[release/7.0] Invalidate buffered data when bypassing the cache

src/libraries/System.IO.FileSystem/tests/FileStream/ReadAsync.cs

@@ -11,7 +11,7 @@ namespace System.IO.Tests
 {
     public abstract class FileStream_AsyncReads : FileSystemTest
     {
-        protected abstract Task<int> ReadAsync(FileStream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken = default);
+        protected abstract Task<int> ReadAsync(Stream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken = default);
 
         [Fact]
         public async Task EmptyFileReadAsyncSucceedSynchronously()
@@ -132,17 +132,72 @@ public async Task IncompleteReadCantSetPositionBeyondEndOfFile(FileShare fileSha
                 Assert.Equal(fileSize, fs.Position);
             }
         }
+
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
+        [InlineData(true, true)]
+        [InlineData(true, false)]
+        [InlineData(false, false)]
+        [InlineData(false, true)]
+        public async Task BypassingCacheInvalidatesCachedData(bool fsIsAsync, bool asyncReads)
+        {
+            const int BufferSize = 4096;
+            const int FileSize = BufferSize * 4;
+            string filePath = GetTestFilePath();
+            byte[] content = RandomNumberGenerator.GetBytes(FileSize);
+            File.WriteAllBytes(filePath, content);
+
+            await Test(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, BufferSize, fsIsAsync));
+            await Test(new BufferedStream(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, 0, fsIsAsync), BufferSize));
+
+            async Task Test(Stream stream)
+            {
+                try
+                {
+                    // 1. Populates the private stream buffer, leaves bufferSize - 1 bytes available for next read.
+                    await ReadAndAssertAsync(stream, 1);
+                    // 2. Consumes all available data from the buffer, reads another bufferSize-many bytes from the disk and copies the 1 missing byte.
+                    await ReadAndAssertAsync(stream, BufferSize);
+                    // 3. Seek back by the number of bytes consumed from the buffer, all buffered data is now available for next read.
+                    stream.Position -= 1;
+                    // 4. Consume all buffered data.
+                    await ReadAndAssertAsync(stream, BufferSize);
+                    // 5. Bypass the cache (all buffered data has been consumed and we need bufferSize-many bytes).
+                    // The cache should get invalidated now!!
+                    await ReadAndAssertAsync(stream,BufferSize);
+                    // 6. Seek back by just a few bytes.
+                    stream.Position -= 9;
+                    // 7. Perform a read, which should not use outdated buffered data.
+                    await ReadAndAssertAsync(stream,BufferSize);
+                }
+                finally
+                {
+                    await stream.DisposeAsync();
+                }
+            }
+
+            async Task ReadAndAssertAsync(Stream stream, int size)
+            {
+                var initialPosition = stream.Position;
+                var buffer = new byte[size];
+
+                var count = asyncReads
+                    ? await ReadAsync(stream, buffer, 0, size)
+                    : stream.Read(buffer);
+
+                Assert.Equal(content.Skip((int)initialPosition).Take(count), buffer.Take(count));
+            }
+        }
     }
 
     public class FileStream_ReadAsync_AsyncReads : FileStream_AsyncReads
     {
-        protected override Task<int> ReadAsync(FileStream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
+        protected override Task<int> ReadAsync(Stream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
             stream.ReadAsync(buffer, offset, count, cancellationToken);
     }
 
     public class FileStream_BeginEndRead_AsyncReads : FileStream_AsyncReads
     {
-        protected override Task<int> ReadAsync(FileStream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
+        protected override Task<int> ReadAsync(Stream stream, byte[] buffer, int offset, int count, CancellationToken cancellationToken) =>
             Task.Factory.FromAsync(
                 (callback, state) => stream.BeginRead(buffer, offset, count, callback, state),
                 iar => stream.EndRead(iar),

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

@@ -326,6 +326,8 @@ public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken
                 {
                     if (_readLen == _readPos && buffer.Length >= _bufferSize)
                     {
+                        // invalidate the buffered data, otherwise certain Seek operation followed by a ReadAsync could try to re-use data from _buffer
+                        _readPos = _readLen = 0;
                         // hot path #1: the read buffer is empty and buffering would not be beneficial
                         // To find out why we are bypassing cache here, please see WriteAsync comments.
                         return _strategy.ReadAsync(buffer, cancellationToken);