runtime: clear mspan.largeType more carefully in the case of arenas The pointer stored in mspan.largeType is an invalid pointer when the span is an arena. We need to make sure that pointer isn't seen by the garbage collector, as it might barf on it. Make sure we zero the pointer using a uintptr write so the old value isn't picked up by the write barrier. The mspan.largeType field itself is in a NotInHeap struct, so a heap scan won't find it. The only way we find it is when writing it, or when reading it and putting it in a GC-reachable location. I think we might need to audit the runtime to make sure these pointers aren't being passed in places where the GC might (non-conservatively) scan a stack frame it lives in. (It might be ok, many such places are either systemstack or nosplit.) Change-Id: Ie059d054e0da4d48a4c4b3be88b8e1e46ffa7d10 Reviewed-on: https://go-review.googlesource.com/c/go/+/548535 Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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