encode: Fix errors for marshalers/encodables with bad output

3 files changed, 129 insertions, 5 deletions

diff --git a/encode.go b/encode.go
index fa337ad..00848ed 100644
--- a/encode.go
+++ b/encode.go
@@ -146,7 +146,11 @@ func encode(w io.Writer, val reflect.Value, escaper BackslashEscaper, quote bool
 			}})
 		}
 		if err := validator.Close(); err != nil && !errors.Is(err, iofs.ErrClosed) {
-			panic(encodeError{err})
+			panic(encodeError{&EncodeMethodError{
+				Type:       val.Type(),
+				SourceFunc: "EncodeJSON",
+				Err:        err,
+			}})
 		}
 
 	case val.Kind() != reflect.Pointer && val.CanAddr() && reflect.PointerTo(val.Type()).Implements(jsonMarshalerType):
@@ -173,10 +177,18 @@ func encode(w io.Writer, val reflect.Value, escaper BackslashEscaper, quote bool
 		// Use a sub-ReEncoder to check that it's a full element.
 		validator := &ReEncoder{Out: w, BackslashEscape: escaper}
 		if _, err := validator.Write(dat); err != nil {
-			panic(encodeError{err})
+			panic(encodeError{&EncodeMethodError{
+				Type:       val.Type(),
+				SourceFunc: "MarshalJSON",
+				Err:        err,
+			}})
 		}
 		if err := validator.Close(); err != nil {
-			panic(encodeError{err})
+			panic(encodeError{&EncodeMethodError{
+				Type:       val.Type(),
+				SourceFunc: "MarshalJSON",
+				Err:        err,
+			}})
 		}
 
 	case val.Kind() != reflect.Pointer && val.CanAddr() && reflect.PointerTo(val.Type()).Implements(textMarshalerType):
diff --git a/errors.go b/errors.go
index d36fc83..fe48723 100644
--- a/errors.go
+++ b/errors.go
@@ -138,8 +138,9 @@ type EncodeTypeError = json.UnsupportedTypeError
 //	}
 type EncodeValueError = json.UnsupportedValueError
 
-// An EncodeMethodError wraps an error that is returned from an
-// object's method when encoding that object to JSON.
+// An EncodeMethodError either wraps an error that is returned from an
+// object's method when encoding that object to JSON, or wraps a
+// *ReEncodeSyntaxError for the method's output.
 type EncodeMethodError struct {
 	Type       reflect.Type // The Go type that the method is on
 	SourceFunc string       // The method: "EncodeJSON", "MarshalJSON", or "MarshalText"
diff --git a/methods_test.go b/methods_test.go
index 5e2209a..46e2601 100644
--- a/methods_test.go
+++ b/methods_test.go
@@ -6,8 +6,10 @@ package lowmemjson_test
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -121,3 +123,112 @@ func TestMethods(t *testing.T) {
 	assert.NoError(t, lowmemjson.NewDecoder(&buf).Decode(&out))
 	assert.Equal(t, in, out)
 }
+
+type strEncoder string
+
+func (s strEncoder) EncodeJSON(w io.Writer) error {
+	_, err := io.WriteString(w, string(s))
+	return err
+}
+
+type strMarshaler string
+
+func (s strMarshaler) MarshalJSON() ([]byte, error) {
+	return []byte(s), nil
+}
+
+type strTextMarshaler struct {
+	str string
+	err string
+}
+
+func (m strTextMarshaler) MarshalText() (txt []byte, err error) {
+	if len(m.str) > 0 {
+		txt = []byte(m.str)
+	}
+	if len(m.err) > 0 {
+		err = errors.New(m.err)
+	}
+	return
+}
+
+func TestMethodsEncode(t *testing.T) {
+	t.Parallel()
+	type testcase struct {
+		In          string
+		ExpectedErr string
+	}
+	testcases := map[string]testcase{
+		"basic": {In: `{}`},
+		"empty": {In: ``, ExpectedErr: `syntax error at input byte 0: EOF`},
+		"short": {In: `{`, ExpectedErr: `syntax error at input byte 1: unexpected EOF`},
+		"long":  {In: `{}{}`, ExpectedErr: `syntax error at input byte 2: invalid character '{' after top-level value`},
+	}
+	t.Run("encodable", func(t *testing.T) {
+		t.Parallel()
+		for tcName, tc := range testcases {
+			tc := tc
+			t.Run(tcName, func(t *testing.T) {
+				t.Parallel()
+				var buf strings.Builder
+				err := lowmemjson.NewEncoder(&buf).Encode([]any{strEncoder(tc.In)})
+				if tc.ExpectedErr == "" {
+					assert.NoError(t, err)
+					assert.Equal(t, "["+tc.In+"]", buf.String())
+				} else {
+					assert.EqualError(t, err,
+						`json: error calling EncodeJSON for type lowmemjson_test.strEncoder: `+
+							tc.ExpectedErr)
+				}
+			})
+		}
+	})
+	t.Run("marshaler", func(t *testing.T) {
+		t.Parallel()
+		for tcName, tc := range testcases {
+			tc := tc
+			t.Run(tcName, func(t *testing.T) {
+				t.Parallel()
+				var buf strings.Builder
+				err := lowmemjson.NewEncoder(&buf).Encode([]any{strMarshaler(tc.In)})
+				if tc.ExpectedErr == "" {
+					assert.NoError(t, err)
+					assert.Equal(t, "["+tc.In+"]", buf.String())
+				} else {
+					assert.EqualError(t, err,
+						`json: error calling MarshalJSON for type lowmemjson_test.strMarshaler: `+
+							tc.ExpectedErr)
+				}
+			})
+		}
+	})
+	t.Run("text", func(t *testing.T) {
+		t.Parallel()
+		type testcase struct {
+			Str string
+			Err string
+		}
+		testcases := map[string]testcase{
+			"basic": {Str: `a`},
+			"err":   {Err: `xxx`},
+			"both":  {Str: `a`, Err: `xxx`},
+		}
+		for tcName, tc := range testcases {
+			tc := tc
+			t.Run(tcName, func(t *testing.T) {
+				t.Parallel()
+				var buf strings.Builder
+				err := lowmemjson.NewEncoder(&buf).Encode([]any{strTextMarshaler{str: tc.Str, err: tc.Err}})
+				if tc.Err == "" {
+					assert.NoError(t, err)
+					assert.Equal(t, `["`+tc.Str+`"]`, buf.String())
+				} else {
+					assert.EqualError(t, err,
+						`json: error calling MarshalText for type lowmemjson_test.strTextMarshaler: `+
+							tc.Err)
+					assert.Equal(t, "[", buf.String())
+				}
+			})
+		}
+	})
+}