sync: disassociate WaitGroups from bubbles on Wait
Fix a race condition in disassociating a WaitGroup in a synctest
bubble from its bubble. We previously disassociated the WaitGroup
when count becomes 0, but this causes problems when an Add call
setting count to 0 races with one incrementing the count.
Instead, disassociate a WaitGroup from its bubble when Wait returns.
Wait must not be called concurrently with an Add call with a
positive delta and a 0 count, so we know that the disassociation
will not race with an Add call trying to create a new association.
Fixes #74386
Change-Id: I9b519519921f7691869a64a245a5ee65d071d054
Reviewed-on: https://go-review.googlesource.com/c/go/+/684635 Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>