Crash when exiting a tab with the debug tap

[j4james]

Windows Terminal version

Commit c52ba7d

Windows build number

10.0.19045.4412

Other Software

No response

Steps to reproduce

I've had this crash quite frequently in the preview build (I think possibly since v1.21), but I couldn't easily reproduce it. However, I've just discovered I can reproduce it consistently when running Windows Terminal from within the debugger with the following steps:

1. Start Windows Terminal.
2. Open a second tab with the debug tap.
3. Type exit to close the second tab.

In both tabs I'm using a WSL bash shell, but I don't think that's relevant. The key thing is typing exit to close the tab rather than clicking on the close button.

Expected Behavior

The tab should close without crashing the terminal.

Actual Behavior

The terminal crashes with an exception in the Pane::_CloseChild method:

Unhandled exception thrown: read access violation.
this->_content.**m_ptr** was nullptr.

On this line:

terminal/src/cascadia/TerminalApp/Pane.cpp

Line 1433 in a7e2b46

const auto& control{ _content.GetRoot() };

And with the following stack trace:

>	TerminalApp.dll!Pane::_CloseChild(const bool closeFirst) Line 1433	C++
 	TerminalApp.dll!winrt::impl::delegate<winrt::Windows::Foundation::EventHandler<winrt::Windows::Foundation::IInspectable>,`Pane::_CloseChildRoutine'::`2'::<lambda_1>>::Invoke(void * sender, void * args) Line 883	C++
 	[External Code]	
 	WindowsTerminal.exe!WindowEmperor::_createNewWindowThread::__l2::<lambda_1>::operator()() Line 222	C++
 	[External Code]	

When I've had this crash in the preview build (assuming it was the same thing), it was usually when I was attempting to compile Windows Terminal and my system was low on memory. At the time I thought the lack of memory might have been a factor, but I suspect now that it might be a timing issue, and the low memory was just slowing the system enough for it to trigger the problem. Either way I'm hoping the trace above is enough for someone to figure out the cause.

I had also initially thought this might be the same bug as #17305, but my dev build has both PR #17333 and PR #17358 applied, so it can't be that.

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[j4james]

Git bisect suggests this broke in commit ce0f8d6 (PR #17358). But that means the crashes I've been getting in the preview build are something else. Unless perhaps this commit isn't the real source of the bug, but just made it more likely to be triggered?

[PankajBhojwani]

This should be fixed by #17450! Could you just double-check that?

[j4james]

Yes, I can confirm that has fixed it. Thank you!

[j4james]

I'm reopening because this issue has returned. It was fixed by PR #17450, but that was reverted in PR #17750, and now it's crashing again. Exact same steps to reproduce as above, and the stack trace is similar:

>	[Inline Frame] TerminalApp.dll!winrt::impl::consume_TerminalApp_IPaneContent<winrt::TerminalApp::IPaneContent>::GetRoot() Line 662	C++
 	TerminalApp.dll!Pane::_CloseChild(const bool closeFirst) Line 1433	C++
 	[Inline Frame] TerminalApp.dll!Pane::_CloseChildRoutine::__l2::<lambda_1>::operator()(const winrt::Windows::Foundation::IInspectable &) Line 1687	C++
 	TerminalApp.dll!winrt::impl::delegate<winrt::Windows::Foundation::EventHandler<winrt::Windows::Foundation::IInspectable>,`Pane::_CloseChildRoutine'::`2'::<lambda_1>>::Invoke(void * sender, void * args) Line 883	C++
 	[External Code]	
 	WindowsTerminal.exe!WindowThread::_messagePump() Line 185	C++
 	WindowsTerminal.exe!WindowEmperor::_createNewWindowThread::__l2::<lambda_1>::operator()() Line 228	C++
 	[External Code]	

Commit 7b39d24 was good.
Commit 056af83 crashes.