Repository: segmentio/encoding (go)

Issue:
Encoding a struct with error type field fails in various ways
While encoding a struct containing error type I get panic or error(`json: unsupported type: `). 

Code:
```go
package main

import (
	"bytes"
	stdjson "encoding/json"
	"errors"
	"fmt"

	segjson "github.com/segmentio/encoding/json"
)

type Column struct {
	Name      string `json:"name"`
	FieldName string `json:"fieldName"`
}

type QueryResult struct {
	Columns []Column                 `json:"columns"`
	Rows    []map[string]interface{} `json:"rows"`
}

type QueryError struct {
	Message string `json:"message"`
	Err     error  `json:"err"`
}

type GroupQueryResult struct {
	GroupId int          `json:"groupId"`
	Data    *QueryResult `json:"data"`
	Error   *QueryError  `json:"error"`
}

func main() {
	v := GroupQueryResult{GroupId: 1, Error: &QueryError{Message: "error message", Err: errors.New("will fail")}}
	fmt.Println("stdEncode: " + stdEncode(v))
	fmt.Println("stdMarshal: " + stdMarshal(v))
	fmt.Println("segEncode: " + segEncode(v))
	fmt.Println("segMarshal: " + segMarshal(v))
}

func stdEncode(v GroupQueryResult) string {
	buf := &bytes.Buffer{}
	enc := stdjson.NewEncoder(buf)
	enc.SetEscapeHTML(true)
	if err := enc.Encode(v); err != nil {
		return err.Error()
	}
	return string(buf.Bytes())
}

func stdMarshal(v GroupQueryResult) string {
	jsonString, err := stdjson.Marshal(v)
	if err != nil {
		return err.Error()
	}
	return string(jsonString)
}

func segEncode(v GroupQueryResult) string {
	buf := &bytes.Buffer{}
	enc := segjson.NewEncoder(buf)
	enc.SetEscapeHTML(true)
	if err := enc.Encode(v); err != nil {
		return err.Error()
	}
	return string(buf.Bytes())
}

func segMarshal(v GroupQueryResult) string {
	jsonString, err := segjson.Marshal(v)
	if err != nil {
		return err.Error()
	}
	return string(jsonString)
}
```


While running given code(go version 1.13.6, library version 0.1.9)
in go playground i get:
```
stdEncode: {"groupId":1,"data":null,"error":{"message":"error message","err":{}}}
stdMarshal: {"groupId":1,"data":null,"error":{"message":"error message","err":{}}}
segEncode: json: unsupported type: 
segMarshal: json: unsupported type:
```
on my pc (windows, amd64) i get:
```
stdEncode: {"groupId":1,"data":null,"error":{"message":"error message","err":{}}}
stdMarshal: {"groupId":1,"data":null,"error":{"message":"error message","err":{}}}
runtime: nameOff 0x50e5e0 out of range 0x4ee000 - 0x55cfe3
fatal error: runtime: name offset out of range

goroutine 1 [running]:
runtime.throw(0x53900d, 0x21)
	C:/Go/src/runtime/panic.go:774 +0x79 fp=0xc000089598 sp=0xc000089568 pc=0x42e639
runtime.resolveNameOff(0x557cc0, 0x1000050e5e0, 0x4f7f78)
	C:/Go/src/runtime/type.go:190 +0x2ee fp=0xc000089600 sp=0xc000089598 pc=0x44e68e
reflect.resolveNameOff(0x557cc0, 0x50e5e0, 0xc0000426d0)
	C:/Go/src/runtime/runtime1.go:478 +0x3a fp=0xc000089628 sp=0xc000089600 pc=0x43ccda
reflect.(*rtype).nameOff(...)
	C:/Go/src/reflect/type.go:691
reflect.(*rtype).String(0x557cc0, 0x52b980, 0x557cc0)
	C:/Go/src/reflect/type.go:761 +0x3d fp=0xc000089660 sp=0xc000089628 pc=0x47914d
reflect.(*rtype).ptrTo(0x557cc0, 0x557cc0)
	C:/Go/src/reflect/type.go:1398 +0x90 fp=0xc000089708 sp=0xc000089660 pc=0x47c5d0
reflect.PtrTo(...)
	C:/Go/src/reflect/type.go:1384
github.com/segmentio/encoding/json.constructCodec(0x55a140, 0x557cc0, 0xc0000897b0, 0x0, 0x0, 0x0)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/codec.go:180 +0x14d fp=0xc000089760 sp=0xc000089708 pc=0x4c9d6d
github.com/segmentio/encoding/json.constructCachedCodec(0x55a140, 0x557cc0, 0xc00006ac00, 0x6320e0, 0x100000000000000)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/codec.go:62 +0xe4 fp=0xc0000898c0 sp=0xc000089760 pc=0x4c9b54
github.com/segmentio/encoding/json.Append(0xc00009c000, 0x42, 0x1000, 0x557cc0, 0xc000042230, 0x3, 0x3a, 0xd, 0x5339e9, 0xc00000a517, ...)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/json.go:132 +0x187 fp=0xc000089958 sp=0xc0000898c0 pc=0x4e3107
github.com/segmentio/encoding/json.encoder.encodeInterface(0x3, 0xc00009c000, 0x42, 0x1000, 0xc0000044f0, 0xc00009c000, 0x3b, 0x1000, 0x0, 0x0)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/encode.go:592 +0x70 fp=0xc0000899c0 sp=0xc000089958 pc=0x4e18e0
github.com/segmentio/encoding/json.encoder.encodeStruct(0x3, 0xc00009c000, 0x21, 0x1000, 0xc0000044e0, 0xc00001a280, 0x51cae0, 0x403b34, 0x6132f0, 0xc00006ac00, ...)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/encode.go:557 +0x1e5 fp=0xc000089a80 sp=0xc0000899c0 pc=0x4e1215
github.com/segmentio/encoding/json.constructStructEncodeFunc.func1(0x3, 0xc00009c000, 0x21, 0x1000, 0xc0000044e0, 0x50f8c0, 0x4fd500, 0x0, 0x0, 0x0)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/codec.go:466 +0x6d fp=0xc000089ae8 sp=0xc000089a80 pc=0x4eb35d
github.com/segmentio/encoding/json.encoder.encodePointer(0x3, 0xc00009c000, 0x21, 0x1000, 0xc000004950, 0x55a140, 0x514e80, 0xc000042740, 0xc00009c000, 0x18, ...)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/encode.go:586 +0x10c fp=0xc000089b50 sp=0xc000089ae8 pc=0x4e180c
github.com/segmentio/encoding/json.constructPointerEncodeFunc.func1(0x3, 0xc00009c000, 0x21, 0x1000, 0xc000004950, 0xc00009c000, 0x18, 0x1000, 0x0, 0x0)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/codec.go:704 +0x85 fp=0xc000089bc8 sp=0xc000089b50 pc=0x4eb765
github.com/segmentio/encoding/json.encoder.encodeStruct(0x3, 0xc00009c000, 0x0, 0x1000, 0xc000004940, 0xc00001a180, 0x0, 0x0, 0x0, 0x0, ...)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/encode.go:557 +0x1e5 fp=0xc000089c88 sp=0xc000089bc8 pc=0x4e1215
github.com/segmentio/encoding/json.constructStructEncodeFunc.func1(0x3, 0xc00009c000, 0x0, 0x1000, 0xc000004940, 0x1000, 0xc00009c000, 0xc00009c000, 0xc000089d78, 0x4653ed)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/codec.go:466 +0x6d fp=0xc000089cf0 sp=0xc000089c88 pc=0x4eb35d
github.com/segmentio/encoding/json.Append(0xc00009c000, 0x0, 0x1000, 0x519de0, 0xc000004940, 0x3, 0xc000089f90, 0xc00008a280, 0xc000089e00, 0x409926, ...)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/json.go:135 +0x107 fp=0xc000089d88 sp=0xc000089cf0 pc=0x4e3087
github.com/segmentio/encoding/json.(*Encoder).Encode(0xc000089e60, 0x519de0, 0xc000004940, 0xc000004940, 0x60cf40)
	C:/Users/Mynde/go/pkg/mod/github.com/segmentio/encoding@v0.1.9/json/json.go:372 +0xb9 fp=0xc000089e10 sp=0xc000089d88 pc=0x4e38a9
main.segEncode(0x1, 0x0, 0xc0000044e0, 0x1, 0x1)
	C:/WorkGo/src/segmenio-json-test/main.go:63 +0x119 fp=0xc000089ec0 sp=0xc000089e10 pc=0x4eda09
main.main()
	C:/WorkGo/src/segmenio-json-test/main.go:37 +0x243 fp=0xc000089f60 sp=0xc000089ec0 pc=0x4ed473
runtime.main()
	C:/Go/src/runtime/proc.go:203 +0x21e fp=0xc000089fe0 sp=0xc000089f60 pc=0x43001e
runtime.goexit()
	C:/Go/src/runtime/asm_amd64.s:1357 +0x1 fp=0xc000089fe8 sp=0xc000089fe0 pc=0x456841
```

Pull request description:
fix issue 28
Fixes #28 

Go does some magic when converting from non-empty to empty interfaces, the casting of `*(*interface{})(p)` for all interface types was not good enough.

This PR addresses it by falling back to the reflect package for this specific case. I don't anticipate this having an impact on performance, non-empty interface types in JSON are rare because they can't really be unmarshaled, and the use of `reflect.NewAt` and conversion of an interface to `interface{}` doesn't add much overhead.

I've added a test which was reproducing the bug and is now passing with the fix.

Patch:
diff --git a/json/codec.go b/json/codec.go
index 1ef84f5..cf7216d 100644
--- a/json/codec.go
+++ b/json/codec.go
@@ -156,7 +156,7 @@ func constructCodec(t reflect.Type, seen map[reflect.Type]*structType, canAddr b
 		c = codec{encode: encoder.encodeString, decode: decoder.decodeString}
 
 	case reflect.Interface:
-		c = codec{encode: encoder.encodeInterface, decode: constructMaybeEmptyInterfaceDecoderFunc(t)}
+		c = constructInterfaceCodec(t)
 
 	case reflect.Array:
 		c = constructArrayCodec(t, seen, canAddr)
@@ -711,6 +711,19 @@ func constructPointerDecodeFunc(t reflect.Type, decode decodeFunc) decodeFunc {
 	}
 }
 
+func constructInterfaceCodec(t reflect.Type) codec {
+	return codec{
+		encode: constructMaybeEmptyInterfaceEncoderFunc(t),
+		decode: constructMaybeEmptyInterfaceDecoderFunc(t),
+	}
+}
+
+func constructMaybeEmptyInterfaceEncoderFunc(t reflect.Type) encodeFunc {
+	return func(e encoder, b []byte, p unsafe.Pointer) ([]byte, error) {
+		return e.encodeMaybeEmptyInterface(b, p, t)
+	}
+}
+
 func constructMaybeEmptyInterfaceDecoderFunc(t reflect.Type) decodeFunc {
 	return func(d decoder, b []byte, p unsafe.Pointer) ([]byte, error) {
 		return d.decodeMaybeEmptyInterface(b, p, t)
diff --git a/json/encode.go b/json/encode.go
index a44544b..75641bd 100644
--- a/json/encode.go
+++ b/json/encode.go
@@ -592,6 +592,10 @@ func (e encoder) encodeInterface(b []byte, p unsafe.Pointer) ([]byte, error) {
 	return Append(b, *(*interface{})(p), e.flags)
 }
 
+func (e encoder) encodeMaybeEmptyInterface(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
+	return Append(b, reflect.NewAt(t, p).Elem().Interface(), e.flags)
+}
+
 func (e encoder) encodeUnsupportedTypeError(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
 	return b, &UnsupportedTypeError{Type: t}
 }


Test patch:
diff --git a/json/json_test.go b/json/json_test.go
index 8c430de..22d9bc6 100644
--- a/json/json_test.go
+++ b/json/json_test.go
@@ -5,6 +5,7 @@ import (
 	"compress/gzip"
 	"encoding"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -1475,3 +1476,16 @@ func TestGithubIssue26(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestGithubIssue28(t *testing.T) {
+	type A struct {
+		Err error `json:"err"`
+	}
+
+	if b, err := Marshal(&A{Err: errors.New("ABC")}); err != nil {
+		t.Error(err)
+	} else if string(b) != `{"err":{}}` {
+		t.Error(string(b))
+	}
+
+}