runtime: emit trace event in direct semaphore handoff
When a goroutine yields the remainder of its time to another goroutine during direct semaphore handoff (as in an Unlock of a sync.Mutex in starvation mode), it needs to signal that change to the execution tracer. The discussion in CL 200577 didn't reach consensus on how best to describe that, but pointed out that "traceEvGoSched / goroutine calls Gosched" could be confusing. Emit a "traceEvGoPreempt / goroutine is preempted" event in this case, to allow the execution tracer to find a consistent event ordering without being both specific and inaccurate about why the active goroutine has changed. Fixes #36186 Change-Id: Ic4ade19325126db2599aff6aba7cba028bb0bee9 Reviewed-on: https://go-review.googlesource.com/c/go/+/211797 Run-TryBot: Dan Scales <danscales@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com>
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