Add job objects to terminate child processes on VM shutdown

[benhillis]

Summary

Add JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE job objects to ensure that child processes spawned by the service (wsldevicehost, wslhost.exe, wslrelay.exe) are terminated when the VM shuts down. This prevents lingering processes from holding DLLs locked during package upgrades.

PR Checklist

• Applies to: WSL Windows components
• Tests: Existing tests pass (cmake --build . -- -m succeeds)

Detailed Description

Problem

During package upgrade, wsldevicehost.dll (and potentially wslhost.exe/wslrelay.exe) can remain running after the service exits, holding file locks that prevent the installer from replacing binaries.

Solution

Create per-VM job objects with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE:

• DeviceHostProxy: Creates a job object and assigns the COM device host process (received via RegisterDeviceHost PID) using AssignProcessToJobObject.
• WslCoreVm: Creates a job object and passes it to child process launches (debug console, kd relay, port relay, interop server) via PROC_THREAD_ATTRIBUTE_JOB_LIST.
• SubProcess: New SetJobObject() method that uses PROC_THREAD_ATTRIBUTE_JOB_LIST for clean job assignment at process creation time.
• helpers: All Launch* functions gain an optional HANDLE JobObject parameter (defaults to nullptr for backward compatibility).

When the VM is destroyed, the job handle closes and Windows automatically terminates all associated processes.

Validation Steps

1. Build succeeds: cmake --build . -- -m (wslservice.exe, wsl.exe)
2. Verified no existing callers are broken (all new parameters default to nullptr)
3. Code review confirmed lifetime safety and correctness

[Copilot]

Pull request overview

This PR introduces per-VM Windows Job Objects configured with JOB_OBJECT_LIMIT_KILL_ON_JOB_CLOSE so that processes spawned by the WSL service (e.g., wslhost.exe, wslrelay.exe, and device host processes) are automatically terminated when the VM shuts down, preventing lingering processes from keeping package binaries/DLLs locked during upgrades.

Changes:

• Add a per-VM job object in WslCoreVm and pass it through process-launch helpers so VM-scoped child processes are tied to VM lifetime.
• Add a SubProcess::SetJobObject() API that wires PROC_THREAD_ATTRIBUTE_JOB_LIST into process creation.
• Add a job object in DeviceHostProxy and assign the registered device host process to it.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/windows/service/exe/WslCoreVm.h	Adds a VM-scoped job handle member to own the kill-on-close job lifetime.
src/windows/service/exe/WslCoreVm.cpp	Creates/configures the job object and passes it to debug/relay/instance launches.
src/windows/service/exe/WslCoreInstance.h	Extends instance construction to accept/store a job handle for subprocess launches.
src/windows/service/exe/WslCoreInstance.cpp	Plumbs the job handle through to interop server launch.
src/windows/common/SubProcess.h	Adds SetJobObject() API and stores an optional job handle.
src/windows/common/SubProcess.cpp	Implements job attribute wiring via PROC_THREAD_ATTRIBUTE_JOB_LIST.
src/windows/common/helpers.hpp	Extends Launch* helper signatures with an optional job handle parameter.
src/windows/common/helpers.cpp	Plumbs job handle into SubProcess launches for wslhost/wslrelay.
src/windows/common/DeviceHostProxy.h	Adds a job handle to manage device-host process lifetime.
src/windows/common/DeviceHostProxy.cpp	Creates/configures the job and assigns the device host PID to it.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.