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>

2 files changed, 31 insertions, 6 deletions

diff --git a/src/testing/internal/testdeps/deps.go b/src/testing/internal/testdeps/deps.go
index 8c0b3fded1..4986898a8e 100644
--- a/src/testing/internal/testdeps/deps.go
+++ b/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 {
diff --git a/src/testing/testing.go b/src/testing/testing.go
index 13937b6ad4..3a4e256b49 100644
--- a/src/testing/testing.go
+++ b/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)