testing: do not crash when m.Run is called twice and -test.testlogfile is used

Tests exist that call m.Run in a loop‽
Now we have one too.

Fixes #23129.

Change-Id: I8cbecb724f239ae14ad45d75e67d12c80e41c994
Reviewed-on: https://go-review.googlesource.com/83956
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -2982,6 +2982,18 @@ func TestGoTestMainAsNormalTest(t *testing.T) {
 	tg.grepBoth(okPattern, "go test did not say ok")
 }
 
+func TestGoTestMainTwice(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOCACHE", tg.tempdir)
+	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))
+	tg.run("test", "-v", "multimain")
+	if strings.Count(tg.getStdout(), "notwithstanding") != 2 {
+		t.Fatal("tests did not run twice")
+	}
+}
+
 func TestGoTestFlagsAfterPackage(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()

src/cmd/go/testdata/src/multimain/multimain_test.go

@@ -0,0 +1,16 @@
+package multimain_test
+
+import "testing"
+
+func TestMain(m *testing.M) {
+	// Some users run m.Run multiple times, changing
+	// some kind of global state between runs.
+	// This used to work so I guess now it has to keep working.
+	// See golang.org/issue/23129.
+	m.Run()
+	m.Run()
+}
+
+func Test(t *testing.T) {
+	t.Log("notwithstanding")
+}

src/testing/internal/testdeps/deps.go

@@ -63,8 +63,9 @@ func (TestDeps) ImportPath() string {
 
 // testLog implements testlog.Interface, logging actions by package os.
 type testLog struct {
-	mu sync.Mutex
-	w  *bufio.Writer
+	mu  sync.Mutex
+	w   *bufio.Writer
+	set bool
 }
 
 func (l *testLog) Getenv(key string) {
@@ -101,14 +102,21 @@ func (l *testLog) add(op, name string) {
 }
 
 var log testLog
+var didSetLogger bool
 
 func (TestDeps) StartTestLog(w io.Writer) {
 	log.mu.Lock()
 	log.w = bufio.NewWriter(w)
-	log.w.WriteString("# test log\n") // known to cmd/go/internal/test/test.go
+	if !log.set {
+		// Tests that define TestMain and then run m.Run multiple times
+		// will call StartTestLog/StopTestLog multiple times.
+		// Checking log.set avoids calling testlog.SetLogger multiple times
+		// (which will panic) and also avoids writing the header multiple times.
+		log.set = true
+		testlog.SetLogger(&log)
+		log.w.WriteString("# test log\n") // known to cmd/go/internal/test/test.go
+	}
 	log.mu.Unlock()
-
-	testlog.SetLogger(&log)
 }
 
 func (TestDeps) StopTestLog() error {

src/testing/testing.go

@@ -914,6 +914,8 @@ type M struct {
 
 	timer     *time.Timer
 	afterOnce sync.Once
+
+	numRun int
 }
 
 // testDeps is an internal interface of functionality that is
@@ -945,6 +947,12 @@ func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchma
 
 // Run runs the tests. It returns an exit code to pass to os.Exit.
 func (m *M) Run() int {
+	// Count the number of calls to m.Run.
+	// We only ever expected 1, but we didn't enforce that,
+	// and now there are tests in the wild that call m.Run multiple times.
+	// Sigh. golang.org/issue/23129.
+	m.numRun++
+
 	// TestMain may have already called flag.Parse.
 	if !flag.Parsed() {
 		flag.Parse()
@@ -1110,7 +1118,16 @@ func (m *M) before() {
 	if *testlog != "" {
 		// Note: Not using toOutputDir.
 		// This file is for use by cmd/go, not users.
-		f, err := os.Create(*testlog)
+		var f *os.File
+		var err error
+		if m.numRun == 1 {
+			f, err = os.Create(*testlog)
+		} else {
+			f, err = os.OpenFile(*testlog, os.O_WRONLY, 0)
+			if err == nil {
+				f.Seek(0, io.SeekEnd)
+			}
+		}
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
 			os.Exit(2)