acquireInstanceLock() — extract exit/dialog seam to enable unit testing

[jamesmarkchan]

Background

App.acquireInstanceLock() uses NIO FileLock to prevent multiple GUI instances from running simultaneously. When a second instance is detected, it calls System.exit(0) inside a SwingUtilities.invokeLater lambda:

SwingUtilities.invokeLater(() -> {
    JOptionPane.showMessageDialog(...);
    System.exit(0);  // ← terminates the test JVM if called during tests
});

This is correct production behavior, but it creates a test hazard: any future test that exercises the GUI startup path and encounters an already-held lock will terminate the entire Maven Surefire JVM rather than failing the individual test cleanly.

Problem

1. System.exit in test JVM — if a test triggers the "already running" branch, the whole test suite crashes with a cryptic Surefire failure instead of a clean test failure.
2. No coverage of the lock lifecycle — there are no tests verifying: first call acquires lock → true; second call finds it held → false; lock releases on channel close; next startup succeeds.
   Proposed fix
   Extract the exit and dialog side-effects behind an injectable strategy (e.g. a Runnable or BiConsumer parameter defaulting to the real System.exit + JOptionPane call). This makes acquireInstanceLock() unit-testable by passing a no-op or capture lambda in tests.

// Example seam
static boolean acquireInstanceLock(Runnable onAlreadyRunning) { ... }
// Production call (unchanged behavior)
acquireInstanceLock(() -> {
    SwingUtilities.invokeLater(() -> {
        JOptionPane.showMessageDialog(...);
        System.exit(0);
    });
});

Acceptance criteria

• acquireInstanceLock() does not call System.exit directly — exit/dialog behavior is injectable
• Unit test: first call acquires lock, returns true
• Unit test: second call with lock held returns false and invokes the onAlreadyRunning callback
• Unit test: releasing the channel allows a subsequent call to succeed
• Existing GUI startup behavior unchanged

References

• Flagged in PR code review (Copilot, medium severity) on icon-base branch, App.java lines 312–314