2));
+
+        // Flushless writes are visible in-memory immediately (no Flush required).
+        effect.Get<string>(first).ShouldBe("a");
+        effect.Get<string>(second).ShouldBe("b");
+        effect.Get<string>(third).ShouldBe("c");
+    }
+
+    [TestMethod]
+    public void CreateNextChildContinuesFromExistingChildren()
+    {
+        var parent = new EffectId([7]);
+        var existing = new List<StoredEffect>
+        {
+            StoredEffect.CreateCompleted(parent.CreateChild(0), alias: null),
+            StoredEffect.CreateCompleted(parent.CreateChild(1), alias: null),
+        };
+        var effect = CreateEffect(existing);
+
+        var next = effect.FlushlessCreateNextChild(parent, content: "x");
+
+        next.ShouldBe(parent.CreateChild(2));
+    }
+
+    [TestMethod]
+    public async Task CreateNextChildKeepsGapAndUsesHighestPlusOne()
+    {
+        var effect = CreateEffect(existingEffects: new List<StoredEffect>());
+        var parent = new EffectId([1]);
+
+        effect.FlushlessCreateNextChild(parent, content: "a"); // [1,0]
+        var middle = effect.FlushlessCreateNextChild(parent, content: "b"); // [1,1]
+        effect.FlushlessCreateNextChild(parent, content: "c"); // [1,2]
+
+        await effect.Clear(middle, flush: true); // remove [1,1], leaving {0,2}
+
+        var next = effect.FlushlessCreateNextChild(parent, content: "d");
+
+        // Highest existing index (2) + 1 -> gap at index 1 is kept.
+        next.ShouldBe(parent.CreateChild(3));
+    }
+
+    [TestMethod]
+    public void CreateNextChildIsScopedToItsParent()
+    {
+        var effect = CreateEffect(existingEffects: new List<StoredEffect>());
+        var parentA = new EffectId([1]);
+        var parentB = new EffectId([2]);
+
+        var a0 = effect.FlushlessCreateNextChild(parentA, content: "a0");
+        var a1 = effect.FlushlessCreateNextChild(parentA, content: "a1");
+        var b0 = effect.FlushlessCreateNextChild(parentB, content: "b0");
+
+        a0.ShouldBe(parentA.CreateChild(0));
+        a1.ShouldBe(parentA.CreateChild(1));
+        b0.ShouldBe(parentB.CreateChild(0));
+    }
+}

Core/Cleipnir.ResilientFunctions/Domain/Effect.cs

@@ -117,6 +117,15 @@ internal T Get<T>(EffectId effectId)
 
     internal IReadOnlyList<EffectId> GetChildren(EffectId effectId) => effectResults.GetChildren(effectId);
 
+    /// <summary>
+    /// Finds the next free child slot under <paramref name="parentId"/> (highest existing direct-child index + 1)
+    /// and records <paramref name="content"/> there as a completed effect, returning the created child's EffectId.
+    /// The write is flushless (in-memory only) - call <see cref="Flush"/> to persist. This is a raw append: each
+    /// call adds a new child, so it is not replay-idempotent on its own.
+    /// </summary>
+    public EffectId FlushlessCreateNextChild<T>(EffectId parentId, T content, string? alias = null)
+        => effectResults.FlushlessCreateNextChild(parentId, content, alias);
+
     #region Implicit ids
 
     public Task Capture(Action work, ResiliencyLevel resiliency = ResiliencyLevel.AtLeastOnce)

Core/Cleipnir.ResilientFunctions/Domain/EffectResults.cs

@@ -231,7 +231,26 @@ public IReadOnlyList<EffectId> GetChildren(EffectId parentId)
                 .Where(id => id.IsDescendant(parentId))
                 .ToList();
     }
-    
+
+    public EffectId FlushlessCreateNextChild<T>(EffectId parentId, T value, string? alias)
+    {
+        lock (_sync)
+        {
+            // Highest existing direct-child index + 1 (append). Index-selection and reservation
+            // happen under a single lock so concurrent callers cannot pick the same index.
+            var nextIndex = 0;
+            foreach (var id in _effectResults.Keys)
+                if (parentId.IsChild(id) && id.Id >= nextIndex)
+                    nextIndex = id.Id + 1;
+
+            var childId = parentId.CreateChild(nextIndex);
+            var serializedValue = _serializer.Serialize(value!, typeof(T));
+            var storedEffect = StoredEffect.CreateCompleted(childId, serializedValue, alias);
+            AddToPendingCore(childId, storedEffect, delete: false, clearChildren: false);
+            return childId;
+        }
+    }
+
     public async Task InnerCapture(EffectId effectId, string? alias, Func<Task> work, ResiliencyLevel resiliency, EffectContext effectContext)
     {
         EffectContext.SetParent(effectId);
@@ -417,6 +436,12 @@ private void AddToPending(IEnumerable<StoredEffect> storedEffects)
     private void AddToPending(EffectId effectId, StoredEffect? storedEffect, bool delete, bool clearChildren)
     {
         lock (_sync)
+            AddToPendingCore(effectId, storedEffect, delete, clearChildren);
+    }
+
+    // Assumes _sync is already held by the caller.
+    private void AddToPendingCore(EffectId effectId, StoredEffect? storedEffect, bool delete, bool clearChildren)
+    {
         {
             if (_effectResults.ContainsKey(effectId))
             {