6455afbc6f
Because we dropg before emitting a trace event in preemptPark, we end up failing to emit a status for the goroutine if this happens to be the first event for it in the generation. We only really see this with multiple subscribers in TestSubscribers. This is for two reasons: 1. If we are missing a status event for a non-initial generation then the trace parser won't validate that (an oversight, but we can only enforce that for new traces because of this bug), and 2. If we're starting the tracer fresh, then we have a STW which effectively guarantees that the first event for a goroutine cannot come from preemptPark. Therefore, we cannot observe this situation unless the first generation manifests the bug, but prior to having flight recording and/or multiple subscribers being able to "cut" the trace data at any point, this was impossible. The fix is simple: dropg only after emitting the trace event. This is also safe, because the tracer doesn't care. The tracer will also start taking a stack trace of the goroutine in this circumstance, but that is also safe, since we are able to generally unwind the stack of asynchronously preempted goroutines, and here we're at the very, very end of asynchronous preemption where all the state to do so is already set up. Fixes #75665. Change-Id: I7ee1142697d0a53b62d4c5647aa53775d2f6976a Reviewed-on: https://go-review.googlesource.com/c/go/+/729400 LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
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