deduplication

Author: baronfel

src/Tasks/Microsoft.NET.Build.Tasks/GenerateDepsFile.cs

@@ -266,28 +266,11 @@ bool ShouldIncludeRuntimeAsset(ITaskItem item)
                 // If file exists, check if content is different using streaming hash comparison
                 if (File.Exists(depsFilePath))
                 {
-                    // Get hash length from a single instance to avoid unnecessary allocations
-                    var hasher = new XxHash64();
-                    var hashLength = hasher.HashLengthInBytes;
-
-                    // Hash existing file content using streaming approach
-                    Span<byte> existingHashBuffer = stackalloc byte[hashLength];
-                    var existingHasher = new XxHash64();
-                    using (var existingStream = File.OpenRead(depsFilePath))
-                    {
-                        existingHasher.Append(existingStream);
-                    }
-                    existingHasher.GetCurrentHash(existingHashBuffer);
-
-                    // Hash new content using streaming approach
-                    Span<byte> newHashBuffer = stackalloc byte[hashLength];
-                    var newHasher = new XxHash64();
-                    contentStream.Position = 0;
-                    newHasher.Append(contentStream);
-                    newHasher.GetCurrentHash(newHashBuffer);
-
+                    // stream positions are reset as part of these utility calls
+                    var existingContentHash = HashingUtils.ComputeXXHash64(File.OpenRead(depsFilePath));
+                    var newContentHash = HashingUtils.ComputeXXHash64(contentStream);
                     // If hashes are equal, content is the same - don't write
-                    if (existingHashBuffer.SequenceEqual(newHashBuffer))
+                    if (existingContentHash.SequenceEqual(newContentHash))
                     {
                         shouldWriteFile = false;
                     }
@@ -298,7 +281,6 @@ bool ShouldIncludeRuntimeAsset(ITaskItem item)
                     // Write the new content to file using CopyTo
                     using (var fileStream = File.Create(depsFilePath))
                     {
-                        contentStream.Position = 0;
                         contentStream.CopyTo(fileStream);
                     }
                 }

src/Tasks/Microsoft.NET.Build.Tasks/GenerateRuntimeConfigurationFiles.cs

@@ -408,39 +408,22 @@ private static void WriteToJsonFile(string fileName, object value)
                 // If file exists, check if content is different using streaming hash comparison
                 if (File.Exists(fileName))
                 {
-                    // Get hash length from a single instance to avoid unnecessary allocations
-                    var hasher = new XxHash64();
-                    var hashLength = hasher.HashLengthInBytes;
-
-                    // Hash existing file content using streaming approach
-                    Span<byte> existingHashBuffer = stackalloc byte[hashLength];
-                    var existingHasher = new XxHash64();
-                    using (var existingStream = File.OpenRead(fileName))
-                    {
-                        existingHasher.Append(existingStream);
-                    }
-                    existingHasher.GetCurrentHash(existingHashBuffer);
-
-                    // Hash new content using streaming approach
-                    Span<byte> newHashBuffer = stackalloc byte[hashLength];
-                    var newHasher = new XxHash64();
-                    contentStream.Position = 0;
-                    newHasher.Append(contentStream);
-                    newHasher.GetCurrentHash(newHashBuffer);
-
+                    // stream positions are reset as part of these utility calls
+                    var existingContentHash = HashingUtils.ComputeXXHash64(File.OpenRead(fileName));
+                    var newContentHash = HashingUtils.ComputeXXHash64(contentStream);
                     // If hashes are equal, content is the same - don't write
-                    if (existingHashBuffer.SequenceEqual(newHashBuffer))
+                    if (existingContentHash.SequenceEqual(newContentHash))
                     {
                         shouldWriteFile = false;
                     }
+
                 }
 
                 if (shouldWriteFile)
                 {
                     // Write the new content to file using CopyTo
                     using (var fileStream = File.Create(fileName))
                     {
-                        contentStream.Position = 0;
                         contentStream.CopyTo(fileStream);
                     }
                 }

src/Tasks/Microsoft.NET.Build.Tasks/HashingUtils.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.IO.Hashing;
+
+namespace Microsoft.NET.Build.Tasks;
+
+public static class HashingUtils
+{
+    /// <summary>
+    /// Computes the XxHash64 hash of a file.
+    /// </summary>
+    /// <param name="content">A stream to read for the hash. If the stream is seekable it will be reset to its incoming position.</param>
+    public static byte[] ComputeXXHash64(Stream content)
+    {
+        var initialPosition = content.CanSeek ? content.Position : 0;
+        var hasher = new XxHash64();
+        hasher.Append(content);
+        if (content.CanSeek)
+        {
+            content.Position = initialPosition;
+        }
+        return hasher.GetCurrentHash();
+    }
+}