test: cover the SafeFileHandle surface and drop the stale exclusions

Directory.Packages.props

@@ -55,8 +55,8 @@
 		<PackageVersion Include="AutoFixture" Version="4.18.1" />
 	</ItemGroup>
 	<ItemGroup>
-		<PackageVersion Include="Testably.Abstractions.Interface" Version="[10.3.0,10.4.0)"/>
-		<PackageVersion Include="Testably.Abstractions" Version="[10.3.0,10.4.0)"/>
+		<PackageVersion Include="Testably.Abstractions.Interface" Version="[10.4.0-pre.1,10.5.0)"/>
+		<PackageVersion Include="Testably.Abstractions" Version="[10.4.0-pre.1,10.5.0)"/>
 		<PackageVersion Include="Testably.Abstractions.Testing" Version="6.6.0"/>
 	</ItemGroup>
 	<ItemGroup>

Docs/pages/docs/file-system/random-access.mdx

@@ -0,0 +1,69 @@
+---
+sidebar_position: 12
+title: RandomAccess
+---
+
+# RandomAccess
+
+`IFileSystem.RandomAccess` abstracts the static methods on `System.IO.RandomAccess`. It reads and writes a file through a `SafeFileHandle` at an explicit offset, without a stream in between.
+
+Every method is positionless: the offset is a parameter, not state carried on the handle. Nothing advances, so two reads at the same offset return the same bytes and concurrent writes at different offsets do not interfere.
+
+```csharp
+MockFileSystem fileSystem = new();
+fileSystem.File.WriteAllBytes("data.bin", [1, 2, 3, 4]);
+
+using SafeFileHandle handle = fileSystem.File.OpenHandle("data.bin",
+    FileMode.Open, FileAccess.ReadWrite);
+
+byte[] buffer = new byte[2];
+fileSystem.RandomAccess.Read(handle, buffer, fileOffset: 1);   // buffer is [2, 3]
+
+fileSystem.RandomAccess.Write(handle, new byte[] { 9 }, fileOffset: 0);
+await Expect.That(fileSystem.File.ReadAllBytes("data.bin"))
+    .IsEqualTo(new byte[] { 9, 2, 3, 4 });
+```
+
+Get the handle from [`IFile.OpenHandle`](./safe-file-handles), so the file system it came from can resolve it.
+
+## What is available
+
+| Member | Available from |
+|---|---|
+| `GetLength` | .NET 6 |
+| `Read`, `ReadAsync` | .NET 6 |
+| `Write`, `WriteAsync` | .NET 6 |
+| `SetLength` | .NET 8 |
+| `FlushToDisk` | .NET 8 |
+
+`Read` and `Write` each have a scatter/gather overload taking an `IReadOnlyList<Memory<byte>>`, which fills or consumes the buffers in order as though they were one.
+
+## Behaviour worth knowing
+
+- A read whose offset is at or past the end of the file returns `0`; a read that runs off the end returns a short count rather than throwing.
+- A write past the end grows the file and zero-fills the gap.
+- `SetLength` truncates, or grows with zeroes.
+- `GetLength` is metadata and works on a write-only handle. Reading a write-only handle, or writing a read-only one, throws `UnauthorizedAccessException`.
+- On a handle opened with `FileMode.Append` the offset is still honoured — `RandomAccess` does not append for you.
+- Operations on a disposed handle throw `ObjectDisposedException`.
+
+## `FlushToDisk` against the mock
+
+`FlushToDisk` asks the operating system to make prior writes durable. The `MockFileSystem` keeps its files in memory and has no write-back cache, so there is nothing to flush — but the call is still resolved against the handle and recorded, so a test can assert that a durability barrier was requested:
+
+```csharp
+MockFileSystem fileSystem = new();
+fileSystem.File.WriteAllText("log", null);
+
+using (SafeFileHandle handle = fileSystem.File.OpenHandle("log",
+    FileMode.Open, FileAccess.Write))
+{
+    fileSystem.RandomAccess.Write(handle, "entry"u8.ToArray(), 0);
+    fileSystem.RandomAccess.FlushToDisk(handle);
+}
+
+await Expect.That(fileSystem.Statistics.RandomAccess.Methods)
+    .Contains(m => m.Name == nameof(IRandomAccess.FlushToDisk));
+```
+
+That is something the real file system cannot tell you, and it is the reason to reach for the abstraction rather than the static class: code that needs a durability barrier can be tested for asking.

Docs/pages/docs/file-system/safe-file-handles.mdx

@@ -5,11 +5,38 @@ title: SafeFileHandle
 
 # SafeFileHandle
 
-`SafeFileHandle` is a wrapper around an OS handle to a real file, and overloads like `File.GetLastAccessTime(SafeFileHandle)` route through it. Because the mock has no kernel handle, you have to tell it how to translate handles into mock paths.
+`SafeFileHandle` is a wrapper around an OS handle to a real file, and overloads like `File.GetLastAccessTime(SafeFileHandle)` route through it.
 
-`MockFileSystem.WithSafeFileHandleStrategy(ISafeFileHandleStrategy)` registers the translation. The strategy maps a `SafeFileHandle` to a `SafeFileHandleMock` that points at a location inside the mock.
+## Handles from the abstraction
 
-The default strategy (`NullSafeFileHandleStrategy`) is registered automatically and throws `NotSupportedException` for any handle - install a custom strategy as soon as your code under test reaches for `SafeFileHandle`.
+`IFile.OpenHandle` returns a handle that the file system it came from understands, so nothing has to be registered:
+
+```csharp
+MockFileSystem fileSystem = new();
+fileSystem.File.WriteAllText("file.txt", "some content");
+
+using SafeFileHandle handle = fileSystem.File.OpenHandle("file.txt");
+
+await Expect.That(fileSystem.File.GetLastAccessTime(handle))
+    .IsEqualTo(fileSystem.File.GetLastAccessTime("file.txt"));
+await Expect.That(fileSystem.RandomAccess.GetLength(handle)).IsEqualTo(12);
+```
+
+The handle refers to the file it was opened on rather than to its name, so reads and writes keep working after the file is renamed, and do not start addressing whatever is later created under the original path.
+
+It also carries the file share and the `FileOptions` it was opened with. `FileOptions.DeleteOnClose` follows the platform: on Unix the name that was opened is unlinked as soon as that handle closes, whatever else still holds the file open, so a file renamed in the meantime survives and a replacement under the old name does not; on Windows the file is removed once nothing holds it open any more, streams included.
+
+Because `SafeFileHandle` is sealed, the mock cannot be notified when one is closed, so released share locks and `DeleteOnClose` take effect the next time the file system is asked whether a file exists or may be opened, rather than at the moment of disposal. A handle that is dropped without being disposed releases its share lock once it is garbage collected, as a real handle does when it is finalized.
+
+A handle the `MockFileSystem` created is not a real OS handle, so passing it to `System.IO.RandomAccess` or to another process fails. Use `IFileSystem.RandomAccess` and the `IFile` overloads instead, which is the point of the abstraction.
+
+## Handles from elsewhere
+
+When a handle comes from somewhere the mock knows nothing about - platform invocation, or a library that hands you one - you still have to tell it which file that handle stands for.
+
+`MockFileSystem.WithSafeFileHandleStrategy(ISafeFileHandleStrategy)` registers the translation. The strategy maps a `SafeFileHandle` to a `SafeFileHandleMock` that points at a location inside the mock, and is consulted for any handle the mock did not create itself.
+
+The default strategy (`NullSafeFileHandleStrategy`) is registered automatically and throws `NotSupportedException` for any handle - install a custom strategy as soon as your code under test reaches for a handle from outside.
 
 ## A complete reference implementation
 
@@ -78,8 +105,8 @@ realFileSystem.File.WriteAllText("real", "some content");
 DateTime expected = new DateTime(2026, 1, 1);
 realFileSystem.File.SetLastAccessTime("real", expected);
 
-// Open a SafeFileHandle on the real file via a small P/Invoke helper
-SafeFileHandle handle = UnmanagedFileLoader.CreateSafeFileHandle("real");
+// Open a SafeFileHandle on the real file
+SafeFileHandle handle = realFileSystem.File.OpenHandle("real");
 
 // Register the mapping
 strategy.AddMapping(handle, "real", new SafeFileHandleMock("mock"));
@@ -92,4 +119,4 @@ await Expect.That(mockFileSystem.File.GetLastAccessTime(handle)).IsEqualTo(expec
 await Expect.That(mockFileSystem.File.GetLastAccessTime("mock")).IsEqualTo(expected);
 ```
 
-`UnmanagedFileLoader` is a small P/Invoke helper that calls the Win32 `CreateFile` API to obtain a `SafeFileHandle` for an existing path.
+Here the handle belongs to the *real* file system, so the mock cannot resolve it on its own and the strategy supplies the mapping. A handle from `mockFileSystem.File.OpenHandle` needs none of this.

Docs/pages/docs/file-system/statistics.mdx

@@ -33,6 +33,7 @@ await Expect.That(stats.File.Methods).Contains(m => m.Name == nameof(IFile.Write
 | `FileSystemWatcher`   | `IPathStatistics<IFileSystemWatcherFactory, IFileSystemWatcher>`    |
 | `FileVersionInfo`     | `IPathStatistics<IFileVersionInfoFactory, IFileVersionInfo>`        |
 | `Path`                | `IStatistics<IPath>`                                                |
+| `RandomAccess`        | `IStatistics<IRandomAccess>`                                        |
 
 `IStatistics<T>` exposes `Methods` and `Properties` collections. Each entry carries the call's name, parameters and a stack-frame counter so you can correlate concurrent activity. `IPathStatistics<TFactory, TItem>` adds an indexer keyed by the path that the factory produced - `stats.FileInfo["a.txt"]` returns just the calls made on that specific `IFileInfo`.
 

Pipeline/Build.cs

@@ -20,7 +20,7 @@ partial class Build : FalloutBuild
 	///     <para />
 	///     Afterward, you can update the package reference in `Directory.Packages.props` and reset this flag.
 	/// </summary>
-	readonly BuildScope BuildScope = BuildScope.CoreOnly;
+	readonly BuildScope BuildScope = BuildScope.Default;
 
 	[Parameter("Configuration to build - Default is 'Debug' (local) or 'Release' (server)")]
 	readonly Configuration Configuration = IsLocalBuild ? Configuration.Debug : Configuration.Release;

Source/Testably.Abstractions.Testing/FileSystem/FileMock.cs

@@ -6,7 +6,7 @@
 using System.Text;
 using Testably.Abstractions.Testing.Helpers;
 using Testably.Abstractions.Testing.Storage;
-#if FEATURE_FILESYSTEM_SAFEFILEHANDLE
+#if FEATURE_FILESYSTEM_SAFEFILEHANDLE || FEATURE_FILESYSTEM_RANDOMACCESS
 using Microsoft.Win32.SafeHandles;
 #endif
 #if FEATURE_FILESYSTEM_ASYNC
@@ -827,6 +827,24 @@ public FileSystemStream Open(string path, FileStreamOptions options)
 	}
 #endif
 
+#if FEATURE_FILESYSTEM_RANDOMACCESS
+	/// <inheritdoc cref="IFile.OpenHandle(string, FileMode, FileAccess, FileShare, FileOptions, long)" />
+	public SafeFileHandle OpenHandle(string path,
+		FileMode mode = FileMode.Open,
+		FileAccess access = FileAccess.Read,
+		FileShare share = FileShare.Read,
+		FileOptions options = FileOptions.None,
+		long preallocationSize = 0)
+	{
+		using IDisposable registration = _fileSystem.StatisticsRegistration
+			.File.RegisterMethod(nameof(OpenHandle),
+				path, mode, access, share, options, preallocationSize);
+
+		return _fileSystem.SafeFileHandleRegistry
+			.Open(path, mode, access, share, options, preallocationSize);
+	}
+#endif
+
 	/// <inheritdoc cref="IFile.OpenRead(string)" />
 	public FileSystemStream OpenRead(string path)
 	{
@@ -1671,20 +1689,7 @@ private IStorageContainer GetContainerFromPath(string path,
 
 #if FEATURE_FILESYSTEM_SAFEFILEHANDLE
 	private IStorageContainer GetContainerFromSafeFileHandle(SafeFileHandle fileHandle)
-	{
-		SafeFileHandleMock safeFileHandleMock = _fileSystem
-			.SafeFileHandleStrategy.MapSafeFileHandle(fileHandle);
-		IStorageContainer container = _fileSystem.Storage
-			.GetContainer(_fileSystem.Storage.GetLocation(
-					safeFileHandleMock.Path)
-				.ThrowExceptionIfNotFound(_fileSystem));
-		if (container is NullContainer)
-		{
-			throw ExceptionFactory.FileNotFound("");
-		}
-
-		return container;
-	}
+		=> _fileSystem.SafeFileHandleRegistry.GetContainer(fileHandle).Container;
 #endif
 
 	private void WriteText(string path, string? contents, Encoding? encoding, bool append = false)

Source/Testably.Abstractions.Testing/FileSystem/FileStreamFactoryMock.cs

@@ -144,17 +144,13 @@ private FileStreamMock New(string path,
 #endif
 
 	/// <inheritdoc cref="IFileStreamFactory.New(SafeFileHandle, FileAccess)" />
-#if NET6_0_OR_GREATER
-	[ExcludeFromCodeCoverage(Justification = "SafeFileHandle cannot be unit tested.")]
-#endif
 	public FileSystemStream New(SafeFileHandle handle, FileAccess access)
 	{
 		using IDisposable registration = _fileSystem.StatisticsRegistration
 			.FileStream.RegisterMethod(nameof(New),
 				handle, access);
 
-		SafeFileHandleMock safeFileHandleMock = _fileSystem
-			.SafeFileHandleStrategy.MapSafeFileHandle(handle);
+		SafeFileHandleMock safeFileHandleMock = MapSafeFileHandle(handle);
 		return New(
 			safeFileHandleMock.Path,
 			safeFileHandleMock.Mode,
@@ -163,17 +159,13 @@ public FileSystemStream New(SafeFileHandle handle, FileAccess access)
 	}
 
 	/// <inheritdoc cref="IFileStreamFactory.New(SafeFileHandle, FileAccess, int)" />
-#if NET6_0_OR_GREATER
-	[ExcludeFromCodeCoverage(Justification = "SafeFileHandle cannot be unit tested.")]
-#endif
 	public FileSystemStream New(SafeFileHandle handle, FileAccess access, int bufferSize)
 	{
 		using IDisposable registration = _fileSystem.StatisticsRegistration
 			.FileStream.RegisterMethod(nameof(New),
 				handle, access, bufferSize);
 
-		SafeFileHandleMock safeFileHandleMock = _fileSystem
-			.SafeFileHandleStrategy.MapSafeFileHandle(handle);
+		SafeFileHandleMock safeFileHandleMock = MapSafeFileHandle(handle);
 		return New(
 			safeFileHandleMock.Path,
 			safeFileHandleMock.Mode,
@@ -183,18 +175,14 @@ public FileSystemStream New(SafeFileHandle handle, FileAccess access, int buffer
 	}
 
 	/// <inheritdoc cref="IFileStreamFactory.New(SafeFileHandle, FileAccess, int, bool)" />
-#if NET6_0_OR_GREATER
-	[ExcludeFromCodeCoverage(Justification = "SafeFileHandle cannot be unit tested.")]
-#endif
 	public FileSystemStream New(SafeFileHandle handle, FileAccess access, int bufferSize,
 		bool isAsync)
 	{
 		using IDisposable registration = _fileSystem.StatisticsRegistration
 			.FileStream.RegisterMethod(nameof(New),
 				handle, access, bufferSize, isAsync);
 
-		SafeFileHandleMock safeFileHandleMock = _fileSystem
-			.SafeFileHandleStrategy.MapSafeFileHandle(handle);
+		SafeFileHandleMock safeFileHandleMock = MapSafeFileHandle(handle);
 		return New(
 			safeFileHandleMock.Path,
 			safeFileHandleMock.Mode,
@@ -234,4 +222,11 @@ public FileSystemStream Wrap(FileStream fileStream)
 	}
 
 	#endregion
+
+	private SafeFileHandleMock MapSafeFileHandle(SafeFileHandle handle)
+#if FEATURE_FILESYSTEM_RANDOMACCESS
+		=> _fileSystem.SafeFileHandleRegistry.Map(handle);
+#else
+		=> _fileSystem.SafeFileHandleStrategy.MapSafeFileHandle(handle);
+#endif
 }

Source/Testably.Abstractions.Testing/FileSystem/FileStreamMock.cs

@@ -227,7 +227,7 @@ FileOptions options
 			path == null ? "" : fileSystem.Execute.Path.GetFullPath(path),
 			(options & FileOptions.Asynchronous) != 0)
 	{
-		ThrowIfInvalidModeAccess(mode, access);
+		FileModeHelper.ThrowIfInvalidModeAccess(mode, access);
 
 		_stream = stream;
 		_fileSystem = fileSystem;
@@ -238,44 +238,8 @@ FileOptions options
 		_initialPosition = base.Position;
 
 		_location = _fileSystem.Storage.GetLocation(base.Name);
-		_location.ThrowExceptionIfNotFound(_fileSystem, true);
-		IStorageContainer file = _fileSystem.Storage.GetContainer(_location);
-		if (file is NullContainer)
-		{
-			if (_mode.Equals(FileMode.Open) ||
-			    _mode.Equals(FileMode.Truncate))
-			{
-				throw ExceptionFactory.FileNotFound(
-					_fileSystem.Execute.Path.GetFullPath(base.Name));
-			}
-
-			file = _fileSystem.Storage.GetOrCreateContainer(_location,
-				InMemoryContainer.NewFile,
-				this);
-		}
-		else if (file.Type == FileSystemTypes.Directory)
-		{
-			if (_fileSystem.Execute.IsWindows)
-			{
-				throw ExceptionFactory.AccessToPathDenied(
-					_fileSystem.Execute.Path.GetFullPath(base.Name));
-			}
-
-			throw ExceptionFactory.FileAlreadyExists(
-				_fileSystem.Execute.Path.GetFullPath(base.Name), 17);
-		}
-		else if (_mode.Equals(FileMode.CreateNew))
-		{
-			throw ExceptionFactory.FileAlreadyExists(
-				_fileSystem.Execute.Path.GetFullPath(Name),
-				_fileSystem.Execute.IsWindows ? -2147024816 : 17);
-		}
-
-		if (file.Attributes.HasFlag(FileAttributes.ReadOnly) &&
-		    access.HasFlag(FileAccess.Write))
-		{
-			throw ExceptionFactory.AccessToPathDenied(_location.FullPath);
-		}
+		IStorageContainer file = FileModeHelper.GetFileContainer(_fileSystem,
+			_location, mode, access, this);
 #if FEATURE_FILESYSTEM_UNIXFILEMODE
 		if (unixFileMode.HasValue)
 		{
@@ -899,27 +863,4 @@ private void ThrowIfDisposed()
 			throw new ObjectDisposedException("", "Cannot access a closed file.");
 		}
 	}
-
-	private static void ThrowIfInvalidModeAccess(FileMode mode, FileAccess access)
-	{
-		if (mode == FileMode.Append)
-		{
-			if (access == FileAccess.Read)
-			{
-				throw ExceptionFactory.InvalidAccessCombination(mode, access);
-			}
-
-			if (access != FileAccess.Write)
-			{
-				throw ExceptionFactory.AppendAccessOnlyInWriteOnlyMode();
-			}
-		}
-
-		if (!access.HasFlag(FileAccess.Write) &&
-		    (mode == FileMode.Truncate || mode == FileMode.CreateNew ||
-		     mode == FileMode.Create || mode == FileMode.Append))
-		{
-			throw ExceptionFactory.InvalidAccessCombination(mode, access);
-		}
-	}
 }