runtime: when crash with panic, call user Error/String methods before freezing the world

Fixes #14432.

Change-Id: I0a92ef86de95de39217df9a664d8034ef685a906
Reviewed-on: https://go-review.googlesource.com/19792
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_cgo_test.go

@@ -178,3 +178,12 @@ func TestCgoCheckBytes(t *testing.T) {
 		t.Errorf("cgo check too slow: got %v, expected at most %v", d1, d2*10)
 	}
 }
+
+func TestCgoPanicDeadlock(t *testing.T) {
+	// test issue 14432
+	got := runTestProg(t, "testprogcgo", "CgoPanicDeadlock")
+	want := "panic: cgo error\n\n"
+	if !strings.HasPrefix(got, want) {
+		t.Fatalf("output does not start with %q:\n%s", want, got)
+	}
+}

src/runtime/crash_test.go

@@ -336,3 +336,19 @@ func TestPanicTraceback(t *testing.T) {
 		output = output[idx[1]:]
 	}
 }
+
+func testPanicDeadlock(t *testing.T, name string, want string) {
+	// test issue 14432
+	output := runTestProg(t, "testprog", name)
+	if !strings.HasPrefix(output, want) {
+		t.Fatalf("output does not start with %q:\n%s", want, output)
+	}
+}
+
+func TestPanicDeadlockGosched(t *testing.T) {
+	testPanicDeadlock(t, "GoschedInPanic", "panic: errorThatGosched\n\n")
+}
+
+func TestPanicDeadlockSyscall(t *testing.T) {
+	testPanicDeadlock(t, "SyscallInPanic", "1\n2\npanic: 3\n\n")
+}

src/runtime/panic.go

@@ -333,6 +333,21 @@ func Goexit() {
 	goexit1()
 }
 
+// Call all Error and String methods before freezing the world.
+// Used when crashing with panicking.
+// This must match types handled by printany.
+func preprintpanics(p *_panic) {
+	for p != nil {
+		switch v := p.arg.(type) {
+		case error:
+			p.arg = v.Error()
+		case stringer:
+			p.arg = v.String()
+		}
+		p = p.link
+	}
+}
+
 // Print all currently active panics.  Used when crashing.
 func printpanics(p *_panic) {
 	if p.link != nil {
@@ -459,6 +474,10 @@ func gopanic(e interface{}) {
 	}
 
 	// ran out of deferred calls - old-school panic now
+	// Because it is unsafe to call arbitrary user code after freezing
+	// the world, we call preprintpanics to invoke all necessary Error
+	// and String methods to prepare the panic strings before startpanic.
+	preprintpanics(gp._panic)
 	startpanic()
 	printpanics(gp._panic)
 	dopanic(0)       // should not return

src/runtime/testdata/testprog/deadlock.go

@@ -30,6 +30,8 @@ func init() {
 	register("PanicAfterGoexit", PanicAfterGoexit)
 	register("RecoveredPanicAfterGoexit", RecoveredPanicAfterGoexit)
 	register("PanicTraceback", PanicTraceback)
+	register("GoschedInPanic", GoschedInPanic)
+	register("SyscallInPanic", SyscallInPanic)
 }
 
 func SimpleDeadlock() {
@@ -152,6 +154,29 @@ func GoexitInPanic() {
 	runtime.Goexit()
 }
 
+type errorThatGosched struct{}
+
+func (errorThatGosched) Error() string {
+	runtime.Gosched()
+	return "errorThatGosched"
+}
+
+func GoschedInPanic() {
+	panic(errorThatGosched{})
+}
+
+type errorThatPrint struct{}
+
+func (errorThatPrint) Error() string {
+	fmt.Println("1")
+	fmt.Println("2")
+	return "3"
+}
+
+func SyscallInPanic() {
+	panic(errorThatPrint{})
+}
+
 func PanicAfterGoexit() {
 	defer func() {
 		panic("hello")

src/runtime/testdata/testprogcgo/deadlock.go

@@ -0,0 +1,30 @@
+// Copyright 2016 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+/*
+char *geterror() {
+	return "cgo error";
+}
+*/
+import "C"
+import (
+	"fmt"
+)
+
+func init() {
+	register("CgoPanicDeadlock", CgoPanicDeadlock)
+}
+
+type cgoError struct{}
+
+func (cgoError) Error() string {
+	fmt.Print("") // necessary to trigger the deadlock
+	return C.GoString(C.geterror())
+}
+
+func CgoPanicDeadlock() {
+	panic(cgoError{})
+}