fix(update): relaunch the new version instead of the old running process

[DamienBattistella]

Description

Clicking "Restart now" / "Redémarrer maintenant" after an update finished downloading kept running the previous version (e.g. it stayed on v1.9.0 instead of switching to v1.10.0). The earlier fix (#67) corrected which binary path the relaunch targets on disk, but two remaining bugs kept the old process alive:

1. RestartApplication never set the quitting flag. QuitApp sets a.quitting.Store(true) before runtime.Quit specifically so beforeClose doesn't intercept the quit. RestartApplication omitted this, so when "Minimize to tray" was enabled, beforeClose ran with quitting == false and returned true (prevent) → the old process merely hid its window instead of quitting, lingering in the tray on the previous version and never releasing the single-instance lock.

2. Single-instance-lock race. cmd.Start() launches the new binary, then runtime.Quit begins a shutdown that takes time (disconnect Discord, stop tray, …). During that window the old process still holds the com.plexcord.app single-instance lock, so the freshly-launched new binary is treated as a second instance — it calls onSecondInstanceLaunch (restores the old window) and exits. The new version never runs.

Fix

• Set a.quitting.Store(true) before runtime.Quit in RestartApplication (mirrors QuitApp), so the old process actually terminates regardless of the "Minimize to tray" setting.
• Pass the old process PID to the relaunched child via PLEXCORD_RELAUNCH_PID. In main(), before wails.Run acquires the single-instance lock, the child waits (bounded to 15s) for that PID to exit and for the OS to release the lock. The updated binary then starts as the primary instance.
• Per-platform process-liveness check: WaitForSingleObject on Windows (relaunch_windows.go), signal 0 on Unix (relaunch_unix.go).

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Related Issues

Fixes the reported issue where the app restarts the current process rather than the freshly downloaded new version.

Testing

• Tested on Windows
• Tested on macOS
• Tested on Linux
• Added/updated unit tests
• All tests pass locally

Added relaunch_test.go covering processExists (self alive, reaped-child dead) and waitForPreviousInstanceExit (no-op without the env marker; returns promptly and clears the env when the predecessor is already gone). Verified go build, go vet, and the full go test ./... suite pass for both linux/amd64 and windows/amd64.

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

Additional Notes

Both fixes are needed together: fix (1) makes the old process actually exit, and fix (2) ensures the new binary doesn't bail out as a "second instance" during the old process's shutdown window.

🤖 Generated with Claude Code

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🎉 This PR is included in version 1.10.1 🎉

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