Fixed leak in object pool (#5521)

Author: harishsk

src/Microsoft.ML.Data/Data/DataViewUtils.cs

@@ -942,8 +942,10 @@ public override void Set(BatchColumn batchCol)
                     public override void Unset()
                     {
                         Contracts.Assert(_index <= _count);
-                        if (Values != null)
-                            _pool.Return(Values);
+                        // Remove all the objects from the pool
+                        // to free up references to those objects
+                        while (_pool.Count > 0)
+                            _pool.Get();
                         Values = null;
                         _count = 0;
                         _index = 0;

test/Microsoft.ML.TestFramework/BaseTestClass.cs

@@ -81,7 +81,7 @@ void IDisposable.Dispose()
             Cleanup();
             Process proc = Process.GetCurrentProcess();
             Console.WriteLine($"Finished test: {FullTestName} " +
-                $"with memory usage {proc.PrivateMemorySize64.ToString("N", CultureInfo.InvariantCulture)}");
+                $"with memory usage {proc.WorkingSet64.ToString("N", CultureInfo.InvariantCulture)}");
         }
 
         protected virtual void Initialize()