Introduce FileStreamStrategy as a first step of FileStream rewrite

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -395,6 +395,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileOptions.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileShare.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamStrategy.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\HandleInheritability.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\InvalidDataException.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\IOException.cs" />
@@ -1611,8 +1613,8 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\HijriCalendar.Win32.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Guid.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\DriveInfoInternal.Windows.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.Windows.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.Win32.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.Windows.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.Win32.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamCompletionSource.Win32.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\Path.Windows.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathHelper.Windows.cs" />
@@ -1816,9 +1818,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\GlobalizationMode.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Globalization\HijriCalendar.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Guid.Unix.cs" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.Lock.OSX.cs" Condition="'$(IsOSXLike)' == 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.Lock.Unix.cs" Condition="'$(IsOSXLike)' != 'true'" />
-    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.Unix.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.Lock.OSX.cs" Condition="'$(IsOSXLike)' == 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.Lock.Unix.cs" Condition="'$(IsOSXLike)' != 'true'" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStreamImpl.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\Path.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathInternal.Unix.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PersistedFiles.Names.Unix.cs" />

src/libraries/System.Private.CoreLib/src/System/IO/FileStreamCompletionSource.Win32.cs

@@ -9,7 +9,7 @@
 
 namespace System.IO
 {
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         // This is an internal object extending TaskCompletionSource with fields
         // for all of the relevant data necessary to complete the IO operation.
@@ -25,7 +25,7 @@ private unsafe class FileStreamCompletionSource : TaskCompletionSource<int>
 
             private static Action<object?>? s_cancelCallback;
 
-            private readonly FileStream _stream;
+            private readonly FileStreamImpl _stream;
             private readonly int _numBufferedBytes;
             private CancellationTokenRegistration _cancellationRegistration;
 #if DEBUG
@@ -35,7 +35,7 @@ private unsafe class FileStreamCompletionSource : TaskCompletionSource<int>
             private long _result; // Using long since this needs to be used in Interlocked APIs
 
             // Using RunContinuationsAsynchronously for compat reasons (old API used Task.Factory.StartNew for continuations)
-            protected FileStreamCompletionSource(FileStream stream, int numBufferedBytes, byte[]? bytes)
+            protected FileStreamCompletionSource(FileStreamImpl stream, int numBufferedBytes, byte[]? bytes)
                 : base(TaskCreationOptions.RunContinuationsAsynchronously)
             {
                 _numBufferedBytes = numBufferedBytes;
@@ -132,8 +132,8 @@ internal static void IOCallback(uint errorCode, uint numBytes, NativeOverlapped*
                 // be directly the FileStreamCompletionSource that's completing (in the case where the preallocated
                 // overlapped was already in use by another operation).
                 object? state = ThreadPoolBoundHandle.GetNativeOverlappedState(pOverlapped);
-                Debug.Assert(state is FileStream || state is FileStreamCompletionSource);
-                FileStreamCompletionSource completionSource = state is FileStream fs ?
+                Debug.Assert(state is FileStreamImpl || state is FileStreamCompletionSource);
+                FileStreamCompletionSource completionSource = state is FileStreamImpl fs ?
                     fs._currentOverlappedOwner! : // must be owned
                     (FileStreamCompletionSource)state!;
                 Debug.Assert(completionSource != null);
@@ -220,7 +220,7 @@ private static void Cancel(object? state)
                 }
             }
 
-            public static FileStreamCompletionSource Create(FileStream stream, int numBufferedBytesRead, ReadOnlyMemory<byte> memory)
+            public static FileStreamCompletionSource Create(FileStreamImpl stream, int numBufferedBytesRead, ReadOnlyMemory<byte> memory)
             {
                 // If the memory passed in is the stream's internal buffer, we can use the base FileStreamCompletionSource,
                 // which has a PreAllocatedOverlapped with the memory already pinned.  Otherwise, we use the derived
@@ -241,7 +241,7 @@ private sealed class MemoryFileStreamCompletionSource : FileStreamCompletionSour
         {
             private MemoryHandle _handle; // mutable struct; do not make this readonly
 
-            internal MemoryFileStreamCompletionSource(FileStream stream, int numBufferedBytes, ReadOnlyMemory<byte> memory) :
+            internal MemoryFileStreamCompletionSource(FileStreamImpl stream, int numBufferedBytes, ReadOnlyMemory<byte> memory) :
                 base(stream, numBufferedBytes, bytes: null) // this type handles the pinning, so null is passed for bytes
             {
                 _handle = memory.Pin();

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.Lock.OSX.cs → src/libraries/System.Private.CoreLib/src/System/IO/FileStreamImpl.Lock.OSX.cs

@@ -3,7 +3,7 @@
 
 namespace System.IO
 {
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         private static void LockInternal(long position, long length)
         {

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.Lock.Unix.cs → src/libraries/System.Private.CoreLib/src/System/IO/FileStreamImpl.Lock.Unix.cs

@@ -3,7 +3,7 @@
 
 namespace System.IO
 {
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         /// <summary>Prevents other processes from reading from or writing to the FileStream.</summary>
         /// <param name="position">The beginning of the range to lock.</param>

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.Unix.cs → src/libraries/System.Private.CoreLib/src/System/IO/FileStreamImpl.Unix.cs

@@ -11,7 +11,7 @@
 namespace System.IO
 {
     /// <summary>Provides an implementation of a file stream for Unix files.</summary>
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         /// <summary>File mode.</summary>
         private FileMode _mode;
@@ -61,7 +61,7 @@ private SafeFileHandle OpenHandle(FileMode mode, FileShare share, FileOptions op
             return SafeFileHandle.Open(_path!, openFlags, (int)OpenPermissions);
         }
 
-        private static bool GetDefaultIsAsync(SafeFileHandle handle) => handle.IsAsync ?? DefaultIsAsync;
+        internal static bool GetDefaultIsAsync(SafeFileHandle handle) => handle.IsAsync ?? DefaultIsAsync;
 
         /// <summary>Initializes a stream for reading or writing a Unix file.</summary>
         /// <param name="mode">How the file should be opened.</param>
@@ -305,7 +305,7 @@ public override ValueTask DisposeAsync()
             // override may already exist on a derived type.
             if (_useAsyncIO && _writePos > 0)
             {
-                return new ValueTask(Task.Factory.StartNew(static s => ((FileStream)s!).Dispose(), this,
+                return new ValueTask(Task.Factory.StartNew(static s => ((FileStreamImpl)s!).Dispose(), this,
                     CancellationToken.None, TaskCreationOptions.DenyChildAttach, TaskScheduler.Default));
             }
 
@@ -530,7 +530,7 @@ private unsafe int ReadNative(Span<byte> buffer)
                 // whereas on Windows it may happen before the write has completed.
 
                 Debug.Assert(t.Status == TaskStatus.RanToCompletion);
-                var thisRef = (FileStream)s!;
+                var thisRef = (FileStreamImpl)s!;
                 Debug.Assert(thisRef._asyncState != null);
                 try
                 {
@@ -691,7 +691,7 @@ private ValueTask WriteAsyncInternal(ReadOnlyMemory<byte> source, CancellationTo
                 // whereas on Windows it may happen before the write has completed.
 
                 Debug.Assert(t.Status == TaskStatus.RanToCompletion);
-                var thisRef = (FileStream)s!;
+                var thisRef = (FileStreamImpl)s!;
                 Debug.Assert(thisRef._asyncState != null);
                 try
                 {
@@ -775,7 +775,7 @@ public override long Seek(long offset, SeekOrigin origin)
         /// <returns>The new position in the stream.</returns>
         private long SeekCore(SafeFileHandle fileHandle, long offset, SeekOrigin origin)
         {
-            Debug.Assert(!fileHandle.IsClosed && (GetType() != typeof(FileStream) || CanSeekCore(fileHandle))); // verify that we can seek, but only if CanSeek won't be a virtual call (which could happen in the ctor)
+            Debug.Assert(!fileHandle.IsClosed && CanSeekCore(fileHandle));
             Debug.Assert(origin >= SeekOrigin.Begin && origin <= SeekOrigin.End);
 
             long pos = CheckFileCall(Interop.Sys.LSeek(fileHandle, offset, (Interop.Sys.SeekWhence)(int)origin)); // SeekOrigin values are the same as Interop.libc.SeekWhence values

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.Win32.cs → src/libraries/System.Private.CoreLib/src/System/IO/FileStreamImpl.Win32.cs

@@ -6,7 +6,7 @@
 
 namespace System.IO
 {
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         private SafeFileHandle OpenHandle(FileMode mode, FileShare share, FileOptions options)
         {
@@ -45,7 +45,7 @@ private unsafe SafeFileHandle CreateFileOpenHandle(FileMode mode, FileShare shar
             }
         }
 
-        private static bool GetDefaultIsAsync(SafeFileHandle handle)
+        internal static bool GetDefaultIsAsync(SafeFileHandle handle)
         {
             return handle.IsAsync ?? !IsHandleSynchronous(handle, ignoreInvalid: true) ?? DefaultIsAsync;
         }

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.Windows.cs → src/libraries/System.Private.CoreLib/src/System/IO/FileStreamImpl.Windows.cs

@@ -39,7 +39,7 @@
 
 namespace System.IO
 {
-    public partial class FileStream : Stream
+    internal sealed partial class FileStreamImpl : FileStreamStrategy
     {
         private bool _canSeek;
         private bool _isPipe;      // Whether to disable async buffering code.
@@ -261,20 +261,15 @@ protected override void Dispose(bool disposing)
             }
         }
 
-        public override ValueTask DisposeAsync() =>
-            GetType() == typeof(FileStream) ?
-                DisposeAsyncCore() :
-                base.DisposeAsync();
-
-        private async ValueTask DisposeAsyncCore()
+        public override async ValueTask DisposeAsync()
         {
             // Same logic as in Dispose(), except with async counterparts.
             // TODO: https://github.com/dotnet/runtime/issues/27643: FlushAsync does synchronous work.
             try
             {
                 if (_fileHandle != null && !_fileHandle.IsClosed && _writePos > 0)
                 {
-                    await FlushAsyncInternal(default).ConfigureAwait(false);
+                    await FlushAsync(default).ConfigureAwait(false);
                 }
             }
             finally
@@ -1247,7 +1242,7 @@ public override Task CopyToAsync(Stream destination, int bufferSize, Cancellatio
             // typical read/write looping.  We also need to take this path if this is a derived
             // instance from FileStream, as a derived type could have overridden ReadAsync, in which
             // case our custom CopyToAsync implementation isn't necessarily correct.
-            if (!_useAsyncIO || GetType() != typeof(FileStream))
+            if (!_useAsyncIO)
             {
                 return base.CopyToAsync(destination, bufferSize, cancellationToken);
             }
@@ -1474,7 +1469,7 @@ private sealed unsafe class AsyncCopyToAwaitable : ICriticalNotifyCompletion
             internal static readonly IOCompletionCallback s_callback = IOCallback;
 
             /// <summary>The FileStream that owns this instance.</summary>
-            internal readonly FileStream _fileStream;
+            internal readonly FileStreamImpl _fileStream;
 
             /// <summary>Tracked position representing the next location from which to read.</summary>
             internal long _position;
@@ -1495,7 +1490,7 @@ private sealed unsafe class AsyncCopyToAwaitable : ICriticalNotifyCompletion
             internal object CancellationLock => this;
 
             /// <summary>Initialize the awaitable.</summary>
-            internal AsyncCopyToAwaitable(FileStream fileStream)
+            internal AsyncCopyToAwaitable(FileStreamImpl fileStream)
             {
                 _fileStream = fileStream;
             }