[Go] Escape single char fields in generated flyweight String output

.gitignore

@@ -79,7 +79,7 @@ gocode/flyweight/src/*/*.go
 !gocode/flyweight/src/*/*_test.go
 gocode/flyweight/src/*/*/*.go
 !gocode/flyweight/src/*/*/*_test.go
-gocode/flyweight/src/otf/code-generation-schema.sbeir
+gocode/flyweight/src/otf/*.sbeir
 
 # csharp
 csharp/*/bin

gocode/flyweight/src/otf/flyweight_single_char_test.go

@@ -0,0 +1,47 @@
+package otf
+
+import (
+	stdjson "encoding/json"
+	"github.com/real-logic/simple-binary-encoding/otf/test/flyweight_single_char"
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+//go:generate java --add-opens java.base/jdk.internal.misc=ALL-UNNAMED -Dsbe.output.dir=. -Dsbe.target.language=golang -Dsbe.go.generate.generate.flyweights=true -Dsbe.generate.ir=true -Dsbe.keyword.append.token=X -jar ../../../${SBE_JAR} ../../../../sbe-tool/src/test/resources/flyweight-single-char-schema.xml
+
+func TestSingleCharFieldsRenderAsValidJson(t *testing.T) {
+	cases := []struct {
+		name     string
+		value    byte
+		expected string
+	}{
+		{"letter", 'A', "A"},
+		{"quote", '"', "\""},
+		{"backslash", '\\', "\\"},
+		{"newline", '\n', "\n"},
+		{"control", 0x01, "\x01"},
+		{"high", 0xe9, "é"},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			buffer := make([]byte, 64)
+			var letter flyweight_single_char.Letter
+			letter.WrapAndApplyHeader(buffer, 0, uint64(len(buffer))).SetCode(c.value)
+			letter.EntriesCount(1).Next().SetSide(c.value)
+
+			var decoder flyweight_single_char.Letter
+			decoder.WrapForDecode(buffer, 8, uint64(letter.SbeBlockLength()), 0, uint64(len(buffer)))
+
+			var decoded struct {
+				Code    string
+				Entries []struct{ Side string }
+			}
+			if err := stdjson.Unmarshal([]byte(decoder.String()), &decoded); err != nil {
+				t.Fatalf("invalid JSON: %v\n%s", err, decoder.String())
+			}
+			assert.Equal(t, c.expected, decoded.Code)
+			assert.Equal(t, c.expected, decoded.Entries[0].Side)
+		})
+	}
+}

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/golang/flyweight/GolangFlyweightGenerator.java

@@ -2947,8 +2947,17 @@ private void writeTokenDisplay(
 
                     if (typeToken.encoding().primitiveType() == PrimitiveType.CHAR)
                     {
-                        sb.append(indent).append("builder.WriteString(fmt.Sprintf(`\"%c\"`, ").append(fieldName)
-                            .append("()))\n");
+                        sb.append(
+                            indent + "builder.WriteString(`\"`)\n" +
+                            indent + "switch c := " + fieldName + "(); {\n" +
+                            indent + "case c == '\"' || c == '\\\\':\n" +
+                            indent + "\tbuilder.WriteString(\"\\\\\" + string(rune(c)))\n" +
+                            indent + "case c < 0x20:\n" +
+                            indent + "\tbuilder.WriteString(fmt.Sprintf(\"\\\\u%04x\", c))\n" +
+                            indent + "default:\n" +
+                            indent + "\tbuilder.WriteString(string(rune(c)))\n" +
+                            indent + "}\n" +
+                            indent + "builder.WriteString(`\"`)\n");
                     }
                     else
                     {

sbe-tool/src/test/resources/flyweight-single-char-schema.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<sbe:messageSchema xmlns:sbe="http://fixprotocol.io/2016/sbe"
+                   package="flyweight_single_char"
+                   id="9"
+                   version="0"
+                   semanticVersion="0"
+                   description="Single char fields rendered by the generated String methods"
+                   byteOrder="littleEndian">
+    <types>
+        <composite name="messageHeader" description="Message identifiers and length of message root">
+            <type name="blockLength" primitiveType="uint16"/>
+            <type name="templateId" primitiveType="uint16"/>
+            <type name="schemaId" primitiveType="uint16"/>
+            <type name="version" primitiveType="uint16"/>
+        </composite>
+        <composite name="groupSizeEncoding" description="Repeating group dimensions">
+            <type name="blockLength" primitiveType="uint16"/>
+            <type name="numInGroup" primitiveType="uint16"/>
+        </composite>
+    </types>
+    <sbe:message name="Letter" id="1">
+        <field name="code" id="1" type="char"/>
+        <group name="entries" id="2" dimensionType="groupSizeEncoding">
+            <field name="side" id="3" type="char"/>
+        </group>
+    </sbe:message>
+</sbe:messageSchema>