testing/synctest, runtime: avoid panic when using linker-alloc WG from bubble

We associate WaitGroups with synctest bubbles by attaching a
special to the WaitGroup. It is not possible to attach a special
to a linker-allocated value, such as:

    var wg sync.WaitGroup

Avoid panicking when accessing a linker-allocated WaitGroup
from a bubble. We have no way to associate these WaitGroups
with a bubble, so just treat them as always unbubbled.
This is probably fine, since the WaitGroup was always
created outside the bubble in this case.

Fixes #74005

Change-Id: Ic71514b0b8d0cecd62e45cc929ffcbeb16f54a55
Reviewed-on: https://go-review.googlesource.com/c/go/+/679695
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/src/testing/synctest/synctest.go b/src/testing/synctest/synctest.go
index 57a6fbfbd6..0911519aab 100644
--- a/src/testing/synctest/synctest.go
+++ b/src/testing/synctest/synctest.go
@@ -93,6 +93,11 @@
 // A [sync.WaitGroup] becomes associated with a bubble on the first
 // call to Add or Go. Once a WaitGroup is associated with a bubble,
 // calling Add or Go from outside that bubble is a fatal error.
+// (As a technical limitation, a WaitGroup defined as a package
+// variable, such as "var wg sync.WaitGroup", cannot be associated
+// with a bubble and operations on it may not be durably blocking.
+// This limitation does not apply to a *WaitGroup stored in a
+// package variable, such as "var wg = new(sync.WaitGroup)".)
 //
 // [sync.Cond.Wait] is durably blocking. Waking a goroutine in a bubble
 // blocked on Cond.Wait from outside the bubble is a fatal error.