From b8520dd854afa28d2ee3d67636feea595eb8c0e1 Mon Sep 17 00:00:00 2001
From: halgari <tbaldridge@gmail.com>
Date: Thu, 8 Aug 2024 12:36:29 -0600
Subject: [PATCH] Fix refcount bug

---
 .../Query/ObservableDatoms.cs                 |  2 +-
 ....RefCountWorksWithObservables.verified.txt |  5 ++
 tests/NexusMods.MnemonicDB.Tests/DbTests.cs   | 46 +++++++++++++++++++
 3 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 tests/NexusMods.MnemonicDB.Tests/DbTests.RefCountWorksWithObservables.verified.txt

diff --git a/src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs b/src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs
index 6ce42cf..8683ff9 100644
--- a/src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs
+++ b/src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs
@@ -49,7 +49,7 @@ public static IObservable<IChangeSet<Datom>> ObserveDatoms(this IConnection conn
         {
             lock (set)
             {
-                if (rev.BasisTxId <= lastTxId)
+                if (rev.BasisTxId <= lastTxId && idx != 0)
                     return ChangeSet<Datom>.Empty;
 
                 lastTxId = rev.BasisTxId;
diff --git a/tests/NexusMods.MnemonicDB.Tests/DbTests.RefCountWorksWithObservables.verified.txt b/tests/NexusMods.MnemonicDB.Tests/DbTests.RefCountWorksWithObservables.verified.txt
new file mode 100644
index 0000000..d877f83
--- /dev/null
+++ b/tests/NexusMods.MnemonicDB.Tests/DbTests.RefCountWorksWithObservables.verified.txt
@@ -0,0 +1,5 @@
+[
+  Test Mod,
+  Test Mod 2,
+  Test Mod 3
+]
\ No newline at end of file
diff --git a/tests/NexusMods.MnemonicDB.Tests/DbTests.cs b/tests/NexusMods.MnemonicDB.Tests/DbTests.cs
index c4bdf90..d4ab760 100644
--- a/tests/NexusMods.MnemonicDB.Tests/DbTests.cs
+++ b/tests/NexusMods.MnemonicDB.Tests/DbTests.cs
@@ -911,4 +911,50 @@ public async Task CollectionAttributesAreSupportedOnModels()
         
         modRO.Tags.Should().BeEquivalentTo(["A", "B", "C"]);
     }
+
+    /// <summary>
+    /// Tests a bug experienced in the app, when RefCount is used with ObserveDatoms, if the last subscription
+    /// is disposed, and then another subscriber attaches, an exception would be thrown. This test ensures that
+    /// this behavior works correctly.
+    /// </summary>
+    [Fact]
+    public async Task RefCountWorksWithObservables()
+    {
+        var tx = Connection.BeginTransaction();
+        var mod = new Mod.New(tx)
+        {
+            Name = "Test Mod",
+            Source = new Uri("http://test.com"),
+            LoadoutId = EntityId.From(0)
+        };
+        var result = await tx.Commit();
+        
+        var modRO = result.Remap(mod);
+
+        var refObs = Mod.Observe(Connection, modRO.Id)
+            .Replay(1)
+            .RefCount();
+        
+        List<Mod.ReadOnly> mods = [];
+        
+        var firstDisp = refObs.Subscribe(mods.Add);
+
+        {
+            var tx2 = Connection.BeginTransaction();
+            tx2.Add(modRO.Id, Mod.Name, "Test Mod 2");
+            await tx2.Commit();
+        }
+        
+        firstDisp.Dispose();
+        
+        var secondDisp = refObs.Subscribe(mods.Add);
+        
+        {
+            var tx2 = Connection.BeginTransaction();
+            tx2.Add(modRO.Id, Mod.Name, "Test Mod 3");
+            await tx2.Commit();
+        }
+        
+        await Verify(mods.Distinct().Select(m => m.Name));
+    }
 }