Avoid distro zombie state when wsl init dies in systemd mode

[chemwolf6922]

Summary of the Pull Request

This PR adds a wsl-init-watcher process to monitor the wsl init when systemd mode is enabled.
The mini_init process monitors the process 1 of a distro to determine if the distro is alive. In non-systemd mode, that is the wsl init. In systemd mode, that will be the systemd init. And if the wsl init dies in systemd mode, the distro ends up in a zombie state. Where all wsl calls from Windows fail with "Catastrophic failure".
This PR works by adding a new process that monitors the wsl init in systemd mode. And requests a shutdown if the wsl init process died.

PR Checklist

• Closes: Link to issue #xxx
• Communication: I've discussed this with core contributors already. If work hasn't been agreed, this work might be rejected
• Tests: Added/updated if needed and all pass
• Localization: All end user facing strings can be localized
• Dev docs: Added/updated if needed
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

Add test: UnitTests::UnitTests::SystemdKillInitTerminatesDistro

[Copilot]

Pull request overview

This PR aims to prevent WSL distributions from getting stuck in a “zombie” state when wsl-init dies while systemd mode is enabled, by introducing a lightweight watcher process that monitors wsl-init and triggers a PID-namespace teardown when it exits.

Changes:

• Adds a new wsl-init-watcher entrypoint in the Linux init multicall binary and forks it when BootInit (systemd mode) is used.
• Implements watcher logic using pidfd_open + poll() to detect wsl-init exit and then calls reboot(RB_POWER_OFF) to tear down the PID namespace.
• Adds a Windows unit test that kills the presumed wsl-init PID in systemd mode and verifies subsequent WSL commands succeed.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
test/windows/UnitTests.cpp	Adds a unit test to validate the distro recovers after wsl-init is killed in systemd mode.
src/shared/inc/lxinitshared.h	Introduces the LX_INIT_WSL_INIT_WATCHER name constant for the new entrypoint.
src/linux/init/init.cpp	Adds wsl-init-watcher dispatch, forks the watcher in systemd mode, and implements watcher behavior via pidfd monitoring and namespace teardown.

[OneBlue]

Do we need a separate entrypoint for this, or could we simplify fork and run that logic inside the forked process ?

[chemwolf6922]

This mainly is for getting a clean state with exec and avoid inheriting states from the init process. Running the logic directly after fork requires carefully managing the inherited stuff. Which may cause problems in the future.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[chemwolf6922]

Hi @OneBlue , could you please take another look? Thanks.