Add Roslyn's Object pools in ServiceStack.Text.Pools namespace and ma…

…ke them public

src/ServiceStack.Text/Pools/ObjectPool.cs

@@ -0,0 +1,278 @@
+namespace ServiceStack.Text.Pools
+{
+    // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+    // define TRACE_LEAKS to get additional diagnostics that can lead to the leak sources. note: it will
+    // make everything about 2-3x slower
+    // 
+    // #define TRACE_LEAKS
+
+    // define DETECT_LEAKS to detect possible leaks
+    // #if DEBUG
+    // #define DETECT_LEAKS  //for now always enable DETECT_LEAKS in debug.
+    // #endif
+
+    using System;
+    using System.Diagnostics;
+    using System.Threading;
+
+#if DETECT_LEAKS
+using System.Runtime.CompilerServices;
+#endif
+    /// <summary>
+    /// Generic implementation of object pooling pattern with predefined pool size limit. The main
+    /// purpose is that limited number of frequently used objects can be kept in the pool for
+    /// further recycling.
+    /// 
+    /// Notes: 
+    /// 1) it is not the goal to keep all returned objects. Pool is not meant for storage. If there
+    ///    is no space in the pool, extra returned objects will be dropped.
+    /// 
+    /// 2) it is implied that if object was obtained from a pool, the caller will return it back in
+    ///    a relatively short time. Keeping checked out objects for long durations is ok, but 
+    ///    reduces usefulness of pooling. Just new up your own.
+    /// 
+    /// Not returning objects to the pool in not detrimental to the pool's work, but is a bad practice. 
+    /// Rationale: 
+    ///    If there is no intent for reusing the object, do not use pool - just use "new". 
+    /// </summary>
+    public class ObjectPool<T> where T : class
+    {
+        [DebuggerDisplay("{Value,nq}")]
+        private struct Element
+        {
+            internal T Value;
+        }
+
+        /// <remarks>
+        /// Not using System.Func{T} because this file is linked into the (debugger) Formatter,
+        /// which does not have that type (since it compiles against .NET 2.0).
+        /// </remarks>
+        internal delegate T Factory();
+
+        // Storage for the pool objects. The first item is stored in a dedicated field because we
+        // expect to be able to satisfy most requests from it.
+        private T _firstItem;
+        private readonly Element[] _items;
+
+        // factory is stored for the lifetime of the pool. We will call this only when pool needs to
+        // expand. compared to "new T()", Func gives more flexibility to implementers and faster
+        // than "new T()".
+        private readonly Factory _factory;
+
+#if DETECT_LEAKS
+        private static readonly ConditionalWeakTable<T, LeakTracker> leakTrackers = new ConditionalWeakTable<T, LeakTracker>();
+
+        private class LeakTracker : IDisposable
+        {
+            private volatile bool disposed;
+
+#if TRACE_LEAKS
+            internal volatile object Trace = null;
+#endif
+
+            public void Dispose()
+            {
+                disposed = true;
+                GC.SuppressFinalize(this);
+            }
+
+            private string GetTrace()
+            {
+#if TRACE_LEAKS
+                return Trace == null ? "" : Trace.ToString();
+#else
+                return "Leak tracing information is disabled. Define TRACE_LEAKS on ObjectPool`1.cs to get more info \n";
+#endif
+            }
+
+            ~LeakTracker()
+            {
+                if (!this.disposed && !Environment.HasShutdownStarted)
+                {
+                    var trace = GetTrace();
+
+                    // If you are seeing this message it means that object has been allocated from the pool 
+                    // and has not been returned back. This is not critical, but turns pool into rather 
+                    // inefficient kind of "new".
+                    Debug.WriteLine($"TRACEOBJECTPOOLLEAKS_BEGIN\nPool detected potential leaking of {typeof(T)}. \n Location of the leak: \n {GetTrace()} TRACEOBJECTPOOLLEAKS_END");
+                }
+            }
+        }
+#endif
+
+        internal ObjectPool(Factory factory)
+            : this(factory, Environment.ProcessorCount * 2)
+        { }
+
+        internal ObjectPool(Factory factory, int size)
+        {
+            Debug.Assert(size >= 1);
+            _factory = factory;
+            _items = new Element[size - 1];
+        }
+
+        private T CreateInstance()
+        {
+            var inst = _factory();
+            return inst;
+        }
+
+        /// <summary>
+        /// Produces an instance.
+        /// </summary>
+        /// <remarks>
+        /// Search strategy is a simple linear probing which is chosen for it cache-friendliness.
+        /// Note that Free will try to store recycled objects close to the start thus statistically 
+        /// reducing how far we will typically search.
+        /// </remarks>
+        internal T Allocate()
+        {
+            // PERF: Examine the first element. If that fails, AllocateSlow will look at the remaining elements.
+            // Note that the initial read is optimistically not synchronized. That is intentional. 
+            // We will interlock only when we have a candidate. in a worst case we may miss some
+            // recently returned objects. Not a big deal.
+            T inst = _firstItem;
+            if (inst == null || inst != Interlocked.CompareExchange(ref _firstItem, null, inst))
+            {
+                inst = AllocateSlow();
+            }
+
+#if DETECT_LEAKS
+            var tracker = new LeakTracker();
+            leakTrackers.Add(inst, tracker);
+
+#if TRACE_LEAKS
+            var frame = CaptureStackTrace();
+            tracker.Trace = frame;
+#endif
+#endif
+            return inst;
+        }
+
+        private T AllocateSlow()
+        {
+            var items = _items;
+
+            for (int i = 0; i < items.Length; i++)
+            {
+                // Note that the initial read is optimistically not synchronized. That is intentional. 
+                // We will interlock only when we have a candidate. in a worst case we may miss some
+                // recently returned objects. Not a big deal.
+                T inst = items[i].Value;
+                if (inst != null)
+                {
+                    if (inst == Interlocked.CompareExchange(ref items[i].Value, null, inst))
+                    {
+                        return inst;
+                    }
+                }
+            }
+
+            return CreateInstance();
+        }
+
+        /// <summary>
+        /// Returns objects to the pool.
+        /// </summary>
+        /// <remarks>
+        /// Search strategy is a simple linear probing which is chosen for it cache-friendliness.
+        /// Note that Free will try to store recycled objects close to the start thus statistically 
+        /// reducing how far we will typically search in Allocate.
+        /// </remarks>
+        internal void Free(T obj)
+        {
+            Validate(obj);
+            ForgetTrackedObject(obj);
+
+            if (_firstItem == null)
+            {
+                // Intentionally not using interlocked here. 
+                // In a worst case scenario two objects may be stored into same slot.
+                // It is very unlikely to happen and will only mean that one of the objects will get collected.
+                _firstItem = obj;
+            }
+            else
+            {
+                FreeSlow(obj);
+            }
+        }
+
+        private void FreeSlow(T obj)
+        {
+            var items = _items;
+            for (int i = 0; i < items.Length; i++)
+            {
+                if (items[i].Value == null)
+                {
+                    // Intentionally not using interlocked here. 
+                    // In a worst case scenario two objects may be stored into same slot.
+                    // It is very unlikely to happen and will only mean that one of the objects will get collected.
+                    items[i].Value = obj;
+                    break;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Removes an object from leak tracking.  
+        /// 
+        /// This is called when an object is returned to the pool.  It may also be explicitly 
+        /// called if an object allocated from the pool is intentionally not being returned
+        /// to the pool.  This can be of use with pooled arrays if the consumer wants to 
+        /// return a larger array to the pool than was originally allocated.
+        /// </summary>
+        [Conditional("DEBUG")]
+        internal void ForgetTrackedObject(T old, T replacement = null)
+        {
+#if DETECT_LEAKS
+            LeakTracker tracker;
+            if (leakTrackers.TryGetValue(old, out tracker))
+            {
+                tracker.Dispose();
+                leakTrackers.Remove(old);
+            }
+            else
+            {
+                var trace = CaptureStackTrace();
+                Debug.WriteLine($"TRACEOBJECTPOOLLEAKS_BEGIN\nObject of type {typeof(T)} was freed, but was not from pool. \n Callstack: \n {trace} TRACEOBJECTPOOLLEAKS_END");
+            }
+
+            if (replacement != null)
+            {
+                tracker = new LeakTracker();
+                leakTrackers.Add(replacement, tracker);
+            }
+#endif
+        }
+
+#if DETECT_LEAKS
+        private static Lazy<Type> _stackTraceType = new Lazy<Type>(() => Type.GetType("System.Diagnostics.StackTrace"));
+
+        private static object CaptureStackTrace()
+        {
+            return Activator.CreateInstance(_stackTraceType.Value);
+        }
+#endif
+
+        [Conditional("DEBUG")]
+        private void Validate(object obj)
+        {
+            Debug.Assert(obj != null, "freeing null?");
+
+            Debug.Assert(_firstItem != obj, "freeing twice?");
+
+            var items = _items;
+            for (int i = 0; i < items.Length; i++)
+            {
+                var value = items[i].Value;
+                if (value == null)
+                {
+                    return;
+                }
+
+                Debug.Assert(value != obj, "freeing twice?");
+            }
+        }
+    }
+}