Unterminated events in Fuchsia format

[Sudha247]

@kayceesrk reported unterminated events on the current main branch and #17 on the default (fuchsia) format.

This is not the case with json format.

[Sudha247]

I spent some time today trying to find the source of this bug. I tried to reproduce this with possible combinations of compiler versions (5.0.0, 5.1.0~beta1), olly version (0.3, trunk) and tracing format (fuchsia, json) on the same program (Eio version of ocurrent's solver service). I could reproduce this with 5.1.0~beta1 + fuchsia, and not json, with both versions of olly. Interestingly, this doesn't occur on 5.0.0!

To make sure the compiler is not skipping emitting some events, I added some GC logs at the start and end of major cycles. From the logs, it's clear that there are no unterminated events.

I suspect something's clobbering writes at the end of event. Will try to dig into this further when I find some time.