runtime: don't discard value from panic while panicking

In issue #17671, there are a endless loop if printing
the panic value panics, CL 30358 has fixed that.

As issue #52257 pointed out, above change should not
discard the value from panic while panicking.

With this CL, when we recover from a panic in error.Error()
or stringer.String(), and the recovered value is string,
then we can print it normally.

Fixes #52257

Change-Id: Icfcc4a1a390635de405eea04904b4607ae9e3055
Reviewed-on: https://go-review.googlesource.com/c/go/+/399874
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

1 files changed, 73 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/crash.go b/src/runtime/testdata/testprog/crash.go
index c4990cdda9..a2294ba149 100644
--- a/src/runtime/testdata/testprog/crash.go
+++ b/src/runtime/testdata/testprog/crash.go
@@ -12,6 +12,13 @@ import (
 func init() {
 	register("Crash", Crash)
 	register("DoublePanic", DoublePanic)
+	register("ErrorPanic", ErrorPanic)
+	register("StringerPanic", StringerPanic)
+	register("DoubleErrorPanic", DoubleErrorPanic)
+	register("DoubleStringerPanic", DoubleStringerPanic)
+	register("StringPanic", StringPanic)
+	register("NilPanic", NilPanic)
+	register("CircularPanic", CircularPanic)
 }
 
 func test(name string) {
@@ -64,3 +71,69 @@ func DoublePanic() {
 	}()
 	panic(P("XXX"))
 }
+
+// Test that panic while panicking discards error message
+// See issue 52257
+type exampleError struct{}
+
+func (e exampleError) Error() string {
+	panic("important error message")
+}
+
+func ErrorPanic() {
+	panic(exampleError{})
+}
+
+type examplePanicError struct{}
+
+func (e examplePanicError) Error() string {
+	panic(exampleError{})
+}
+
+func DoubleErrorPanic() {
+	panic(examplePanicError{})
+}
+
+type exampleStringer struct{}
+
+func (s exampleStringer) String() string {
+	panic("important stringer message")
+}
+
+func StringerPanic() {
+	panic(exampleStringer{})
+}
+
+type examplePanicStringer struct{}
+
+func (s examplePanicStringer) String() string {
+	panic(exampleStringer{})
+}
+
+func DoubleStringerPanic() {
+	panic(examplePanicStringer{})
+}
+
+func StringPanic() {
+	panic("important string message")
+}
+
+func NilPanic() {
+	panic(nil)
+}
+
+type exampleCircleStartError struct {}
+
+func (e exampleCircleStartError) Error() string {
+	panic(exampleCircleEndError{})
+}
+
+type exampleCircleEndError struct {}
+
+func (e exampleCircleEndError) Error() string {
+	panic(exampleCircleStartError{})
+}
+
+func CircularPanic() {
+	panic(exampleCircleStartError{})
+}
\ No newline at end of file