Fix memory leak in MemoryCache

[verdie-g]

WeakReference<T>.TryGetTarget apparently returns false in the target object's finalizer. Here is an example demonstrating that:

CreateTest();
GC.Collect(GC.MaxGeneration);
GC.WaitForPendingFinalizers();

void CreateTest() => _ = new Test();

class Test
{
    private readonly WeakReference<Test> _wr;

    public Test()
    {
        _wr = new WeakReference<Test>(this);
        Console.WriteLine(_wr.TryGetTarget(out _)); // True
    }

    ~Test()
    {
        Console.WriteLine(_wr.TryGetTarget(out _)); // False
    }
}

In MemoryCache that would cause the Stats in the _allStats list to never be cleaned out.

MemoryCache c = new(new MemoryCacheOptions { TrackStatistics = true });
for (int i = 0; i < 100; i += 1)
{
    CreateThread();
}

for (int i = 0; i < 10; i += 1)
{
    Thread.Sleep(100);
    GC.Collect(GC.MaxGeneration);
    GC.WaitForPendingFinalizers();
}

Console.WriteLine(c.GetCurrentStatistics());

void CreateThread()
{
    Thread t = new(() => { c.Get(""); });
    t.Start();
    t.Join();
}

At the end of this program, c._allStats.Count == 100.

[ghost]

Tagging subscribers to this area: @dotnet/area-extensions-caching
See info in area-owners.md if you want to be subscribed.

Issue Details

---

WeakReference<T>.TryGetTarget apparently returns false in the target object's finalizer. Here is an example demonstrating that:

CreateTest();
GC.Collect(GC.MaxGeneration);
GC.WaitForPendingFinalizers();

void CreateTest() => _ = new Test();

class Test
{
    private readonly WeakReference<Test> _wr;

    public Test()
    {
        _wr = new WeakReference<Test>(this);
        Console.WriteLine(_wr.TryGetTarget(out _)); // True
    }

    ~Test()
    {
        Console.WriteLine(_wr.TryGetTarget(out _)); // False
    }
}

In MemoryCache that would cause the Stats in the _allStats list to never be cleaned out.

Author:	verdie-g
Assignees:	-
Labels:	area-Extensions-Caching, community-contribution
Milestone:	-

[jozkee]

Do you see any downsides to use this aggressive approach on RemoveFromStats?

[verdie-g]

Do you see any downsides to use this aggressive approach on RemoveFromStats?

It makes the intention a little less clear imo but I don't see any important downsides.

[jozkee]

Thanks, @verdie-g.