testing: fix panic in t.Log

If a testing.TB is no longer on the stack, t.Log would panic because
its outputWriter is nil. Check for nil and drop the write, which
is the previous behavior.

Change-Id: Ifde97997a3aa26ae604ac9c218588c1980110cbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/673215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>

2 files changed, 34 insertions, 9 deletions

diff --git a/src/testing/sub_test.go b/src/testing/sub_test.go
index 1a6cfed594..d3d383427d 100644
--- a/src/testing/sub_test.go
+++ b/src/testing/sub_test.go
@@ -280,7 +280,6 @@ func TestTRun(t *T) {
 					t.Run("c", func(t *T) {
 						t.Parallel()
 					})
-
 				})
 			})
 		},
@@ -305,7 +304,6 @@ func TestTRun(t *T) {
 									time.Sleep(time.Nanosecond)
 								})
 							}
-
 						})
 					}
 				})
@@ -841,7 +839,7 @@ func TestBenchmarkOutput(t *T) {
 }
 
 func TestBenchmarkStartsFrom1(t *T) {
-	var first = true
+	first := true
 	Benchmark(func(b *B) {
 		if first && b.N != 1 {
 			panic(fmt.Sprintf("Benchmark() first N=%v; want 1", b.N))
@@ -851,7 +849,7 @@ func TestBenchmarkStartsFrom1(t *T) {
 }
 
 func TestBenchmarkReadMemStatsBeforeFirstRun(t *T) {
-	var first = true
+	first := true
 	Benchmark(func(b *B) {
 		if first && (b.startAllocs == 0 || b.startBytes == 0) {
 			panic("ReadMemStats not called before first run")
@@ -1250,3 +1248,21 @@ func TestOutputWriteAfterComplete(t *T) {
 		t.Error(s)
 	}
 }
+
+// Verify that logging to an inactive top-level testing.T does not panic.
+// These tests can run in either order.
+
+func TestOutputEscape1(t *T) { testOutputEscape(t) }
+func TestOutputEscape2(t *T) { testOutputEscape(t) }
+
+var global *T
+
+func testOutputEscape(t *T) {
+	if global == nil {
+		// Store t in a global, to set up for the second execution.
+		global = t
+	} else {
+		// global is inactive here.
+		global.Log("hello")
+	}
+}
diff --git a/src/testing/testing.go b/src/testing/testing.go
index efbcd59dc0..e0f8247e3b 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -904,8 +904,10 @@ type TB interface {
 	private()
 }
 
-var _ TB = (*T)(nil)
-var _ TB = (*B)(nil)
+var (
+	_ TB = (*T)(nil)
+	_ TB = (*B)(nil)
+)
 
 // T is a type passed to Test functions to manage test state and support formatted test logs.
 //
@@ -1119,6 +1121,11 @@ type outputWriter struct {
 // Write writes a log message to the test's output stream, properly formatted and
 // indented. It may not be called after a test function and all its parents return.
 func (o *outputWriter) Write(p []byte) (int, error) {
+	// o can be nil if this is called from a top-level *TB that is no longer active.
+	// Just ignore the message in that case.
+	if o == nil || o.c == nil {
+		return 0, nil
+	}
 	if o.c.destination() == nil {
 		panic("Write called after " + o.c.name + " has completed")
 	}
@@ -1369,7 +1376,7 @@ func (c *common) TempDir() string {
 	}
 
 	dir := fmt.Sprintf("%s%c%03d", c.tempDir, os.PathSeparator, seq)
-	if err := os.Mkdir(dir, 0777); err != nil {
+	if err := os.Mkdir(dir, 0o777); err != nil {
 		c.Fatalf("TempDir: %v", err)
 	}
 	return dir
@@ -2132,8 +2139,10 @@ func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchma
 	}
 }
 
-var testingTesting bool
-var realStderr *os.File
+var (
+	testingTesting bool
+	realStderr     *os.File
+)
 
 // Run runs the tests. It returns an exit code to pass to os.Exit.
 // The exit code is zero when all tests pass, and non-zero for any kind