fix: prevent crash when force-closing during PDF download (#59)

[happycola233]

Motivation

• 用户在下载 PDF 时强行点叉或关闭窗口会导致程序崩溃，因为后台下载线程在窗口销毁后仍然尝试调用 Tk API（如 root.after）。(终止下载导致崩溃 #59)

Description

• 将下载/工作线程设为守护线程（t.daemon = True）以避免后台线程阻止进程退出；修改点在 src/tchMaterial-parser.pyw 中的 thread_it。
• 增加全局标志 app_closing 并在 ui_call 中先检查该标志以避免在关闭过程中调度 UI 更新，同时对 root.after 抛错做安全吞掉处理。
• 改造窗口关闭处理 on_closing，增加幂等性保护，先设置 app_closing = True，结束子进程并尝试 root.destroy()，替代直接调用 sys.exit(0)，从而安全完成资源清理。
• 修改影响的文件：src/tchMaterial-parser.pyw。

Testing

• 已执行 python -m py_compile src/tchMaterial-parser.pyw 校验语法，结果成功。

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Codex Task

[Copilot]

Pull request overview

This PR fixes a crash that occurs when users force-close the application window during a PDF download. The issue was caused by background download threads attempting to call Tkinter UI update methods after the main window was destroyed.

Changes:

• Enabled daemon threads to allow the process to exit cleanly even with running background threads
• Added app_closing flag with guards in UI update calls to prevent post-destruction UI access
• Replaced sys.exit(0) with root.destroy() for proper resource cleanup during window closure

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[Copilot]

The lambda function uses (not app_closing) and func(*args, **kwargs) which will return the result of func(*args, **kwargs) when app_closing is False, but will return False when app_closing is True. This is correct for preventing execution, but the lambda will still be queued even if app_closing becomes True between line 706 and line 710. Additionally, this creates a redundant check since line 706-707 already guards against app_closing. Consider simplifying to root.after(0, lambda: func(*args, **kwargs)) since the early return on line 706-707 already handles the app_closing check.

Suggested change

	root.after(0, lambda: (not app_closing) and func(*args, **kwargs))
	root.after(0, lambda: func(*args, **kwargs))

[Copilot]

The global flag app_closing is accessed and modified across multiple threads without synchronization primitives (e.g., threading.Lock). In Python, while simple boolean operations are generally atomic due to the GIL, there's no guarantee that all threads will immediately see the updated value due to potential memory caching. This could lead to race conditions where background threads continue to call ui_call even after app_closing is set to True. Consider using threading.Event instead, which provides proper memory barriers and thread synchronization. For example: app_closing = threading.Event(), then use app_closing.is_set() to check and app_closing.set() to signal closure.

[Copilot]

Using a bare except Exception: here silently swallows all exceptions, which could hide legitimate errors beyond just the expected case where root has been destroyed. While this may work for the intended use case, it makes debugging more difficult. Consider being more specific about what exception you expect (e.g., except tk.TclError: for Tkinter-related errors when the window is destroyed), or at minimum, log the exception for debugging purposes.

[Copilot]

Using a bare except Exception: here silently swallows all exceptions during window destruction. While this provides robustness, it could hide legitimate errors. Consider being more specific (e.g., except tk.TclError:) or at minimum, log the exception using print_error(e) to aid debugging, consistent with error handling elsewhere in the codebase (e.g., lines 28-29, 383).

Suggested change

	except Exception:
	pass
	except Exception as e:
	print_error(e)