fix: handle shared file contents across handles

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockFileStream.cs

@@ -38,6 +38,8 @@ public NullFileSystemStream() : base(Null, ".", true)
     private readonly MockFileData fileData;
     private bool disposed;
 
+    private byte[] lastKnownContents;
+
     /// <inheritdoc />
     public MockFileStream(
         IMockFileDataAccessor mockFileDataAccessor,
@@ -104,6 +106,7 @@ public MockFileStream(
         mockFileDataAccessor.FileHandles.AddHandle(path, guid, access, share);
         this.access = access;
         this.share = share;
+        lastKnownContents = fileData.Contents;
     }
 
     private static void ThrowIfInvalidModeAccess(FileMode mode, FileAccess access)
@@ -138,11 +141,46 @@ private static void ThrowIfInvalidModeAccess(FileMode mode, FileAccess access)
     /// <inheritdoc />
     public override int Read(byte[] buffer, int offset, int count)
     {
+        RefreshBufferFromSharedFileData();
         mockFileDataAccessor.AdjustTimes(fileData,
             TimeAdjustments.LastAccessTime);
         return base.Read(buffer, offset, count);
     }
 
+#if FEATURE_SPAN
+        /// <inheritdoc />
+        public override int Read(Span<byte> buffer)
+        {
+            RefreshBufferFromSharedFileData();
+            return base.Read(buffer);
+        }
+#endif
+
+    /// <inheritdoc />
+    public override int ReadByte()
+    {
+        RefreshBufferFromSharedFileData();
+        return base.ReadByte();
+    }
+
+    /// <inheritdoc />
+    public override Task<int> ReadAsync(byte[] buffer, int offset, int count,
+        CancellationToken cancellationToken)
+    {
+        RefreshBufferFromSharedFileData();
+        return base.ReadAsync(buffer, offset, count, cancellationToken);
+    }
+
+#if FEATURE_SPAN
+        /// <inheritdoc />
+        public override ValueTask<int> ReadAsync(Memory<byte> buffer,
+            CancellationToken cancellationToken = new())
+        {
+            RefreshBufferFromSharedFileData();
+            return base.ReadAsync(buffer, cancellationToken);
+        }
+#endif
+
     /// <inheritdoc />
     protected override void Dispose(bool disposing)
     {
@@ -296,16 +334,63 @@ private void InternalFlush()
             /* reset back to the beginning .. */
             var position = Position;
             Seek(0, SeekOrigin.Begin);
-            /* .. read everything out */
+            /* .. read everything out (bypassing the read-refresh, so we capture this handle's own buffer) */
             var data = new byte[Length];
-            _ = Read(data, 0, (int)Length);
+            _ = base.Read(data, 0, (int)Length);
             /* restore to original position */
             Seek(position, SeekOrigin.Begin);
-            /* .. put it in the mock system */
-            mockFileData.Contents = data;
+            /* .. put it in the mock system, but only if this handle actually changed the contents since it last synchronized. */
+            if (!ContentsEqual(data, lastKnownContents))
+            {
+                mockFileData.Contents = data;
+                lastKnownContents = data;
+            }
         }
     }
 
+    private static bool ContentsEqual(byte[] left, byte[] right)
+    {
+        if (ReferenceEquals(left, right))
+        {
+            return true;
+        }
+
+        if (left == null || right == null || left.Length != right.Length)
+        {
+            return false;
+        }
+
+        for (var i = 0; i < left.Length; i++)
+        {
+            if (left[i] != right[i])
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private void RefreshBufferFromSharedFileData()
+    {
+        var sharedContents = fileData.Contents;
+        if (ReferenceEquals(sharedContents, lastKnownContents))
+        {
+            return;
+        }
+
+        var position = base.Position;
+        base.Position = 0;
+        base.SetLength(sharedContents.Length);
+        if (sharedContents.Length > 0)
+        {
+            base.Write(sharedContents, 0, sharedContents.Length);
+        }
+
+        base.Position = position;
+        lastKnownContents = sharedContents;
+    }
+
     private void OnClose()
     {
         if (options.HasFlag(FileOptions.DeleteOnClose) && mockFileDataAccessor.FileExists(path))

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_net10.0.txt

@@ -404,7 +404,11 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
+        public override int Read(System.Span<byte> buffer) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.ValueTask<int> ReadAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken = default) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_net472.txt

@@ -316,6 +316,8 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }
         public override void Write(byte[] buffer, int offset, int count) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_net6.0.txt

@@ -366,7 +366,11 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
+        public override int Read(System.Span<byte> buffer) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.ValueTask<int> ReadAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken = default) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_net8.0.txt

@@ -390,7 +390,11 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
+        public override int Read(System.Span<byte> buffer) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.ValueTask<int> ReadAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken = default) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_net9.0.txt

@@ -404,7 +404,11 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
+        public override int Read(System.Span<byte> buffer) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.ValueTask<int> ReadAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken = default) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         [System.Runtime.Versioning.SupportedOSPlatform("windows")]
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_netstandard2.0.txt

@@ -316,6 +316,8 @@ namespace System.IO.Abstractions.TestingHelpers
         public object GetAccessControl() { }
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }
         public override void Write(byte[] buffer, int offset, int count) { }

tests/TestableIO.System.IO.Abstractions.Api.Tests/Expected/TestableIO.System.IO.Abstractions.TestingHelpers_netstandard2.1.txt

@@ -341,7 +341,11 @@ namespace System.IO.Abstractions.TestingHelpers
         public override System.Threading.Tasks.Task FlushAsync(System.Threading.CancellationToken cancellationToken) { }
         public object GetAccessControl() { }
         public object GetAccessControl(System.IO.Abstractions.IFileSystemAclSupport.AccessControlSections includeSections) { }
+        public override int Read(System.Span<byte> buffer) { }
         public override int Read(byte[] buffer, int offset, int count) { }
+        public override System.Threading.Tasks.ValueTask<int> ReadAsync(System.Memory<byte> buffer, System.Threading.CancellationToken cancellationToken = default) { }
+        public override System.Threading.Tasks.Task<int> ReadAsync(byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken) { }
+        public override int ReadByte() { }
         public void SetAccessControl(object value) { }
         public override void SetLength(long value) { }
         public override void Write(System.ReadOnlySpan<byte> buffer) { }

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockFileStreamTests.cs

@@ -10,6 +10,39 @@
 [TestFixture]
 public class MockFileStreamTests
 {
+    [Test]
+    public async Task MockFileStream_SharedReadWrite_SecondHandleSeesWritesFromFirstHandle()
+    {
+        // Arrange
+        var filename = XFS.Path(@"C:\temp\shared.bin");
+        var fileSystem = new MockFileSystem();
+        fileSystem.AddEmptyFile(filename);
+        var results = new List<int>();
+
+        // Act
+        using (var file1 = fileSystem.FileStream.New(filename, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite))
+        using (var file2 = fileSystem.FileStream.New(filename, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite))
+        {
+            var buffer = new byte[4];
+            for (int ix = 0; ix < 10; ix++)
+            {
+                var bytes = BitConverter.GetBytes(ix);
+                file1.Position = 0;
+                file1.Write(bytes, 0, bytes.Length);
+                file1.Flush();
+
+                file2.Position = 0;
+                file2.Flush();
+                var bytesRead = file2.Read(buffer, 0, buffer.Length);
+                await That(bytesRead).IsEqualTo(buffer.Length);
+                results.Add(BitConverter.ToInt32(buffer, 0));
+            }
+        }
+
+        // Assert
+        await That(results).IsEqualTo(new[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 });
+    }
+
     [Test]
     public async Task MockFileStream_Flush_WritesByteToFile()
     {