From d5e94b9be344d6ca5391bb4471b49842b711d101 Mon Sep 17 00:00:00 2001
From: Kevin Hahn <kevin_hahn@sil.org>
Date: Wed, 22 Jan 2025 15:45:41 +0700
Subject: [PATCH] avoid reference cycles between ComplexForms by marking the
 component as deleted if creating it would result in a reference cycle

---
 .../Entries/AddEntryComponentChange.cs        | 34 ++++++++++++++++---
 backend/harmony                               |  2 +-
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/backend/FwLite/LcmCrdt/Changes/Entries/AddEntryComponentChange.cs b/backend/FwLite/LcmCrdt/Changes/Entries/AddEntryComponentChange.cs
index f12bb9d85..b6fe66f2f 100644
--- a/backend/FwLite/LcmCrdt/Changes/Entries/AddEntryComponentChange.cs
+++ b/backend/FwLite/LcmCrdt/Changes/Entries/AddEntryComponentChange.cs
@@ -38,7 +38,10 @@ public override async ValueTask<ComplexFormComponent> NewEntity(Commit commit, C
         Sense? componentSense = null;
         if (ComponentSenseId is not null)
             componentSense = await context.GetCurrent<Sense>(ComponentSenseId.Value);
-        return new ComplexFormComponent
+        var shouldBeDeleted = (complexFormEntry?.DeletedAt is not null ||
+                               componentEntry?.DeletedAt is not null ||
+                               (ComponentSenseId.HasValue && componentSense?.DeletedAt is not null));
+        var component = new ComplexFormComponent
         {
             Id = EntityId,
             ComplexFormEntryId = ComplexFormEntryId,
@@ -46,11 +49,34 @@ public override async ValueTask<ComplexFormComponent> NewEntity(Commit commit, C
             ComponentEntryId = ComponentEntryId,
             ComponentHeadword = componentEntry?.Headword(),
             ComponentSenseId = ComponentSenseId,
-            DeletedAt = (complexFormEntry?.DeletedAt is not null ||
-                         componentEntry?.DeletedAt is not null ||
-                         (ComponentSenseId.HasValue && componentSense?.DeletedAt is not null))
+            DeletedAt = shouldBeDeleted
                 ? commit.DateTime
                 : (DateTime?)null,
         };
+        if (component.DeletedAt is null && await HasReferenceCycle(component, context)) component.DeletedAt = commit.DateTime;
+        return component;
+    }
+
+    private static async ValueTask<bool> HasReferenceCycle(ComplexFormComponent parent, ChangeContext context)
+    {
+        if (parent.ComplexFormEntryId == parent.ComponentEntryId) return true;
+        //used to avoid checking the same ComplexFormComponent multiple times
+        HashSet<Guid> visited = [parent.Id];
+        Queue<ComplexFormComponent> queue = new Queue<ComplexFormComponent>();
+        queue.Enqueue(parent);
+        while (queue.Count > 0)
+        {
+            var current = queue.Dequeue();
+            if (current.ComplexFormEntryId == parent.ComponentEntryId) return true;
+            await foreach (var o in context.GetObjectsReferencing(current.ComplexFormEntryId))
+            {
+                if (o is not ComplexFormComponent cfc) continue;
+                if (visited.Contains(cfc.Id)) continue;
+                if (cfc.ComplexFormEntryId == parent.ComponentEntryId) return true;
+                queue.Enqueue(cfc);
+                visited.Add(cfc.Id);
+            }
+        }
+        return false;
     }
 }
diff --git a/backend/harmony b/backend/harmony
index 8a0bd5d8a..5d62145dc 160000
--- a/backend/harmony
+++ b/backend/harmony
@@ -1 +1 @@
-Subproject commit 8a0bd5d8a503c6e391950122e22e683e0945bd7c
+Subproject commit 5d62145dcd6a1ee574666ae5684e199457db163c