Use ReadAtLeastAsync to handle short-reads.

Seeks to beginning of streams if CanSeek is true.
Added remarks about stream position.
Add test coverage for short-reads.
Fix fast-path tests to actually test the fast path.
Also fix class comment.

2 files changed, 259 insertions, 14 deletions

diff --git a/src/Jellyfin.Extensions/StreamExtensions.cs b/src/Jellyfin.Extensions/StreamExtensions.cs
index fb3fd2eac1..15b44d8f40 100644
--- a/src/Jellyfin.Extensions/StreamExtensions.cs
+++ b/src/Jellyfin.Extensions/StreamExtensions.cs
@@ -11,7 +11,7 @@ using System.Threading.Tasks;
 namespace Jellyfin.Extensions
 {
     /// <summary>
-    /// Class BaseExtensions.
+    /// Extension methods for the <see cref="Stream"/> class.
     /// </summary>
     public static class StreamExtensions
     {
@@ -74,7 +74,11 @@ namespace Jellyfin.Extensions
         /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
         /// <returns>True if the stream and file are identical; otherwise false.</returns>
         /// <exception cref="ArgumentException"><paramref name="stream"/> does not support seeking.</exception>
-        public static async Task<bool> IsFileIdenticalAsync(this Stream stream, string path, CancellationToken cancellationToken)
+        /// <remarks>
+        /// The entire stream is compared against the file from the beginning (the position is reset to 0 on entry)
+        /// and restored to its original value after the call.
+        /// </remarks>
+        public static async Task<bool> IsFileIdenticalAsync(this Stream stream, string path, CancellationToken cancellationToken = default)
         {
             ArgumentNullException.ThrowIfNull(stream);
             ArgumentException.ThrowIfNullOrEmpty(path);
@@ -114,11 +118,31 @@ namespace Jellyfin.Extensions
         /// <param name="b">The second stream to compare.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
         /// <returns>True if the streams are identical; otherwise false.</returns>
-        public static async Task<bool> IsStreamIdenticalAsync(this Stream a, Stream b, CancellationToken cancellationToken)
+        /// <remarks>
+        /// Seekable streams are compared from the beginning (their position is reset to 0 on entry).
+        /// Non-seekable streams are compared from their current read position. Stream positions are not
+        /// restored after the call.
+        /// </remarks>
+        public static async Task<bool> IsStreamIdenticalAsync(this Stream a, Stream b, CancellationToken cancellationToken = default)
         {
             ArgumentNullException.ThrowIfNull(a);
             ArgumentNullException.ThrowIfNull(b);
 
+            if (ReferenceEquals(a, b))
+            {
+                return true;
+            }
+
+            if (a.CanSeek)
+            {
+                a.Position = 0;
+            }
+
+            if (b.CanSeek)
+            {
+                b.Position = 0;
+            }
+
             if (a.CanSeek && b.CanSeek && b.Length != a.Length)
             {
                 return false;
@@ -145,9 +169,9 @@ namespace Jellyfin.Extensions
                     var memoryB = bufferB.AsMemory();
                     int offset = 0;
                     int bytesRead;
-                    while ((bytesRead = await b.ReadAsync(memoryB, cancellationToken).ConfigureAwait(false)) > 0)
+                    while ((bytesRead = await b.ReadAtLeastAsync(memoryB, memoryB.Length, throwOnEndOfStream: false, cancellationToken).ConfigureAwait(false)) > 0)
                     {
-                        if (!segmentA.AsSpan(offset, bytesRead).SequenceEqual(memoryB.Span[..bytesRead]))
+                        if (offset + bytesRead > segmentA.Count || !segmentA.AsSpan(offset, bytesRead).SequenceEqual(memoryB.Span[..bytesRead]))
                         {
                             return false;
                         }
@@ -174,8 +198,8 @@ namespace Jellyfin.Extensions
                     {
                         cancellationToken.ThrowIfCancellationRequested();
 
-                        var bytesReadA = await a.ReadAsync(memoryA, cancellationToken).ConfigureAwait(false);
-                        var bytesReadB = await b.ReadAsync(memoryB, cancellationToken).ConfigureAwait(false);
+                        var bytesReadA = await a.ReadAtLeastAsync(memoryA, memoryA.Length, throwOnEndOfStream: false, cancellationToken).ConfigureAwait(false);
+                        var bytesReadB = await b.ReadAtLeastAsync(memoryB, memoryB.Length, throwOnEndOfStream: false, cancellationToken).ConfigureAwait(false);
 
                         if (bytesReadA != bytesReadB)
                         {
diff --git a/tests/Jellyfin.Extensions.Tests/StreamExtensionsTests.cs b/tests/Jellyfin.Extensions.Tests/StreamExtensionsTests.cs
index f7efee1e6c..cdbf2f8b1d 100644
--- a/tests/Jellyfin.Extensions.Tests/StreamExtensionsTests.cs
+++ b/tests/Jellyfin.Extensions.Tests/StreamExtensionsTests.cs
@@ -2,7 +2,6 @@ using System;
 using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
-using Jellyfin.Extensions;
 using Xunit;
 
 namespace Jellyfin.Extensions.Tests;
@@ -65,8 +64,12 @@ public class StreamExtensionsTests
         }
     }
 
-    [Fact]
-    public async Task IsFileIdenticalAsync_UsesStartOfStreamAndRestoresPosition_OnMatch()
+    // Both publiclyVisible values are exercised so the test runs once under the fast path
+    // (TryGetBuffer succeeds) and once under the slow path (TryGetBuffer returns false).
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task IsFileIdenticalAsync_UsesStartOfStreamAndRestoresPosition_OnMatch(bool publiclyVisible)
     {
         var cancellationToken = TestContext.Current.CancellationToken;
         var path = Path.Join(Path.GetTempPath(), Path.GetRandomFileName());
@@ -75,7 +78,7 @@ public class StreamExtensionsTests
 
         try
         {
-            await using var stream = new MemoryStream(bytes);
+            await using var stream = CreateMemoryStream(bytes, publiclyVisible);
             stream.Position = 3;
 
             var result = await stream.IsFileIdenticalAsync(path, cancellationToken);
@@ -89,8 +92,10 @@ public class StreamExtensionsTests
         }
     }
 
-    [Fact]
-    public async Task IsFileIdenticalAsync_RestoresPosition_OnMismatch()
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task IsFileIdenticalAsync_RestoresPosition_OnMismatch(bool publiclyVisible)
     {
         var cancellationToken = TestContext.Current.CancellationToken;
         var path = Path.Join(Path.GetTempPath(), Path.GetRandomFileName());
@@ -98,7 +103,7 @@ public class StreamExtensionsTests
 
         try
         {
-            await using var stream = new MemoryStream(new byte[] { 10, 20, 30, 40, 50 });
+            await using var stream = CreateMemoryStream(new byte[] { 10, 20, 30, 40, 50 }, publiclyVisible);
             stream.Position = 2;
 
             var result = await stream.IsFileIdenticalAsync(path, cancellationToken);
@@ -112,6 +117,96 @@ public class StreamExtensionsTests
         }
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task IsStreamIdenticalAsync_BothMemoryStreams_NonZeroPositions_SeeksToStart(bool publiclyVisible)
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        await using var a = CreateMemoryStream(new byte[] { 1, 2, 3, 4, 5 }, publiclyVisible);
+        await using var b = CreateMemoryStream(new byte[] { 1, 2, 3, 4, 5 }, publiclyVisible);
+        a.Position = 3;
+        b.Position = 1;
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.True(result);
+    }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task IsStreamIdenticalAsync_MemoryStreamPairedWithSeekableNonMemoryStream_NonZeroPositions_SeeksToStart(bool publiclyVisible)
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        await using var a = CreateMemoryStream(new byte[] { 1, 2, 3, 4 }, publiclyVisible);
+        await using var b = new SeekableNonMemoryStream(new byte[] { 1, 2, 3, 4 });
+        a.Position = 2;
+        b.Position = 3;
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.True(result);
+    }
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task IsStreamIdenticalAsync_NonMemoryStreamPairedWithMemoryStream_Swaps_ReturnsTrue(bool publiclyVisible)
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        await using var a = new SeekableNonMemoryStream(new byte[] { 1, 2, 3, 4 });
+        await using var b = CreateMemoryStream(new byte[] { 1, 2, 3, 4 }, publiclyVisible);
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.True(result);
+    }
+
+    [Fact]
+    public async Task IsStreamIdenticalAsync_BothSeekableNonMemoryStreams_NonZeroPositions_SeeksToStart()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        await using var a = new SeekableNonMemoryStream(new byte[] { 1, 2, 3, 4 });
+        await using var b = new SeekableNonMemoryStream(new byte[] { 1, 2, 3, 4 });
+        a.Position = 1;
+        b.Position = 2;
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.True(result);
+    }
+
+    [Fact]
+    public async Task IsStreamIdenticalAsync_NonSeekableShortReads_Identical_ReturnsTrue()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        var data = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
+        await using var a = new ShortReadingNonSeekableStream(data, maxReadSize: 3);
+        await using var b = new ShortReadingNonSeekableStream(data, maxReadSize: 5);
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.True(result);
+    }
+
+    [Fact]
+    public async Task IsStreamIdenticalAsync_NonSeekableShortReads_DifferentLengths_ReturnsFalse()
+    {
+        var cancellationToken = TestContext.Current.CancellationToken;
+        await using var a = new ShortReadingNonSeekableStream(new byte[] { 1, 2, 3, 4 }, maxReadSize: 3);
+        await using var b = new ShortReadingNonSeekableStream(new byte[] { 1, 2, 3, 4, 5 }, maxReadSize: 5);
+
+        var result = await a.IsStreamIdenticalAsync(b, cancellationToken);
+
+        Assert.False(result);
+    }
+
+    private static MemoryStream CreateMemoryStream(byte[] data, bool publiclyVisible)
+        => publiclyVisible
+            ? new MemoryStream(data, 0, data.Length, writable: false, publiclyVisible: true)
+            : new MemoryStream(data);
+
     private sealed class NonSeekableReadStream : Stream
     {
         private readonly Stream _inner;
@@ -173,4 +268,130 @@ public class StreamExtensionsTests
             await base.DisposeAsync();
         }
     }
+
+    private sealed class SeekableNonMemoryStream : Stream
+    {
+        private readonly MemoryStream _inner;
+
+        public SeekableNonMemoryStream(byte[] data)
+        {
+            _inner = new MemoryStream(data, writable: false);
+        }
+
+        public override bool CanRead => true;
+
+        public override bool CanSeek => true;
+
+        public override bool CanWrite => false;
+
+        public override long Length => _inner.Length;
+
+        public override long Position
+        {
+            get => _inner.Position;
+            set => _inner.Position = value;
+        }
+
+        public override void Flush()
+        {
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+            => _inner.Read(buffer, offset, count);
+
+        public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
+            => _inner.ReadAsync(buffer, cancellationToken);
+
+        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            => _inner.ReadAsync(buffer.AsMemory(offset, count), cancellationToken).AsTask();
+
+        public override long Seek(long offset, SeekOrigin origin)
+            => _inner.Seek(offset, origin);
+
+        public override void SetLength(long value)
+            => throw new NotSupportedException();
+
+        public override void Write(byte[] buffer, int offset, int count)
+            => throw new NotSupportedException();
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                _inner.Dispose();
+            }
+
+            base.Dispose(disposing);
+        }
+
+        public override async ValueTask DisposeAsync()
+        {
+            await _inner.DisposeAsync();
+            await base.DisposeAsync();
+        }
+    }
+
+    private sealed class ShortReadingNonSeekableStream : Stream
+    {
+        private readonly Stream _inner;
+        private readonly int _maxReadSize;
+
+        public ShortReadingNonSeekableStream(byte[] data, int maxReadSize)
+        {
+            _inner = new MemoryStream(data, writable: false);
+            _maxReadSize = maxReadSize;
+        }
+
+        public override bool CanRead => true;
+
+        public override bool CanSeek => false;
+
+        public override bool CanWrite => false;
+
+        public override long Length => throw new NotSupportedException();
+
+        public override long Position
+        {
+            get => throw new NotSupportedException();
+            set => throw new NotSupportedException();
+        }
+
+        public override void Flush()
+        {
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+            => _inner.Read(buffer, offset, Math.Min(count, _maxReadSize));
+
+        public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
+            => _inner.ReadAsync(buffer[..Math.Min(buffer.Length, _maxReadSize)], cancellationToken);
+
+        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+            => _inner.ReadAsync(buffer.AsMemory(offset, Math.Min(count, _maxReadSize)), cancellationToken).AsTask();
+
+        public override long Seek(long offset, SeekOrigin origin)
+            => throw new NotSupportedException();
+
+        public override void SetLength(long value)
+            => throw new NotSupportedException();
+
+        public override void Write(byte[] buffer, int offset, int count)
+            => throw new NotSupportedException();
+
+        protected override void Dispose(bool disposing)
+        {
+            if (disposing)
+            {
+                _inner.Dispose();
+            }
+
+            base.Dispose(disposing);
+        }
+
+        public override async ValueTask DisposeAsync()
+        {
+            await _inner.DisposeAsync();
+            await base.DisposeAsync();
+        }
+    }
 }