NuCache: fix panic exception

src/Umbraco.Tests/Cache/SnapDictionaryTests.cs

@@ -1098,6 +1098,87 @@ namespace Umbraco.Tests.Cache
             Assert.AreEqual("four", all[3]);
         }
 
+        [Test]
+        public void DontPanic()
+        {
+            var d = new SnapDictionary<int, string>();
+            d.Test.CollectAuto = false;
+
+            Assert.IsNull(d.Test.GenObj); // set with first snapshot or first lock, then never null
+
+            // gen 1
+            d.Set(1, "one");
+            Assert.IsTrue(d.Test.NextGen);
+            Assert.AreEqual(1, d.Test.LiveGen);
+            Assert.IsNotNull(d.Test.GenObj); // set with lock
+
+            var s1 = d.CreateSnapshot();
+            Assert.IsFalse(d.Test.NextGen);
+            Assert.AreEqual(1, d.Test.LiveGen);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(1, d.Test.GenObj.Gen);
+
+            Assert.AreEqual(1, s1.Gen);
+            Assert.AreEqual("one", s1.Get(1));
+
+            d.Set(1, "uno");
+            Assert.IsTrue(d.Test.NextGen);
+            Assert.AreEqual(2, d.Test.LiveGen);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(1, d.Test.GenObj.Gen);
+
+            var scopeContext = new ScopeContext();
+            var scopeProvider = GetScopeProvider(scopeContext);
+
+            // scopeProvider.Context == scopeContext -> writer is scoped
+            // writer is scope contextual and scoped
+            //  when disposed, nothing happens
+            //  when the context exists, the writer is released
+            using (d.GetWriter(scopeProvider))
+            {
+                d.Set(1, "ein");
+                Assert.IsTrue(d.Test.NextGen);
+                Assert.AreEqual(3, d.Test.LiveGen);
+                Assert.IsNotNull(d.Test.GenObj);
+                Assert.AreEqual(2, d.Test.GenObj.Gen);
+            }
+
+            // writer has not released
+            Assert.AreEqual(1, d.Test.WLocked);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(2, d.Test.GenObj.Gen);
+
+            // nothing changed
+            Assert.IsTrue(d.Test.NextGen);
+            Assert.AreEqual(3, d.Test.LiveGen);
+
+            // panic!
+            var s2 = d.CreateSnapshot();
+
+            Assert.AreEqual(1, d.Test.WLocked);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(2, d.Test.GenObj.Gen);
+            Assert.AreEqual(3, d.Test.LiveGen);
+            Assert.IsTrue(d.Test.NextGen);
+
+            // release writer
+            scopeContext.ScopeExit(true);
+
+            Assert.AreEqual(0, d.Test.WLocked);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(2, d.Test.GenObj.Gen);
+            Assert.AreEqual(3, d.Test.LiveGen);
+            Assert.IsTrue(d.Test.NextGen);
+
+            var s3 = d.CreateSnapshot();
+
+            Assert.AreEqual(0, d.Test.WLocked);
+            Assert.IsNotNull(d.Test.GenObj);
+            Assert.AreEqual(3, d.Test.GenObj.Gen);
+            Assert.AreEqual(3, d.Test.LiveGen);
+            Assert.IsFalse(d.Test.NextGen);
+        }
+
         private IScopeProvider GetScopeProvider(ScopeContext scopeContext = null)
         {
             var scopeProvider = Mock.Of<IScopeProvider>();

src/Umbraco.Web/PublishedCache/NuCache/ContentStore.cs

@@ -133,11 +133,12 @@ namespace Umbraco.Web.PublishedCache.NuCache
                 {
                     _wlocked++;
                     lockInfo.Count = true;
-                    if (_nextGen == false || (forceGen && _wlocked == 1)) // if true already... ok to have "holes" in generation objects
+                    if (_nextGen == false || (forceGen && _wlocked == 1))
                     {
                         // because we are changing things, a new generation
                         // is created, which will trigger a new snapshot
                         _nextGen = true;
+                        _genObjs.Enqueue(_genObj = new GenObj(_liveGen));
                         _liveGen += 1;
                     }
                 }
@@ -214,7 +215,6 @@ namespace Umbraco.Web.PublishedCache.NuCache
                 else
                     dictionary.TryUpdate(key, link.Next, link);
             }
-
         }
 
         #endregion

src/Umbraco.Web/PublishedCache/NuCache/SnapDictionary.cs

@@ -139,18 +139,12 @@ namespace Umbraco.Web.PublishedCache.NuCache
                     _wlocked++;
                     lockInfo.Count = true;
 
-                    // fixme - this comment: "ok to have holes in generation objects" is annoying
-                    // 'cos if you have a hole, then doing _liveGen-1 in some places would be wrong
-                    // besides, forceGen is used only when getting a scoped write lock, which is not reentrant?
-                    //
-                    // but... if _wlocked ==1 and we just incremented it, it means it was 0, so it wasnt locked, so wtf?
-                    // this is the only place where _nextGen would turn true so ... this makes no sense at all!
-
-                    if (_nextGen == false || (forceGen && _wlocked == 1)) // if true already... ok to have "holes" in generation objects
+                    if (_nextGen == false || (forceGen && _wlocked == 1))
                     {
                         // because we are changing things, a new generation
                         // is created, which will trigger a new snapshot
                         _nextGen = true; // this is the ONLY place where _nextGen becomes true
+                        _genObjs.Enqueue(_genObj = new GenObj(_liveGen));
                         _liveGen += 1;
                     }
                 }
@@ -379,9 +373,6 @@ namespace Umbraco.Web.PublishedCache.NuCache
                     if (_genObj == null)
                         _genObjs.Enqueue(_genObj = new GenObj(snapGen));
 
-                    // fixme - getting a panic exception here
-                    // means _genObj != null, means _nextGen == true
-
                     // if we have one already, ensure it's consistent
                     else if (_genObj.Gen != snapGen)
                         throw new Exception("panic");
@@ -551,6 +542,8 @@ namespace Umbraco.Web.PublishedCache.NuCache
                 set => _dict._collectAuto = value;
             }
 
+            public GenObj GenObj => _dict._genObj;
+
             public ConcurrentQueue<GenObj> GenObjs => _dict._genObjs;
 
             public Snapshot LiveSnapshot => new Snapshot(_dict, _dict._liveGen);