Remove unnecessary allocations in Hash task. (#7162)

Fixes #7086

### Context
`Hash.Execute()` allocates a string which gets to the large object heap. This could be avoided without changing the resulting hash function.

### Changes Made
Hash function is rewritten.

### Testing
Unit tests & manual testing

src/Tasks.UnitTests/Hash_Tests.cs

@@ -45,6 +45,78 @@ public void HashTaskEmptyInputTest()
             Assert.Null(zeroLengthItemsHash);
         }
 
+        [Fact]
+        public void HashTaskLargeInputCountTest()
+        {
+            // This hash was pre-computed. If the implementation changes it may need to be adjusted.
+            var expectedHash = "8a996bbcb5e481981c2fba7ac408e20d0b4360a5";
+
+            ITaskItem[] itemsToHash = new ITaskItem[1000];
+            for (int i = 0; i < itemsToHash.Length; i++)
+            {
+                itemsToHash[i] = new TaskItem($"Item{i}");
+            }
+
+            var actualHash = ExecuteHashTask(itemsToHash);
+            Assert.Equal(expectedHash, actualHash);
+        }
+
+        [Fact]
+        public void HashTaskLargeInputSizeTest()
+        {
+            // This hash was pre-computed. If the implementation changes it may need to be adjusted.
+            var expectedHash = "0509142dd3d3a733f30a52a0eec37cd727d46122";
+
+            string[] array = new string[1000];
+            for (int i = 0; i < array.Length; i++)
+            {
+                array[i] = $"Item{i}";
+            }
+            ITaskItem[] itemsToHash = new ITaskItem[] { new TaskItem(string.Join("", array)) };
+
+            var actualHash = ExecuteHashTask(itemsToHash);
+            Assert.Equal(expectedHash, actualHash);
+        }
+
+#pragma warning disable CA5350
+        // This test verifies that hash computes correctly for various numbers of characters.
+        // We would like to process edge of the buffer use cases regardless on the size of the buffer.
+        [Fact]
+        public void HashTaskDifferentInputSizesTest()
+        {
+            int maxInputSize = 2000;
+            string input = "";
+            using (var sha1 = System.Security.Cryptography.SHA1.Create())
+            {
+                var stringBuilder = new System.Text.StringBuilder(sha1.HashSize);
+                MockEngine mockEngine = new();
+                for (int i = 0; i < maxInputSize; i++)
+                {
+                    input += "a";
+
+                    Hash hashTask = new()
+                    {
+                        BuildEngine = mockEngine,
+                        ItemsToHash = new ITaskItem[] { new TaskItem(input) },
+                        IgnoreCase = false
+                    };
+                    Assert.True(hashTask.Execute());
+                    string actualHash = hashTask.HashResult;
+
+                    byte[] hash = sha1.ComputeHash(System.Text.Encoding.UTF8.GetBytes(input + '\u2028'));
+                    stringBuilder.Clear();
+                    foreach (var b in hash)
+                    {
+                        stringBuilder.Append(b.ToString("x2"));
+                    }
+                    string expectedHash = stringBuilder.ToString();
+
+                    Assert.Equal(expectedHash, actualHash);
+                }
+            }
+        }
+#pragma warning restore CA5350
+
         [Fact]
         public void HashTaskIgnoreCaseTest()
         {

src/Tasks/Hash.cs

@@ -5,7 +5,6 @@
 using System.Security.Cryptography;
 using System.Text;
 using Microsoft.Build.Framework;
-using Microsoft.Build.Shared;
 
 #nullable disable
 
@@ -24,6 +23,16 @@ namespace Microsoft.Build.Tasks
     public class Hash : TaskExtension
     {
         private const char ItemSeparatorCharacter = '\u2028';
+        private static readonly Encoding s_encoding = Encoding.UTF8;
+        private static readonly byte[] s_itemSeparatorCharacterBytes = s_encoding.GetBytes(new char[] { ItemSeparatorCharacter });
+
+        // Size of buffer where bytes of the strings are stored until sha1.TransformBlock is to be run on them.
+        // It is needed to get a balance between amount of costly sha1.TransformBlock calls and amount of allocated memory.
+        private const int Sha1BufferSize = 512;
+
+        // Size of chunks in which ItemSpecs would be cut.
+        // We have chosen this length so itemSpecChunkByteBuffer rented from ArrayPool will be close but not bigger than 512.
+        private const int MaxInputChunkLength = 169;
 
         /// <summary>
         /// Items from which to generate a hash.
@@ -52,52 +61,100 @@ public override bool Execute()
             {
                 using (var sha1 = SHA1.Create())
                 {
-                    var concatenatedItemStringSize = ComputeStringSize(ItemsToHash);
+                    // Buffer in which bytes of the strings are to be stored until their number reaches the limit size.
+                    // Once the limit is reached, the sha1.TransformBlock is to be run on all the bytes of this buffer.
+                    byte[] sha1Buffer = null;
 
-                    var hashStringSize = sha1.HashSize;
+                    // Buffer in which bytes of items' ItemSpec are to be stored.
+                    byte[] itemSpecChunkByteBuffer = null;
 
-                    using (var stringBuilder = new ReuseableStringBuilder(Math.Max(concatenatedItemStringSize, hashStringSize)))
+                    try
                     {
-                        foreach (var item in ItemsToHash)
+                        sha1Buffer = System.Buffers.ArrayPool<byte>.Shared.Rent(Sha1BufferSize);
+                        itemSpecChunkByteBuffer = System.Buffers.ArrayPool<byte>.Shared.Rent(s_encoding.GetMaxByteCount(MaxInputChunkLength));
+
+                        int sha1BufferPosition = 0;
+                        for (int i = 0; i < ItemsToHash.Length; i++)
                         {
-                            string itemSpec = item.ItemSpec;
-                            stringBuilder.Append(IgnoreCase ? itemSpec.ToUpperInvariant() : itemSpec);
-                            stringBuilder.Append(ItemSeparatorCharacter);
-                        }
+                            string itemSpec = IgnoreCase ? ItemsToHash[i].ItemSpec.ToUpperInvariant() : ItemsToHash[i].ItemSpec;
 
-                        var hash = sha1.ComputeHash(Encoding.UTF8.GetBytes(stringBuilder.ToString()));
+                            // Slice the itemSpec string into chunks of reasonable size and add them to sha1 buffer.
+                            for (int itemSpecPosition = 0; itemSpecPosition < itemSpec.Length; itemSpecPosition += MaxInputChunkLength)
+                            {
+                                int charsToProcess = Math.Min(itemSpec.Length - itemSpecPosition, MaxInputChunkLength);
+                                int byteCount = s_encoding.GetBytes(itemSpec, itemSpecPosition, charsToProcess, itemSpecChunkByteBuffer, 0);
 
-                        stringBuilder.Clear();
+                                sha1BufferPosition = AddBytesToSha1Buffer(sha1, sha1Buffer, sha1BufferPosition, Sha1BufferSize, itemSpecChunkByteBuffer, byteCount);
+                            }
 
-                        foreach (var b in hash)
-                        {
-                            stringBuilder.Append(b.ToString("x2"));
+                            sha1BufferPosition = AddBytesToSha1Buffer(sha1, sha1Buffer, sha1BufferPosition, Sha1BufferSize, s_itemSeparatorCharacterBytes, s_itemSeparatorCharacterBytes.Length);
                         }
 
-                        HashResult = stringBuilder.ToString();
+                        sha1.TransformFinalBlock(sha1Buffer, 0, sha1BufferPosition);
+
+                        using (var stringBuilder = new ReuseableStringBuilder(sha1.HashSize))
+                        {
+                            foreach (var b in sha1.Hash)
+                            {
+                                stringBuilder.Append(b.ToString("x2"));
+                            }
+                            HashResult = stringBuilder.ToString();
+                        }
+                    }
+                    finally
+                    {
+                        if (sha1Buffer != null)
+                        {
+                            System.Buffers.ArrayPool<byte>.Shared.Return(sha1Buffer);
+                        }
+                        if (itemSpecChunkByteBuffer != null)
+                        {
+                            System.Buffers.ArrayPool<byte>.Shared.Return(itemSpecChunkByteBuffer);
+                        }
                     }
                 }
             }
-
             return true;
         }
 
-        private int ComputeStringSize(ITaskItem[] itemsToHash)
+        /// <summary>
+        /// Add bytes to the sha1 buffer. Once the limit size is reached, sha1.TransformBlock is called and the buffer is flushed.
+        /// </summary>
+        /// <param name="sha1">Hashing algorithm sha1.</param>
+        /// <param name="sha1Buffer">The sha1 buffer which stores bytes of the strings. Bytes should be added to this buffer.</param>
+        /// <param name="sha1BufferPosition">Number of used bytes of the sha1 buffer.</param>
+        /// <param name="sha1BufferSize">The size of sha1 buffer.</param>
+        /// <param name="byteBuffer">Bytes buffer which contains bytes to be written to sha1 buffer.</param>
+        /// <param name="byteCount">Amount of bytes that are to be added to sha1 buffer.</param>
+        /// <returns>Updated sha1BufferPosition.</returns>
+        private int AddBytesToSha1Buffer(SHA1 sha1, byte[] sha1Buffer, int sha1BufferPosition, int sha1BufferSize, byte[] byteBuffer, int byteCount)
         {
-            if (itemsToHash.Length == 0)
+            int bytesProcessed = 0;
+            while (sha1BufferPosition + byteCount >= sha1BufferSize)
             {
-                return 0;
-            }
+                int sha1BufferFreeSpace = sha1BufferSize - sha1BufferPosition;
 
-            var totalItemSize = 0;
+                if (sha1BufferPosition == 0)
+                {
+                    // If sha1 buffer is empty and bytes number is big enough there is no need to copy bytes to sha1 buffer.
+                    // Pass the bytes to TransformBlock right away.
+                    sha1.TransformBlock(byteBuffer, bytesProcessed, sha1BufferSize, null, 0);
+                }
+                else
+                {
+                    Array.Copy(byteBuffer, bytesProcessed, sha1Buffer, sha1BufferPosition, sha1BufferFreeSpace);
+                    sha1.TransformBlock(sha1Buffer, 0, sha1BufferSize, null, 0);
+                    sha1BufferPosition = 0;
+                }
 
-            foreach (var item in itemsToHash)
-            {
-                totalItemSize += item.ItemSpec.Length;
+                bytesProcessed += sha1BufferFreeSpace;
+                byteCount -= sha1BufferFreeSpace;
             }
 
-            // Add one ItemSeparatorCharacter per item
-            return totalItemSize + itemsToHash.Length;
+            Array.Copy(byteBuffer, bytesProcessed, sha1Buffer, sha1BufferPosition, byteCount);
+            sha1BufferPosition += byteCount;
+
+            return sha1BufferPosition;
         }
     }
 }