Breaking changes for FileStream

docs/core/compatibility/6.0.md

@@ -42,6 +42,8 @@ If you're migrating an app to .NET 6, the breaking changes listed here might aff
 | [API obsoletions with non-default diagnostic IDs](core-libraries/6.0/obsolete-apis-with-custom-diagnostics.md) | Preview 1 |
 | [Changes to nullable reference type annotations](core-libraries/6.0/nullable-ref-type-annotation-changes.md) | Preview 1-2 |
 | [Environment.ProcessorCount behavior on Windows](core-libraries/6.0/environment-processorcount-on-windows.md) | Preview 2 |
+| [FileStream no longer synchronizes file offset with OS](core-libraries/6.0/filestream-doesnt-sync-offset-with-os.md) | Preview 4 |
+| [FileStream.Position updates after ReadAsync or WriteAsync completes](core-libraries/6.0/filestream-position-updates-after-readasync-writeasync-completion.md) | Preview 4 |
 | [New diagnostic IDs for obsoleted APIs](core-libraries/6.0/diagnostic-id-change-for-obsoletions.md) | Preview 5 |
 | [New System.Linq.Queryable method overloads](core-libraries/6.0/additional-linq-queryable-method-overloads.md) | Preview 3-4 |
 | [Older framework versions dropped from package](core-libraries/6.0/older-framework-versions-dropped.md) | Preview 5 |

docs/core/compatibility/core-libraries/6.0/filestream-doesnt-sync-offset-with-os.md

@@ -0,0 +1,87 @@
+---
+title: ".NET 6 breaking change: FileStream doesn't synchronize file offset with OS"
+description: Learn about the .NET 6 breaking change in core .NET libraries where FileStream doesn't synchronize the file offset with the operating system.
+ms.date: 05/05/2021
+---
+# FileStream no longer synchronizes file offset with OS
+
+To improve performance, <xref:System.IO.FileStream> no longer synchronizes the file offset with the operating system.
+
+## Change description
+
+In previous .NET versions, <xref:System.IO.FileStream> synchronizes the file offset with the Windows operating system (OS) when it reads or writes to a file. It synchronizes the offset by calling [SetFilePointer](/windows/win32/api/fileapi/nf-fileapi-setfilepointer), which is an expensive system call. Starting in .NET 6, <xref:System.IO.FileStream> no longer synchronizes the file offset, and instead just keeps the offset in memory. <xref:System.IO.FileStream.Position?displayProperty=nameWithType> always returns the current offset, but if you obtain the file handle from <xref:System.IO.FileStream.SafeFileHandle?displayProperty=nameWithType> and query the OS for the current file offset using a system call, the offset value will be 0.
+
+The following code shows how the file offset differs between previous .NET versions and .NET 6.
+
+```csharp
+[DllImport("kernel32.dll")]
+private static extern bool SetFilePointerEx(SafeFileHandle hFile, long liDistanceToMove, out long lpNewFilePointer, uint dwMoveMethod);
+
+byte[] bytes = new byte[10_000];
+string path = Path.Combine(Path.GetTempPath(), Path.GetTempFileName());
+
+using (FileStream fs = new FileStream(path, FileMode.Create, FileAccess.ReadWrite, FileShare.None, bufferSize: 4096, useAsync: true))
+{
+    SafeFileHandle handle = fs.SafeFileHandle;
+
+    await fs.WriteAsync(bytes, 0, bytes.Length);
+    Console.WriteLine(fs.Position); // 10000 in all versions
+
+    if (SetFilePointerEx(handle, 0, out long currentOffset, 1 /* get current offset */))
+    {
+        Console.WriteLine(currentOffset);  // 10000 in .NET 5, 0 in .NET 6
+    }
+}
+```
+
+## Version introduced
+
+6.0 Preview 4
+
+## Reason for change
+
+This change was introduced to improve the performance of asynchronous reads and writes and to address the following issues:
+
+- [Win32 FileStream will issue a seek on every ReadAsync call](https://github.com/dotnet/runtime/issue/16354)
+- [FileStream.Windows useAsync WriteAsync calls blocking APIs](https://github.com/dotnet/runtime/issue/25905)
+
+With this change, <xref:System.IO.FileStream.ReadAsync%2A> operations are up to two times faster, and <xref:System.IO.FileStream.WriteAsync%2A> operations are up to five times faster.
+
+## Recommended action
+
+- Modify any code that relied on the offset being synchronized.
+
+- To enable the .NET 5 behavior in .NET 6, specify an `AppContext` switch or an environment variable. By setting the switch to `true`, you opt out of all performance improvements made to `FileStream` in .NET 6.
+
+  ```xml
+  {
+      "configProperties": {
+          "System.IO.UseNet5CompatFileStream": true
+      }
+  }
+  ```
+
+  ```cmd
+  set DOTNET_SYSTEM_IO_USENET5COMPATFILESTREAM=1
+  ```
+
+## Affected APIs
+
+None.
+
+## See also
+
+- [SetFilePointer function](/windows/win32/api/fileapi/nf-fileapi-setfilepointer)
+- [SetFilePointerEx function](/windows/win32/api/fileapi/nf-fileapi-setfilepointerex)
+
+<!--
+
+### Category
+
+- Core .NET libraries
+
+### Affected APIS
+
+Not detectible via API analysis.
+
+-->

docs/core/compatibility/core-libraries/6.0/filestream-position-updates-after-readasync-writeasync-completion.md

@@ -0,0 +1,85 @@
+---
+title: ".NET 6 breaking change: FileStream.Position updated after ReadAsync or WriteAsync completion"
+description: Learn about the .NET 6 breaking change in core .NET libraries where FileStream.Position is updated after ReadAsync or WriteAsync completion.
+ms.date: 05/05/2021
+---
+# FileStream.Position updates after ReadAsync or WriteAsync completes
+
+<xref:System.IO.FileStream.Position?displayProperty=nameWithType> is now updated after <xref:System.IO.FileStream.ReadAsync%2A> or <xref:System.IO.FileStream.WriteAsync%2A> completes.
+
+## Change description
+
+In previous .NET versions on Windows, <xref:System.IO.FileStream.Position?displayProperty=nameWithType> is updated after the asynchronous read or write operation starts. Starting in .NET 6, <xref:System.IO.FileStream.Position?displayProperty=nameWithType> is updated after those operations complete.
+
+The following code shows how the value of <xref:System.IO.FileStream.Position?displayProperty=nameWithType> differs between previous .NET versions and .NET 6.
+
+```csharp
+byte[] bytes = new byte[10_000];
+string path = Path.Combine(Path.GetTempPath(), Path.GetTempFileName());
+
+using (FileStream fs = new FileStream(path, FileMode.Create, FileAccess.ReadWrite, FileShare.None, bufferSize: 4096, useAsync: true))
+{
+    Task[] writes = new Task[3];
+
+    writes[0] = fs.WriteAsync(bytes, 0, bytes.Length);
+    Console.WriteLine(fs.Position);  // 10000 in .NET 5, 0 in .NET 6
+
+    writes[1] = fs.WriteAsync(bytes, 0, bytes.Length);
+    Console.WriteLine(fs.Position);  // 20000 in .NET 5, 0 in .NET 6
+
+    writes[2] = fs.WriteAsync(bytes, 0, bytes.Length);
+    Console.WriteLine(fs.Position);  // 30000 in .NET 5, 0 in .NET 6
+
+    await Task.WhenAll(writes);
+    Console.WriteLine(fs.Position);  // 30000 in all versions
+}
+```
+
+## Version introduced
+
+6.0 Preview 4
+
+## Reason for change
+
+<xref:System.IO.FileStream> has never been thread-safe, but until .NET 6, .NET has tried to support multiple concurrent calls to its asynchronous methods (<xref:System.IO.FileStream.ReadAsync%2A> and <xref:System.IO.FileStream.WriteAsync%2A>) on Windows.
+
+This change was introduced to allow for 100% asynchronous file I/O with <xref:System.IO.FileStream> and to fix the following issues:
+
+- [FileStream.FlushAsync ends up doing synchronous writes](https://github.com/dotnet/runtime/issue/27643)
+- [Win32 FileStream turns async reads into sync reads](https://github.com/dotnet/runtime/issue/16341)
+
+Now, when buffering is enabled (that is, the `bufferSize` argument that's passed to the [FileStream constructor](xref:System.IO.FileStream.%23ctor%2A) is greater than 1), every <xref:System.IO.FileStream.ReadAsync%2A> and <xref:System.IO.FileStream.WriteAsync%2A> operation is serialized.
+
+## Recommended action
+
+- Modify any code that relied on the position being set before operations completed.
+
+- To enable the .NET 5 behavior in .NET 6, specify an `AppContext` switch or an environment variable. By setting the switch to `true`, you opt out of all performance improvements made to `FileStream` in .NET 6.
+
+  ```xml
+  {
+      "configProperties": {
+          "System.IO.UseNet5CompatFileStream": true
+      }
+  }
+  ```
+
+  ```cmd
+  set DOTNET_SYSTEM_IO_USENET5COMPATFILESTREAM=1
+  ```
+
+## Affected APIs
+
+- <xref:System.IO.FileStream.Position?displayProperty=fullName>
+
+<!--
+
+### Category
+
+- Core .NET libraries
+
+### Affected APIs
+
+- `P:System.IO.FileStream.Position`
+
+-->

docs/core/compatibility/toc.yml

@@ -65,6 +65,10 @@ items:
           href: core-libraries/6.0/obsolete-apis-with-custom-diagnostics.md
         - name: Environment.ProcessorCount behavior on Windows
           href: core-libraries/6.0/environment-processorcount-on-windows.md
+        - name: FileStream no longer synchronizes offset with OS
+          href: core-libraries/6.0/filestream-doesnt-sync-offset-with-os.md
+        - name: FileStream.Position updated after completion
+          href: core-libraries/6.0/filestream-position-updates-after-readasync-writeasync-completion.md
         - name: New diagnostic IDs for obsoleted APIs
           href: core-libraries/6.0/diagnostic-id-change-for-obsoletions.md
         - name: New Queryable method overloads
@@ -533,6 +537,10 @@ items:
           href: core-libraries/6.0/obsolete-apis-with-custom-diagnostics.md
         - name: Environment.ProcessorCount behavior on Windows
           href: core-libraries/6.0/environment-processorcount-on-windows.md
+        - name: FileStream no longer synchronizes offset with OS
+          href: core-libraries/6.0/filestream-doesnt-sync-offset-with-os.md
+        - name: FileStream.Position updated after completion
+          href: core-libraries/6.0/filestream-position-updates-after-readasync-writeasync-completion.md
         - name: New diagnostic IDs for obsoleted APIs
           href: core-libraries/6.0/diagnostic-id-change-for-obsoletions.md
         - name: New Queryable method overloads