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/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)