Add kill-tree helper and runtime sidecar PID registry

[sakina1303]

Adds a best-effort process-tree killer and a lightweight runtime sidecar PID registry, ensuring that descendant processes spawned by sidecars are properly terminated when their parent sidecar is killed.

This resolves the core issue described in #14360 by introducing both API and runtime cleanup mechanisms for sidecar process management.

Details:

• tauri::process::kill_process_tree(pid: u32) — POSIX shell & PowerShell based, best-effort.
• Sidecar PID registry via AppHandle::register_sidecar(pid) / unregister_sidecar(pid) for cleanup on exit.
• App::cleanup_before_exit() drains registry and calls the kill helper.
• CLI dev-run now attempts process-tree cleanup when stopping dev children.

Files Changed:

• process.rs: Added kill_process_tree() (POSIX + PowerShell).
• mod.rs: Added sidecar PID tracking + helper methods.
• app.rs: Added register/unregister APIs; updated cleanup.
• desktop.rs: Added kill-tree logic in dev-run path.
• CHANGELOG.md / PR_DRAFT.md: Updated with notes.

Testing:

• [ ✅ ] cargo test -p tauri, all tests passed.
• [ ✅ ] Manual: verified registry and cleanup invoke kill helper.
• Runtime cleanup is opt-in:
  Call app_handle.register_sidecar(child.id() as u32) after spawning.

Fixes #14360

[github-actions]

Package Changes Through ed8d624

There are 1 changes which include @tauri-apps/api with patch

Planned Package Versions

The following package releases are the planned based on the context of changes in this pull request.

package	current	next
@tauri-apps/api	2.9.0	2.9.1

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[Legend-Master]

Thanks for working on this!

[Legend-Master]

We would also want to implement this for the TauriPlugin so that tauri::plugin::Builder could have access to this API

[sakina1303]

for sure! I’ll add this to the TauriPlugin trait as well so it’s accessible via tauri::plugin::Builder.

[FabianLars]

since this is a generic function and not bound to this app instance imo this should be moved into https://github.com/tauri-apps/plugins-workspace/tree/v2/plugins/process (and in theory that plugin's exit and restart should probably be part of the app core plugin instead but that's a discussion for another day).

[sakina1303]

Would it be okay if I leave a TODO for now and move it in a follow-up PR?

[FabianLars]

No, we at least have to decide how to proceed here because once this is added in tauri we cannot remove it without a major release since removing apis is a breaking change.

[sakina1303]

Should I move kill_process_tree to the process plugin now as you suggested? I’ll update this PR to avoid a future breaking change.

[FabianLars]

i think that would make sense, yes. we can always move it back if needed (that way around works)

[Legend-Master]

I think we have now gathered a few different problems around this now:

• Only the side car process is killed on app exit, not the entire process tree
  • Just from testing, it seems like some of the processes are killed while some others are not???
• tauri dev doesn't kill side cars (it kills the app forcefully by sending SIGKILL on mac and linux, TerminateProcess on Windows to cargo)
  • It seems to me that it does kill the side car but not always the entire process tree???
• NSIS installers don't kill side cars, they kill the app forcefully through TerminateProcess (the Wix .msi installers do, they send WM_QUERYENDSESSION to the app which is now handled after fix(windows): emit LoopDestroyed on WM_ENDSESSION tao#1126 no, I thought they do but what actually happened is just it killed the tray icon by sending WM_CLOSE)
• Updater plugin exits the app through std::process::exit which doesn't trigger the Exit event so side cars are not killed

My brain is basically fried right now, no more processes 😭

[FabianLars]

1. and perhaps 2) should imo be fixed via [shell] Add option to spawn command in process group plugins-workspace#1332 and not by trying to kill a process tree. prob doesn't help with the rest though.