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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Threading;
using Microsoft.Build.TaskHost.Utilities;
namespace Microsoft.Build.TaskHost.Collections;
// The following class is back-ported from .NET 4.X CoreFX library because
// MSBuildTaskHost requires 3.5 .NET Framework. Only GetOrAdd method kept.
internal class ConcurrentDictionary<TKey, TValue>
where TKey : notnull
{
/// <summary>
/// Tables that hold the internal state of the ConcurrentDictionary
///
/// Wrapping the three tables in a single object allows us to atomically
/// replace all tables at once.
/// </summary>
private sealed class Tables
{
internal readonly Node[] _buckets; // A singly-linked list for each bucket.
internal readonly object[] _locks; // A set of locks, each guarding a section of the table.
internal volatile int[] _countPerLock; // The number of elements guarded by each lock.
internal Tables(Node[] buckets, object[] locks, int[] countPerLock)
{
_buckets = buckets;
_locks = locks;
_countPerLock = countPerLock;
}
}
private volatile Tables _tables; // Internal tables of the dictionary
private readonly IEqualityComparer<TKey> _comparer; // Key equality comparer
private readonly bool _growLockArray; // Whether to dynamically increase the size of the striped lock
private int _budget; // The maximum number of elements per lock before a resize operation is triggered
// The default capacity, i.e. the initial # of buckets. When choosing this value, we are making
// a trade-off between the size of a very small dictionary, and the number of resizes when
// constructing a large dictionary. Also, the capacity should not be divisible by a small prime.
private const int DefaultCapacity = 31;
// The maximum size of the striped lock that will not be exceeded when locks are automatically
// added as the dictionary grows. However, the user is allowed to exceed this limit by passing
// a concurrency level larger than MaxLockNumber into the constructor.
private const int MaxLockNumber = 1024;
// Whether TValue is a type that can be written atomically (i.e., with no danger of torn reads)
private static readonly bool s_isValueWriteAtomic = IsValueWriteAtomic();
/// <summary>
/// Determines whether type TValue can be written atomically
/// </summary>
private static bool IsValueWriteAtomic()
{
// Section 12.6.6 of ECMA CLI explains which types can be read and written atomically without
// the risk of tearing.
//
// See http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-335.pdf
Type valueType = typeof(TValue);
if (!valueType.IsValueType)
{
return true;
}
switch (Type.GetTypeCode(valueType))
{
case TypeCode.Boolean:
case TypeCode.Byte:
case TypeCode.Char:
case TypeCode.Int16:
case TypeCode.Int32:
case TypeCode.SByte:
case TypeCode.Single:
case TypeCode.UInt16:
case TypeCode.UInt32:
return true;
case TypeCode.Int64:
case TypeCode.Double:
case TypeCode.UInt64:
return IntPtr.Size == 8;
default:
return false;
}
}
/// <summary>
/// Initializes a new instance of the <see
/// cref="ConcurrentDictionary{TKey,TValue}"/>
/// class that is empty, has the default concurrency level, has the default initial capacity, and
/// uses the default comparer for the key type.
/// </summary>
public ConcurrentDictionary(IEqualityComparer<TKey>? comparer = null)
{
int concurrencyLevel = NativeMethods.GetLogicalCoreCount();
int capacity = DefaultCapacity;
// The capacity should be at least as large as the concurrency level. Otherwise, we would have locks that don't guard
// any buckets.
if (capacity < concurrencyLevel)
{
capacity = concurrencyLevel;
}
object[] locks = new object[concurrencyLevel];
for (int i = 0; i < locks.Length; i++)
{
locks[i] = new object();
}
int[] countPerLock = new int[locks.Length];
Node[] buckets = new Node[capacity];
_tables = new Tables(buckets, locks, countPerLock);
_comparer = comparer ?? EqualityComparer<TKey>.Default;
_growLockArray = true;
_budget = buckets.Length / locks.Length;
}
private bool TryGetValueInternal(TKey key, int hashcode, [MaybeNullWhen(false)] out TValue value)
{
Debug.Assert(_comparer.GetHashCode(key) == hashcode);
// We must capture the _buckets field in a local variable. It is set to a new table on each table resize.
Tables tables = _tables;
int bucketNo = GetBucket(hashcode, tables._buckets.Length);
// We can get away w/out a lock here.
// The Volatile.Read ensures that we have a copy of the reference to tables._buckets[bucketNo].
// This protects us from reading fields ('_hashcode', '_key', '_value' and '_next') of different instances.
Thread.MemoryBarrier();
Node? n = tables._buckets[bucketNo];
while (n != null)
{
if (hashcode == n._hashcode && _comparer.Equals(n._key, key))
{
value = n._value;
return true;
}
n = n._next;
}
value = default;
return false;
}
/// <summary>
/// Shared internal implementation for inserts and updates.
/// If key exists, we always return false; and if updateIfExists == true we force update with value;
/// If key doesn't exist, we always add value and return true;
/// </summary>
private bool TryAddInternal(TKey key, int hashcode, TValue value, bool updateIfExists, bool acquireLock, out TValue resultingValue)
{
Debug.Assert(_comparer.GetHashCode(key) == hashcode);
while (true)
{
int bucketNo, lockNo;
Tables tables = _tables;
GetBucketAndLockNo(hashcode, out bucketNo, out lockNo, tables._buckets.Length, tables._locks.Length);
bool resizeDesired = false;
bool lockTaken = false;
try
{
if (acquireLock)
{
lockTaken = Monitor.TryEnter(tables._locks[lockNo]);
}
// If the table just got resized, we may not be holding the right lock, and must retry.
// This should be a rare occurrence.
if (tables != _tables)
{
continue;
}
// Try to find this key in the bucket
Node? prev = null;
for (Node? node = tables._buckets[bucketNo]; node != null; node = node._next)
{
Debug.Assert((prev == null && node == tables._buckets[bucketNo]) || prev!._next == node);
if (hashcode == node._hashcode && _comparer.Equals(node._key, key))
{
// The key was found in the dictionary. If updates are allowed, update the value for that key.
// We need to create a new node for the update, in order to support TValue types that cannot
// be written atomically, since lock-free reads may be happening concurrently.
if (updateIfExists)
{
if (s_isValueWriteAtomic)
{
node._value = value;
}
else
{
Node newNode = new Node(node._key, value, hashcode, node._next);
if (prev == null)
{
Interlocked.Exchange(ref tables._buckets[bucketNo], newNode);
}
else
{
prev._next = newNode;
}
}
resultingValue = value;
}
else
{
resultingValue = node._value;
}
return false;
}
prev = node;
}
// The key was not found in the bucket. Insert the key-value pair.
Interlocked.Exchange(ref tables._buckets[bucketNo], new Node(key, value, hashcode, tables._buckets[bucketNo]));
checked
{
tables._countPerLock[lockNo]++;
}
// If the number of elements guarded by this lock has exceeded the budget, resize the bucket table.
// It is also possible that GrowTable will increase the budget but won't resize the bucket table.
// That happens if the bucket table is found to be poorly utilized due to a bad hash function.
if (tables._countPerLock[lockNo] > _budget)
{
resizeDesired = true;
}
}
finally
{
if (lockTaken)
{
Monitor.Exit(tables._locks[lockNo]);
}
}
// The fact that we got here means that we just performed an insertion. If necessary, we will grow the table.
//
// Concurrency notes:
// - Notice that we are not holding any locks at when calling GrowTable. This is necessary to prevent deadlocks.
// - As a result, it is possible that GrowTable will be called unnecessarily. But, GrowTable will obtain lock 0
// and then verify that the table we passed to it as the argument is still the current table.
if (resizeDesired)
{
GrowTable(tables);
}
resultingValue = value;
return true;
}
}
[DoesNotReturn]
private static void ThrowKeyNullException()
=> throw new ArgumentNullException("key");
/// <summary>
/// Adds a key/value pair to the <see cref="ConcurrentDictionary{TKey,TValue}"/>
/// if the key does not already exist.
/// </summary>
/// <param name="key">The key of the element to add.</param>
/// <param name="valueFactory">The function used to generate a value for the key.</param>
/// <exception cref="ArgumentNullException"><paramref name="key"/> is a null reference
/// (Nothing in Visual Basic).</exception>
/// <exception cref="ArgumentNullException"><paramref name="valueFactory"/> is a null reference
/// (Nothing in Visual Basic).</exception>
/// <exception cref="OverflowException">The dictionary contains too many
/// elements.</exception>
/// <returns>The value for the key. This will be either the existing value for the key if the
/// key is already in the dictionary, or the new value for the key as returned by valueFactory
/// if the key was not in the dictionary.</returns>
public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)
{
if (key == null)
{
ThrowKeyNullException();
}
if (valueFactory == null)
{
throw new ArgumentNullException(nameof(valueFactory));
}
int hashcode = _comparer.GetHashCode(key);
if (!TryGetValueInternal(key, hashcode, out TValue? resultingValue))
{
TryAddInternal(key, hashcode, valueFactory(key), updateIfExists: false, acquireLock: true, out resultingValue);
}
return resultingValue;
}
/// <summary>
/// Replaces the bucket table with a larger one. To prevent multiple threads from resizing the
/// table as a result of races, the Tables instance that holds the table of buckets deemed too
/// small is passed in as an argument to GrowTable(). GrowTable() obtains a lock, and then checks
/// the Tables instance has been replaced in the meantime or not.
/// </summary>
private void GrowTable(Tables tables)
{
const int MaxArrayLength = 0X7FEFFFFF;
int locksAcquired = 0;
try
{
// The thread that first obtains _locks[0] will be the one doing the resize operation
AcquireLocks(0, 1, ref locksAcquired);
// Make sure nobody resized the table while we were waiting for lock 0:
if (tables != _tables)
{
// We assume that since the table reference is different, it was already resized (or the budget
// was adjusted). If we ever decide to do table shrinking, or replace the table for other reasons,
// we will have to revisit this logic.
return;
}
// Compute the (approx.) total size. Use an Int64 accumulation variable to avoid an overflow.
long approxCount = 0;
for (int i = 0; i < tables._countPerLock.Length; i++)
{
approxCount += tables._countPerLock[i];
}
// If the bucket array is too empty, double the budget instead of resizing the table
if (approxCount < tables._buckets.Length / 4)
{
_budget = 2 * _budget;
if (_budget < 0)
{
_budget = int.MaxValue;
}
return;
}
// Compute the new table size. We find the smallest integer larger than twice the previous table size, and not divisible by
// 2,3,5 or 7. We can consider a different table-sizing policy in the future.
int newLength = 0;
bool maximizeTableSize = false;
try
{
checked
{
// Double the size of the buckets table and add one, so that we have an odd integer.
newLength = (tables._buckets.Length * 2) + 1;
// Now, we only need to check odd integers, and find the first that is not divisible
// by 3, 5 or 7.
while (newLength % 3 == 0 || newLength % 5 == 0 || newLength % 7 == 0)
{
newLength += 2;
}
Debug.Assert(newLength % 2 != 0);
if (newLength > MaxArrayLength)
{
maximizeTableSize = true;
}
}
}
catch (OverflowException)
{
maximizeTableSize = true;
}
if (maximizeTableSize)
{
newLength = MaxArrayLength;
// We want to make sure that GrowTable will not be called again, since table is at the maximum size.
// To achieve that, we set the budget to int.MaxValue.
//
// (There is one special case that would allow GrowTable() to be called in the future:
// calling Clear() on the ConcurrentDictionary will shrink the table and lower the budget.)
_budget = int.MaxValue;
}
// Now acquire all other locks for the table
AcquireLocks(1, tables._locks.Length, ref locksAcquired);
object[] newLocks = tables._locks;
// Add more locks
if (_growLockArray && tables._locks.Length < MaxLockNumber)
{
newLocks = new object[tables._locks.Length * 2];
Array.Copy(tables._locks, 0, newLocks, 0, tables._locks.Length);
for (int i = tables._locks.Length; i < newLocks.Length; i++)
{
newLocks[i] = new object();
}
}
Node[] newBuckets = new Node[newLength];
int[] newCountPerLock = new int[newLocks.Length];
// Copy all data into a new table, creating new nodes for all elements
for (int i = 0; i < tables._buckets.Length; i++)
{
Node? current = tables._buckets[i];
while (current != null)
{
Node? next = current._next;
int newBucketNo, newLockNo;
GetBucketAndLockNo(current._hashcode, out newBucketNo, out newLockNo, newBuckets.Length, newLocks.Length);
newBuckets[newBucketNo] = new Node(current._key, current._value, current._hashcode, newBuckets[newBucketNo]);
checked
{
newCountPerLock[newLockNo]++;
}
current = next;
}
}
// Adjust the budget
_budget = Math.Max(1, newBuckets.Length / newLocks.Length);
// Replace tables with the new versions
_tables = new Tables(newBuckets, newLocks, newCountPerLock);
}
finally
{
// Release all locks that we took earlier
ReleaseLocks(0, locksAcquired);
}
}
/// <summary>
/// Computes the bucket for a particular key.
/// </summary>
private static int GetBucket(int hashcode, int bucketCount)
{
int bucketNo = (hashcode & 0x7fffffff) % bucketCount;
Debug.Assert(bucketNo >= 0 && bucketNo < bucketCount);
return bucketNo;
}
/// <summary>
/// Computes the bucket and lock number for a particular key.
/// </summary>
private static void GetBucketAndLockNo(int hashcode, out int bucketNo, out int lockNo, int bucketCount, int lockCount)
{
bucketNo = (hashcode & 0x7fffffff) % bucketCount;
lockNo = bucketNo % lockCount;
Debug.Assert(bucketNo >= 0 && bucketNo < bucketCount);
Debug.Assert(lockNo >= 0 && lockNo < lockCount);
}
/// <summary>
/// Acquires a contiguous range of locks for this hash table, and increments locksAcquired
/// by the number of locks that were successfully acquired. The locks are acquired in an
/// increasing order.
/// </summary>
private void AcquireLocks(int fromInclusive, int toExclusive, ref int locksAcquired)
{
Debug.Assert(fromInclusive <= toExclusive);
object[] locks = _tables._locks;
for (int i = fromInclusive; i < toExclusive; i++)
{
bool lockTaken = false;
try
{
lockTaken = Monitor.TryEnter(locks[i]);
}
finally
{
if (lockTaken)
{
locksAcquired++;
}
}
}
}
/// <summary>
/// Releases a contiguous range of locks.
/// </summary>
private void ReleaseLocks(int fromInclusive, int toExclusive)
{
Debug.Assert(fromInclusive <= toExclusive);
for (int i = fromInclusive; i < toExclusive; i++)
{
Monitor.Exit(_tables._locks[i]);
}
}
/// <summary>
/// A node in a singly-linked list representing a particular hash table bucket.
/// </summary>
private sealed class Node
{
internal readonly TKey _key;
internal TValue _value;
internal volatile Node? _next;
internal readonly int _hashcode;
internal Node(TKey key, TValue value, int hashcode, Node? next)
{
_key = key;
_value = value;
_next = next;
_hashcode = hashcode;
}
}
}
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