Fix orphaned modal sheet deadlock when parent window is destroyed

ModernTests/NSAlert_iTermTests.swift

@@ -0,0 +1,42 @@
+//
+//  NSAlert_iTermTests.swift
+//  iTerm2 ModernTests
+//
+//  Verifies that -[NSAlert(iTerm) runSheetModalForWindow:] does not
+//  hang when the parent window is destroyed while the sheet is shown.
+//
+
+import XCTest
+@testable import iTerm2SharedARC
+
+final class NSAlert_iTermTests: XCTestCase {
+
+    func test_modalReturnsWhenParentWindowCloses() {
+        let window = NSWindow(
+            contentRect: NSRect(x: 0, y: 0, width: 100, height: 100),
+            styleMask: [.titled, .closable],
+            backing: .buffered,
+            defer: false)
+
+        let alert = NSAlert()
+        alert.messageText = "test"
+        alert.addButton(withTitle: "OK")
+
+        // Close the window from a background queue shortly after the
+        // modal begins.  The nested run loop inside runModalForWindow:
+        // will deliver NSWindowWillCloseNotification, our observer
+        // will fire abortModal, and the modal must unwind — proving
+        // we are not deadlocked.
+        DispatchQueue.global().async {
+            // Give the modal a moment to enter its run loop.
+            Thread.sleep(forTimeInterval: 0.1)
+            window.close()
+        }
+
+        let response = alert.runSheetModalForWindow(window)
+        // Any return value means the fix is working — the call did not
+        // block forever.
+        XCTAssertNotEqual(response, NSModalResponseContinue,
+                          "Modal should not return NSModalResponseContinue when aborted")
+    }
+}

sources/Categories/NSAlert+iTerm.m

@@ -14,7 +14,32 @@ - (NSModalResponse)runSheetModalForWindow:(NSWindow *)window {
     DLog(@"Run sheet modal for window %@", window);
 
     [NSApp activateIgnoringOtherApps:YES];
+
+    // If the parent window is closed before the user dismisses the sheet,
+    // the completion handler will never fire and the modal loop in
+    // runModalForWindow: will run forever, blocking the entire app.
+    // Observe the parent window closing so we can abort the modal.
+    __block BOOL stopped = NO;
+    __block id observer = nil;
+    observer = [[NSNotificationCenter defaultCenter] addObserverForName:NSWindowWillCloseNotification
+                                                                 object:window
+                                                                  queue:nil
+                                                             usingBlock:^(NSNotification *note) {
+        if (stopped) {
+            return;
+        }
+        stopped = YES;
+        DLog(@"Parent window closed while sheet modal was active — aborting modal");
+        [[NSNotificationCenter defaultCenter] removeObserver:observer];
+        [NSApp abortModal];
+    }];
+
     [self beginSheetModalForWindow:window completionHandler:^(NSModalResponse returnCode) {
+        if (stopped) {
+            return;
+        }
+        stopped = YES;
+        [[NSNotificationCenter defaultCenter] removeObserver:observer];
         [NSApp stopModalWithCode:returnCode];
     }];
     return [NSApp runModalForWindow:[self window]];