runtime, testing/synctest: breaking bubble isolation with Cond is fatal

sync.Cond.Wait is durably blocking. Waking a goroutine out of Cond.Wait
from outside its bubble panics.

Make this panic a fatal panic, since it leaves the notifyList in an
inconsistent state. We could do some work to make this a recoverable
panic, but the complexity doesn't seem worth the outcome.

For #67434

Change-Id: I88874c1519c2e5c0063175297a9b120cedabcd07
Reviewed-on: https://go-review.googlesource.com/c/go/+/675617
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Damien Neil <dneil@google.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/synctest.go b/src/runtime/testdata/testprog/synctest.go
new file mode 100644
index 0000000000..dd3a6df8a0
--- /dev/null
+++ b/src/runtime/testdata/testprog/synctest.go
@@ -0,0 +1,58 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"internal/synctest"
+	"sync"
+)
+
+func init() {
+	register("SynctestCond/signal/no_bubble", func() {
+		synctestCond(func(cond *sync.Cond) {
+			cond.Signal()
+		})
+	})
+	register("SynctestCond/broadcast/no_bubble", func() {
+		synctestCond(func(cond *sync.Cond) {
+			cond.Broadcast()
+		})
+	})
+	register("SynctestCond/signal/other_bubble", func() {
+		synctestCond(func(cond *sync.Cond) {
+			synctest.Run(cond.Signal)
+		})
+	})
+	register("SynctestCond/broadcast/other_bubble", func() {
+		synctestCond(func(cond *sync.Cond) {
+			synctest.Run(cond.Broadcast)
+		})
+	})
+}
+
+func synctestCond(f func(*sync.Cond)) {
+	var (
+		mu     sync.Mutex
+		cond   = sync.NewCond(&mu)
+		readyc = make(chan struct{})
+		wg     sync.WaitGroup
+	)
+	defer wg.Wait()
+	wg.Go(func() {
+		synctest.Run(func() {
+			go func() {
+				mu.Lock()
+				defer mu.Unlock()
+				cond.Wait()
+			}()
+			synctest.Wait()
+			<-readyc // #1: signal that cond.Wait is waiting
+			<-readyc // #2: wait to continue
+			cond.Signal()
+		})
+	})
+	readyc <- struct{}{}
+	f(cond)
+}