runtime/race: test that close synchronizes with read

Add a test to ensure that the race detector sees that closing a
channel synchronizes with a read from that channel.
This test case failed when CL 181543 was in the tree.
CL 181543 was reverted in CL 216158; this adds a test to make
sure that we don't re-introduce the problem at a later date.

For #32529
For #36714

Change-Id: I5a40f744c67c3f8191d6ad822710c180880a7375
Reviewed-on: https://go-review.googlesource.com/c/go/+/216099
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/runtime/race/testdata/chan_test.go

@@ -737,3 +737,29 @@ func TestNoRaceBlockedSelectSendSync(t *testing.T) {
 	case <-make(chan int):
 	}
 }
+
+// Test that close synchronizes with a read from the empty closed channel.
+// See https://golang.org/issue/36714.
+func TestNoRaceCloseHappensBeforeRead(t *testing.T) {
+	for i := 0; i < 100; i++ {
+		var loc int
+		var write = make(chan struct{})
+		var read = make(chan struct{})
+
+		go func() {
+			select {
+			case <-write:
+				_ = loc
+			default:
+			}
+			close(read)
+		}()
+
+		go func() {
+			loc = 1
+			close(write)
+		}()
+
+		<-read
+	}
+}