Terminal: Crashes if zoom font size up and down rapidly with ctrl + scroll wheel

[guyrleech]

Environment

Win10 1909 x64

Windows Terminal version (if applicable): 0.11.1121.0

Steps to reproduce

Increase and decrease the font size rapidly in the first tab using ctrl and mouse scroll wheel

Expected behavior

Not to crash

Actual behavior

Crashes

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Crashes every time when I use Ctrl and the mouse wheel to increase and decrease the font rapidly. Only happens with the first tab and regardless of what is running in there. I have crash dumps. Doesn't crash when I try and record a video of it in Snagit.

[skyline75489]

It's very easy to reproduce this crash with Release build. The root cause I think is that the render thread clashes with the main thread.

When font size up or down, the main thread will call DxEngine::UpdateFont, in which the render related thing(_dwriteFontFace, for example) is effectively updated. In the meantime, the render thread is happily refreshing the content using PaintBufferLine, which also uses _dwriteFontFace and friends to construct CustomTextLayout. That gives us a race condition. For some reason _dwriteFontFace became a nullptr during the process, causing the crash.

A naive (but acutally working) fix would be adding guard-return for those DX properties to prevent any null-value access. However this is far from ideal. If we going to face these kind of race issue fearlessly, maybe explicit locking is the way to go.

[zadjii-msft]

Oh that's so on me for not locking that call. I'm pretty sure the right solution here is to have the caller get the write lock before calling TriggerFontChange. Thanks for helping debug this!