runtime: avoid endless loop if printing the panic value panics

Change-Id: I56de359a5ccdc0a10925cd372fa86534353c6ca0
Reviewed-on: https://go-review.googlesource.com/30358
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_test.go

@@ -444,6 +444,13 @@ func TestPanicDeadlockSyscall(t *testing.T) {
 	testPanicDeadlock(t, "SyscallInPanic", "1\n2\npanic: 3\n\n")
 }
 
+func TestPanicLoop(t *testing.T) {
+	output := runTestProg(t, "testprog", "PanicLoop")
+	if want := "panic while printing panic value"; !strings.Contains(output, want) {
+		t.Errorf("output does not contain %q:\n%s", want, output)
+	}
+}
+
 func TestMemPprof(t *testing.T) {
 	testenv.MustHaveGoRun(t)
 

src/runtime/panic.go

@@ -376,6 +376,11 @@ func Goexit() {
 // Used when crashing with panicking.
 // This must match types handled by printany.
 func preprintpanics(p *_panic) {
+	defer func() {
+		if recover() != nil {
+			throw("panic while printing panic value")
+		}
+	}()
 	for p != nil {
 		switch v := p.arg.(type) {
 		case error:

src/runtime/testdata/testprog/deadlock.go

@@ -32,6 +32,7 @@ func init() {
 	register("PanicTraceback", PanicTraceback)
 	register("GoschedInPanic", GoschedInPanic)
 	register("SyscallInPanic", SyscallInPanic)
+	register("PanicLoop", PanicLoop)
 }
 
 func SimpleDeadlock() {
@@ -214,3 +215,13 @@ func pt2() {
 	}()
 	panic("hello")
 }
+
+type panicError struct{}
+
+func (*panicError) Error() string {
+	panic("double error")
+}
+
+func PanicLoop() {
+	panic(&panicError{})
+}