encoding/json: make error capture logic in recover more type safe

Rather than only ignoring runtime.Error panics, which are a very
narrow set of possible panic values, switch it such that the json
package only captures panic values that have been properly wrapped
in a jsonError struct. This ensures that only intentional panics
originating from the json package are captured.

Fixes #23012

Change-Id: I5e85200259edd2abb1b0512ce6cc288849151a6d
Reviewed-on: https://go-review.googlesource.com/94019
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/decode.go

@@ -14,7 +14,6 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
-	"runtime"
 	"strconv"
 	"unicode"
 	"unicode/utf16"
@@ -168,13 +167,19 @@ func (e *InvalidUnmarshalError) Error() string {
 	return "json: Unmarshal(nil " + e.Type.String() + ")"
 }
 
+// jsonError is an error wrapper type for internal use only.
+// Panics with errors are wrapped in jsonError so that the top-level recover
+// can distinguish intentional panics from this package.
+type jsonError struct{ error }
+
 func (d *decodeState) unmarshal(v interface{}) (err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			if _, ok := r.(runtime.Error); ok {
+			if je, ok := r.(jsonError); ok {
+				err = je.error
+			} else {
 				panic(r)
 			}
-			err = r.(error)
 		}
 	}()
 
@@ -295,9 +300,9 @@ func (d *decodeState) init(data []byte) *decodeState {
 	return d
 }
 
-// error aborts the decoding by panicking with err.
+// error aborts the decoding by panicking with err wrapped in jsonError.
 func (d *decodeState) error(err error) {
-	panic(d.addErrorContext(err))
+	panic(jsonError{d.addErrorContext(err)})
 }
 
 // saveError saves the first err it is called with,

src/encoding/json/decode_test.go

@@ -2166,3 +2166,17 @@ func TestUnmarshalEmbeddedPointerUnexported(t *testing.T) {
 		}
 	}
 }
+
+type unmarshalPanic struct{}
+
+func (unmarshalPanic) UnmarshalJSON([]byte) error { panic(0xdead) }
+
+func TestUnmarshalPanic(t *testing.T) {
+	defer func() {
+		if got := recover(); !reflect.DeepEqual(got, 0xdead) {
+			t.Errorf("panic() = (%T)(%v), want 0xdead", got, got)
+		}
+	}()
+	Unmarshal([]byte("{}"), &unmarshalPanic{})
+	t.Fatalf("Unmarshal should have panicked")
+}

src/encoding/json/encode.go

@@ -17,7 +17,6 @@ import (
 	"fmt"
 	"math"
 	"reflect"
-	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -286,21 +285,20 @@ func newEncodeState() *encodeState {
 func (e *encodeState) marshal(v interface{}, opts encOpts) (err error) {
 	defer func() {
 		if r := recover(); r != nil {
-			if _, ok := r.(runtime.Error); ok {
+			if je, ok := r.(jsonError); ok {
+				err = je.error
+			} else {
 				panic(r)
 			}
-			if s, ok := r.(string); ok {
-				panic(s)
-			}
-			err = r.(error)
 		}
 	}()
 	e.reflectValue(reflect.ValueOf(v), opts)
 	return nil
 }
 
+// error aborts the encoding by panicking with err wrapped in jsonError.
 func (e *encodeState) error(err error) {
-	panic(err)
+	panic(jsonError{err})
 }
 
 func isEmptyValue(v reflect.Value) bool {

src/encoding/json/encode_test.go

@@ -981,3 +981,17 @@ func TestMarshalRawMessageValue(t *testing.T) {
 		}
 	}
 }
+
+type marshalPanic struct{}
+
+func (marshalPanic) MarshalJSON() ([]byte, error) { panic(0xdead) }
+
+func TestMarshalPanic(t *testing.T) {
+	defer func() {
+		if got := recover(); !reflect.DeepEqual(got, 0xdead) {
+			t.Errorf("panic() = (%T)(%v), want 0xdead", got, got)
+		}
+	}()
+	Marshal(&marshalPanic{})
+	t.Error("Marshal should have panicked")
+}