fix: Adds Gen2 callback for each STArrayPool (#971)

Adds. Gen2Callback for STArrayPool so it can purge the internal array stacks properly. This only happens if significant timed has passed, 2 Gen 2's have been run, and memory pressure exceeds a threshold.

Projects/Server/GarbageCollection/Gen2GcCallback.cs

@@ -0,0 +1,75 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using System.Runtime.ConstrainedExecution;
+using System.Runtime.InteropServices;
+
+namespace System;
+
+/// <summary>
+/// Schedules a callback roughly every gen 2 GC (you may see a Gen 0 an Gen 1 but only once)
+/// (We can fix this by capturing the Gen 2 count at startup and testing, but I mostly don't care)
+/// </summary>
+internal sealed class Gen2GcCallback : CriticalFinalizerObject
+{
+    private readonly Func<object, bool> _callback;
+    private GCHandle _weakTargetObj;
+
+    private Gen2GcCallback(Func<object, bool> callback, object targetObj)
+    {
+        _callback = callback;
+        _weakTargetObj = GCHandle.Alloc(targetObj, GCHandleType.Weak);
+    }
+
+    /// <summary>
+    /// Schedule 'callback' to be called in the next GC.  If the callback returns true it is
+    /// rescheduled for the next Gen 2 GC.  Otherwise the callbacks stop.
+    ///
+    /// NOTE: This callback will be kept alive until either the callback function returns false,
+    /// or the target object dies.
+    /// </summary>
+    public static void Register(Func<object, bool> callback, object targetObj)
+    {
+        // Create a unreachable object that remembers the callback function and target object.
+        new Gen2GcCallback(callback, targetObj);
+    }
+
+    ~Gen2GcCallback()
+    {
+        if (_weakTargetObj.IsAllocated)
+        {
+            // Check to see if the target object is still alive.
+            object? targetObj = _weakTargetObj.Target;
+            if (targetObj == null)
+            {
+                // The target object is dead, so this callback object is no longer needed.
+                _weakTargetObj.Free();
+                return;
+            }
+
+            // Execute the callback method.
+            try
+            {
+                Debug.Assert(_callback != null);
+                if (_callback?.Invoke(targetObj) != true)
+                {
+                    // If the callback returns false, this callback object is no longer needed.
+                    _weakTargetObj.Free();
+                    return;
+                }
+            }
+            catch
+            {
+                // Ensure that we still get a chance to resurrect this object, even if the callback throws an exception.
+#if DEBUG
+                    // Except in DEBUG, as we really shouldn't be hitting any exceptions here.
+                    throw;
+#endif
+            }
+        }
+
+        // Resurrect ourselves by re-registering for finalization.
+        GC.ReRegisterForFinalize(this);
+    }
+}