Add defaultVhdType configuration setting to .wslconfig

[obrobrio2000]

Summary

This PR implements a new defaultVhdType configuration setting for .wslconfig and improves how VHD size and type defaults are applied when creating new WSL 2 distributions. This addresses the feature request in issue #13529.

Problem Statement

Currently, when creating a WSL 2 distribution with a fixed-size VHD, users must always specify both --vhd-size and --fixed-vhd, even if they have already configured a defaultVhdSize in their .wslconfig file:

wsl --install Ubuntu-24.04 --vhd-size 10GB --fixed-vhd

Additionally, there was no way to configure the default VHD type (dynamic vs. fixed) in .wslconfig, requiring users to always specify --fixed-vhd explicitly for every distribution they wanted to create with a fixed VHD.

Solution

This PR adds:

1. New defaultVhdType configuration setting - Users can now specify their preferred VHD type in .wslconfig:

   [wsl2]
   defaultVhdSize=10GB
   defaultVhdType=fixed

2. Improved command-line logic - The --vhd-size parameter is now optional when defaultVhdSize is configured in .wslconfig, and the VHD type is determined by:

   • Explicit --fixed-vhd flag (highest priority)
   • defaultVhdType configuration setting (if no explicit flag)
   • Dynamic (backward compatible default)

3. Simplified installation commands:

   # With .wslconfig configured, both of these are now valid:
   wsl --install Ubuntu-24.04 --fixed-vhd  # Uses defaultVhdSize from config
   wsl --install Ubuntu-24.04               # Uses both defaults from config

Changes Made

Configuration System

• Added VhdType enum with Dynamic and Fixed values to WslCoreConfig.h
• Added VhdDefaultType field to the Config struct (defaults to Dynamic)
• Added parsing for wsl2.defaultVhdType in config file parser
• Updated telemetry to track VhdDefaultType usage

Command-Line Handling

• Removed strict validation requiring --vhd-size with --fixed-vhd
• Added comment explaining that the service layer will validate and apply defaults

VHD Creation Logic

• Updated LxssUserSession::RegisterDistribution to apply defaultVhdSize when not explicitly provided
• Modified VHD type determination to consider both explicit flag and config default
• Maintains full backward compatibility

API Interfaces

• Extended WslCoreConfigInterface.h with VhdType config entry and VhdTypeConfiguration enum
• Implemented Get/Set handlers in WslCoreConfigInterface.cpp
• Updated C# bindings in LibWsl.cs with renumbered enum values and new VhdType enum

Testing

• Added comprehensive test DefaultVhdTypeConfiguration that validates:
  • Installation with --fixed-vhd uses defaultVhdSize from config
  • defaultVhdType=fixed creates Fixed VHDs
  • defaultVhdType=dynamic creates Dynamic VHDs
  • Proper VHD type verification via PowerShell Get-VHD

Documentation

• Updated wsl-config.md with new defaultVhdType setting documentation
• Included usage examples and value descriptions

Testing

The implementation has been tested with:

• New unit test validating VHD type configuration behavior
• Backward compatibility verified (existing behavior unchanged when config not set)
• Both dynamic and fixed VHD creation tested
• Configuration file parsing validated

Backward Compatibility

✅ This PR maintains full backward compatibility:

• Existing behavior unchanged when defaultVhdType is not configured (defaults to dynamic)
• Explicit command-line flags continue to work as before
• --vhd-size can still be specified explicitly and takes precedence

Related Issue

Fixes #13529

Example Usage

Configuration in %USERPROFILE%\.wslconfig:

[wsl2]
defaultVhdSize=10GB
defaultVhdType=fixed

Installation commands:

# Uses both defaults from config (10GB fixed VHD)
wsl --install Ubuntu-24.04

# Explicit override of size but uses type from config
wsl --install Ubuntu-24.04 --vhd-size 20GB

# Explicit override forcing dynamic type
wsl --install Ubuntu-24.04 --vhd-size 20GB  # Omitting --fixed-vhd uses dynamic

# Using --fixed-vhd with default size from config
wsl --install Ubuntu-24.04 --fixed-vhd

Checklist

• Implementation complete
• Unit tests added and passing
• Documentation updated
• Backward compatibility maintained
• Telemetry added
• Configuration interfaces updated
• C# bindings updated

[Copilot]

Pull Request Overview

This PR implements a new defaultVhdType configuration setting for .wslconfig files and improves the handling of VHD size and type defaults when creating WSL 2 distributions. The change allows users to configure their preferred VHD type (dynamic vs. fixed) in .wslconfig and makes the --vhd-size parameter optional when defaults are configured.

Key changes include:

• Added VhdType enum and defaultVhdType configuration support across the codebase
• Relaxed command-line validation to allow --fixed-vhd without explicit --vhd-size when defaults exist
• Enhanced VHD creation logic to apply both size and type defaults from configuration

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/windows/UnitTests.cpp	Added comprehensive test for VHD type configuration validation
src/windows/wslsettings/LibWsl.cs	Updated C# bindings with renumbered enums and new VhdType enum
src/windows/service/exe/LxssUserSession.cpp	Modified VHD creation to apply config defaults and determine type
src/windows/libwsl/WslCoreConfigInterface.cpp	Implemented Get/Set handlers for new VhdType configuration
src/windows/inc/WslCoreConfigInterface.h	Added VhdTypeConfiguration enum and struct member
src/windows/common/WslCoreConfig.h	Added VhdType enum, string mappings, and config struct field
src/windows/common/WslCoreConfig.cpp	Added config file parsing support for defaultVhdType
src/windows/common/WslClient.cpp	Removed strict validation requiring --vhd-size with --fixed-vhd

Comments suppressed due to low confidence (1)

src/windows/libwsl/WslCoreConfigInterface.cpp:1

• Missing break; statement after the VhdSizeBytes case. This will cause fall-through to the VhdType case, leading to incorrect execution.

/*++

[OneBlue]

We should avoid to depend on networking in the tests, since that can create unstable test results.

Instead of downloading ubuntu, let' use the test distro tar. Something like this should do:

std::format(L"--install --from-file \"{}\" --no-launch --name test_vhd_flag --fixed-vhd", g_testDistroPath));

[OneBlue]

Thank you for doing this ! Added a couple comments

[OneBlue]

Let's use a different name for the distribution we use for this test so we don't have to unregister the main one

[OneBlue]

nit: it might be worth extracting this logic into a lambda. Something like:

auto validateVhdType = [](const char* Expected){...};

So the test can just do
validateVhdType("Fixed");

[OneBlue]

wslsettings.exe doesn't actually display this setting, so let's remove the changes in libwsl and wslsettings