Fix recurisve Monitor.TryEnter to not return false due to a conditi…

…on (#77243)

A race condition when the object header is transitioning to a sync block can cause a concurrent recursive `TryEnter` to return false.

Fixes #76660

src/coreclr/vm/syncblk.inl

@@ -636,10 +636,10 @@ FORCEINLINE AwareLock::EnterHelperResult ObjHeader::EnterObjMonitorHelper(Thread
         return AwareLock::EnterHelperResult_Contention;
     }
 
-    // The header is transitioning - treat this as if the lock was taken
+    // The header is transitioning - use the slow path
     if (oldValue & BIT_SBLK_SPIN_LOCK)
     {
-        return AwareLock::EnterHelperResult_Contention;
+        return AwareLock::EnterHelperResult_UseSlowPath;
     }
 
     // Here we know we have the "thin lock" layout, but the lock is not free.

src/libraries/System.Threading/tests/MonitorTests.cs

@@ -456,5 +456,35 @@ public static void Enter_HasToWait_LockContentionCountTest()
             Enter_HasToWait();
             Assert.True(Monitor.LockContentionCount - initialLockContentionCount >= 2);
         }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
+        public static void ObjectHeaderSyncBlockTransitionTryEnterRaceTest()
+        {
+            var threadStarted = new AutoResetEvent(false);
+            var startTest = new AutoResetEvent(false);
+            var obj = new object();
+            var t = ThreadTestHelpers.CreateGuardedThread(out _, () =>
+            {
+                threadStarted.Set();
+                startTest.CheckedWait();
+                Monitor.TryEnter(obj, 100); // likely to perform a full wait, which may involve some sort of transition
+            });
+            t.IsBackground = true;
+            t.Start();
+            threadStarted.CheckedWait();
+
+            lock (obj)
+            {
+                startTest.Set();
+                do
+                {
+                    for (int i = 0; i < 1000; i++)
+                    {
+                        Assert.True(Monitor.TryEnter(obj)); // this could race with the transition happening on the other thread
+                        Monitor.Exit(obj);
+                    }
+                } while (!t.Join(0));
+            }
+        }
     }
 }