compat/json: Compact, Indent: Clear the output if there's an error

3 files changed, 72 insertions, 2 deletions

diff --git a/ReleaseNotes.md b/ReleaseNotes.md
index 613ea0c..e72a664 100644
--- a/ReleaseNotes.md
+++ b/ReleaseNotes.md
@@ -14,6 +14,9 @@
       + compat/json.Valid: No longer considers truncated JSON
         documents to be valid.
 
+      + compat/json.Compact, compat/json.Indent: Don't write to the
+        destination buffer if there is a syntax error.
+
 # v0.3.6 (2023-02-16)
 
   Theme: Architectural improvements
diff --git a/compat/json/compat.go b/compat/json/compat.go
index 300ab2f..1cdbf0b 100644
--- a/compat/json/compat.go
+++ b/compat/json/compat.go
@@ -157,18 +157,28 @@ func reencode(dst io.Writer, src []byte, cfg lowmemjson.ReEncoderConfig) error {
 }
 
 func Compact(dst *bytes.Buffer, src []byte) error {
-	return reencode(dst, src, lowmemjson.ReEncoderConfig{
+	start := dst.Len()
+	err := reencode(dst, src, lowmemjson.ReEncoderConfig{
 		Compact:         true,
 		BackslashEscape: lowmemjson.EscapePreserve,
 	})
+	if err != nil {
+		dst.Truncate(start)
+	}
+	return err
 }
 
 func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
-	return reencode(dst, src, lowmemjson.ReEncoderConfig{
+	start := dst.Len()
+	err := reencode(dst, src, lowmemjson.ReEncoderConfig{
 		Indent:          indent,
 		Prefix:          prefix,
 		BackslashEscape: lowmemjson.EscapePreserve,
 	})
+	if err != nil {
+		dst.Truncate(start)
+	}
+	return err
 }
 
 func Valid(data []byte) bool {
diff --git a/compat/json/compat_test.go b/compat/json/compat_test.go
index 5c8f3ee..d513c27 100644
--- a/compat/json/compat_test.go
+++ b/compat/json/compat_test.go
@@ -5,6 +5,7 @@
 package json
 
 import (
+	"bytes"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -32,3 +33,59 @@ func TestCompatValid(t *testing.T) {
 		})
 	}
 }
+
+func TestCompatCompact(t *testing.T) {
+	t.Parallel()
+	type testcase struct {
+		In  string
+		Out string
+		Err string
+	}
+	testcases := map[string]testcase{
+		"trunc":  {In: `{`, Out: ``, Err: `unexpected end of JSON input`},
+		"object": {In: `{}`, Out: `{}`},
+	}
+	for tcName, tc := range testcases {
+		tc := tc
+		t.Run(tcName, func(t *testing.T) {
+			t.Parallel()
+			t.Logf("in=%q", tc.In)
+			var out bytes.Buffer
+			err := Compact(&out, []byte(tc.In))
+			assert.Equal(t, tc.Out, out.String())
+			if tc.Err == "" {
+				assert.NoError(t, err)
+			} else {
+				assert.EqualError(t, err, tc.Err)
+			}
+		})
+	}
+}
+
+func TestCompatIndent(t *testing.T) {
+	t.Parallel()
+	type testcase struct {
+		In  string
+		Out string
+		Err string
+	}
+	testcases := map[string]testcase{
+		"trunc":  {In: `{`, Out: ``, Err: `unexpected end of JSON input`},
+		"object": {In: `{}`, Out: `{}`},
+	}
+	for tcName, tc := range testcases {
+		tc := tc
+		t.Run(tcName, func(t *testing.T) {
+			t.Parallel()
+			t.Logf("in=%q", tc.In)
+			var out bytes.Buffer
+			err := Indent(&out, []byte(tc.In), ">", ".")
+			assert.Equal(t, tc.Out, out.String())
+			if tc.Err == "" {
+				assert.NoError(t, err)
+			} else {
+				assert.EqualError(t, err, tc.Err)
+			}
+		})
+	}
+}