Copy fields map if it exists in order to not affect parent context

Closes PumpkinSeed#5

context.go

@@ -16,10 +16,20 @@ func WithValue(parent context.Context, key string, val any) context.Context {
 		panic("cannot create context from nil parent")
 	}
 	if v, ok := parent.Value(fields).(*sync.Map); ok {
-		v.Store(key, val)
-		return context.WithValue(parent, fields, v)
+		mapCopy := copySyncMap(v)
+		mapCopy.Store(key, val)
+		return context.WithValue(parent, fields, mapCopy)
 	}
 	v := &sync.Map{}
 	v.Store(key, val)
 	return context.WithValue(parent, fields, v)
 }
+
+func copySyncMap(m *sync.Map) *sync.Map {
+	var cp sync.Map
+	m.Range(func(k, v interface{}) bool {
+		cp.Store(k, v)
+		return true
+	})
+	return &cp
+}

handler_test.go

@@ -2,9 +2,11 @@ package slogcontext
 
 import (
 	"context"
+	"encoding/json"
 	"io"
 	"log/slog"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -25,6 +27,22 @@ func TestHandler(t *testing.T) {
 	logger.ErrorContext(ctx, "this is an error")
 }
 
+func TestHandler_WithValue_ShouldNotAffectParent(t *testing.T) {
+	stringBuilder := strings.Builder{}
+	handler := NewHandler(slog.NewJSONHandler(&stringBuilder, nil))
+	logger := slog.New(handler)
+	ctx := WithValue(context.Background(), "k1", "v1")
+	WithValue(ctx, "k2", "v2")
+
+	logger.InfoContext(ctx, "this is an log")
+
+	logMessage := jsonToMap(t, stringBuilder.String())
+	_, ok := logMessage["k2"]
+	if ok {
+		t.Errorf("Log message shouldn't contain key added to different context")
+	}
+}
+
 func TestHandlerConcurrent(t *testing.T) {
 	ctx := WithValue(context.Background(), "number", 12)
 	ctx = WithValue(ctx, "string", "data")
@@ -60,3 +78,12 @@ func BenchmarkHandler(b *testing.B) {
 		logger.ErrorContext(ctx, "this is an error")
 	}
 }
+
+func jsonToMap(t *testing.T, jsonStr string) map[string]any {
+	result := make(map[string]any)
+	err := json.Unmarshal([]byte(jsonStr), &result)
+	if err != nil {
+		t.Errorf("Failed serializing log message into a map: %v", err)
+	}
+	return result
+}