testing: fix data race between parallel subtests

This CL fixes a race condition if there are two subtests, and
one finishing but the other is panicking.

Fixes #37551

Change-Id: Ic33963eb338aec228964b95f7c34a0d207b91e00
Reviewed-on: https://go-review.googlesource.com/c/go/+/221322
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/testdata/script/test_main_panic.txt

@@ -0,0 +1,30 @@
+[short] skip
+[!race] skip
+
+! go test -v -race main_panic/testmain_parallel_sub_panic_test.go
+! stdout 'DATA RACE'
+-- main_panic/testmain_parallel_sub_panic_test.go --
+package testmain_parallel_sub_panic_test
+
+import "testing"
+
+func setup()    { println("setup()") }
+func teardown() { println("teardown()") }
+func TestA(t *testing.T) {
+	t.Run("1", func(t *testing.T) {
+		t.Run("1", func(t *testing.T) {
+			t.Parallel()
+			panic("A/1/1 panics")
+		})
+		t.Run("2", func(t *testing.T) {
+			t.Parallel()
+			println("A/1/2 is ok")
+		})
+	})
+}
+
+func TestMain(m *testing.M) {
+	setup()
+	defer teardown()
+	m.Run()
+}

src/testing/testing.go

@@ -928,16 +928,15 @@ func tRunner(t *T, fn func(t *T)) {
 				t.Logf("cleanup panicked with %v", r)
 			}
 			// Flush the output log up to the root before dying.
-			t.mu.Lock()
+			for root := &t.common; root.parent != nil; root = root.parent {
-			root := &t.common
+				root.mu.Lock()
-			for ; root.parent != nil; root = root.parent {
 				root.duration += time.Since(root.start)
-				fmt.Fprintf(root.parent.w, "--- FAIL: %s (%s)\n", root.name, fmtDuration(root.duration))
+				d := root.duration
+				root.mu.Unlock()
+				root.flushToParent("--- FAIL: %s (%s)\n", root.name, fmtDuration(d))
 				if r := root.parent.runCleanup(recoverAndReturnPanic); r != nil {
 					fmt.Fprintf(root.parent.w, "cleanup panicked with %v", r)
 				}
-				root.parent.mu.Lock()
-				io.Copy(root.parent.w, bytes.NewReader(root.output))
 			}
 			panic(err)
 		}