From d596bc0e819aec2cf65a61d8b92f6bec8e0a7124 Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Fri, 9 May 2025 16:18:49 -0700
Subject: [PATCH] runtime: disallow closing bubbled chans from outside bubble

A chan created within a synctest bubble may not be
operated on from outside the bubble.
We panicked on send and receive, but not close.
Panic on close as well.

For #67434

Change-Id: I98d39e0cf7baa1a679aca1fb325453d69c535308
Reviewed-on: https://go-review.googlesource.com/c/go/+/671960
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>
---
 src/internal/synctest/synctest_test.go | 40 ++++++++++++++++++++++++++
 src/runtime/chan.go                    |  3 ++
 2 files changed, 43 insertions(+)

diff --git a/src/internal/synctest/synctest_test.go b/src/internal/synctest/synctest_test.go
index c0e126e3fc..7c8fd7ef9e 100644
--- a/src/internal/synctest/synctest_test.go
+++ b/src/internal/synctest/synctest_test.go
@@ -263,6 +263,46 @@ func TestChannelFromOutsideBubble(t *testing.T) {
 	}
 }
 
+func TestChannelMovedOutOfBubble(t *testing.T) {
+	for _, test := range []struct {
+		desc      string
+		f         func(chan struct{})
+		wantPanic string
+	}{{
+		desc: "receive",
+		f: func(ch chan struct{}) {
+			<-ch
+		},
+		wantPanic: "receive on synctest channel from outside bubble",
+	}, {
+		desc: "send",
+		f: func(ch chan struct{}) {
+			ch <- struct{}{}
+		},
+		wantPanic: "send on synctest channel from outside bubble",
+	}, {
+		desc: "close",
+		f: func(ch chan struct{}) {
+			close(ch)
+		},
+		wantPanic: "close of synctest channel from outside bubble",
+	}} {
+		t.Run(test.desc, func(t *testing.T) {
+			donec := make(chan struct{})
+			ch := make(chan chan struct{})
+			go func() {
+				defer close(donec)
+				defer wantPanic(t, test.wantPanic)
+				test.f(<-ch)
+			}()
+			synctest.Run(func() {
+				ch <- make(chan struct{})
+			})
+			<-donec
+		})
+	}
+}
+
 func TestTimerFromInsideBubble(t *testing.T) {
 	for _, test := range []struct {
 		desc      string
diff --git a/src/runtime/chan.go b/src/runtime/chan.go
index cb2737d096..63d8044b44 100644
--- a/src/runtime/chan.go
+++ b/src/runtime/chan.go
@@ -415,6 +415,9 @@ func closechan(c *hchan) {
 	if c == nil {
 		panic(plainError("close of nil channel"))
 	}
+	if c.synctest && getg().bubble == nil {
+		panic(plainError("close of synctest channel from outside bubble"))
+	}
 
 	lock(&c.lock)
 	if c.closed != 0 {
-- 
2.51.0