Log::AsyncDispatcher#close not fiber-safe?

[xendk]

I know this is a bit vague, but I believe I ran into: https://github.com/crystal-lang/crystal/blob/a5a617d5499772b1a5bedef70e5dc67ec00bf709/src/log/dispatch.cr#L68C55-L68C55

I'm not clear on how to reproduce, as it doesn't fail in spec, but does when used in a Kemal app. I've implemented my own Log::Backend (which just spreads entries over different files) and it triggers:

Invalid memory access (signal 11) at address 0x100000006
[0x5613ce8fb2c6] *Exception::CallStack::print_backtrace:Nil +118 in ./bin/server
[0x5613ce8ce36e] ~procProc(Int32, Pointer(LibC::SiginfoT), Pointer(Void), Nil) +366 in ./bin/server
[0x7f3d6a468510] ?? +139901752739088 in /lib/x86_64-linux-gnu/libc.so.6
[0x5613ceaf3a83] *Log::AsyncDispatcher#close:Nil +19 in ./bin/server
[0x7f3d6a319c80] ???

Adding super(dispatch_mode: Log::DispatchMode::Sync) to my initialize fixes the problem, so I believe this pin-points the problem to Log::AsyncDispatcher#close, but I don't know why close is called in the first place.

Hopefully someone with deeper insight can figure out what the problem is.