refactor(log): migrate from log to log/slog for structured logging

Replace all usages of the standard log package with log/slog across
backend and swift worker. Configure Echo's request logger to emit
structured log attributes via slog instead of the default format.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

5 files changed, 42 insertions, 15 deletions

diff --git a/backend/admin/handler.go b/backend/admin/handler.go
index a18e32a..ef19f21 100644
--- a/backend/admin/handler.go
+++ b/backend/admin/handler.go
@@ -3,7 +3,7 @@ package admin
 import (
 	"context"
 	"errors"
-	"log"
+	"log/slog"
 	"net/http"
 	"strconv"
 	"time"
@@ -268,7 +268,7 @@ func (h *Handler) postUserFetchIcon(c echo.Context) error {
 	go func() {
 		err := account.FetchIcon(context.Background(), h.q, int(row.UserID))
 		if err != nil {
-			log.Printf("%v", err)
+			slog.Error("failed to fetch icon", "error", err)
 			// The failure is intentionally ignored. Retry manually if needed.
 		}
 	}()
diff --git a/backend/api/handler.go b/backend/api/handler.go
index 9f8849c..05a185a 100644
--- a/backend/api/handler.go
+++ b/backend/api/handler.go
@@ -4,7 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
-	"log"
+	"log/slog"
 	"net/http"
 	"strconv"
 	"time"
@@ -47,7 +47,7 @@ func (h *Handler) PostLogin(ctx context.Context, request PostLoginRequestObject)
 	password := request.Body.Password
 	userID, err := auth.Login(ctx, h.q, username, password)
 	if err != nil {
-		log.Printf("login failed: %v", err)
+		slog.Error("login failed", "error", err)
 		var msg string
 		if errors.Is(err, auth.ErrForteeLoginTimeout) {
 			msg = "ログインに失敗しました"
diff --git a/backend/auth/auth.go b/backend/auth/auth.go
index d5264ec..babbe68 100644
--- a/backend/auth/auth.go
+++ b/backend/auth/auth.go
@@ -3,7 +3,7 @@ package auth
 import (
 	"context"
 	"errors"
-	"log"
+	"log/slog"
 	"time"
 
 	"github.com/jackc/pgx/v5"
@@ -93,7 +93,7 @@ func signup(
 	go func() {
 		err := account.FetchIcon(context.Background(), queries, int(userID))
 		if err != nil {
-			log.Printf("%v", err)
+			slog.Error("failed to fetch icon", "error", err)
 			// The failure is intentionally ignored. Retry manually if needed.
 		}
 	}()
diff --git a/backend/game/hub.go b/backend/game/hub.go
index 910911a..9ae725a 100644
--- a/backend/game/hub.go
+++ b/backend/game/hub.go
@@ -2,7 +2,8 @@ package game
 
 import (
 	"context"
-	"log"
+	"log/slog"
+	"os"
 	"regexp"
 	"strings"
 
@@ -29,7 +30,8 @@ func NewGameHub(q *db.Queries, taskQueue *taskqueue.Queue, taskWorker *taskqueue
 func (hub *Hub) Run() {
 	go func() {
 		if err := hub.taskWorker.Run(); err != nil {
-			log.Fatal(err)
+			slog.Error("task worker failed", "error", err)
+			os.Exit(1)
 		}
 	}()
 
diff --git a/backend/main.go b/backend/main.go
index c1b670a..ac996f0 100644
--- a/backend/main.go
+++ b/backend/main.go
@@ -3,8 +3,9 @@ package main
 import (
 	"context"
 	"fmt"
-	"log"
+	"log/slog"
 	"net/http"
+	"os"
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -39,12 +40,14 @@ func main() {
 	var err error
 	conf, err := config.NewConfigFromEnv()
 	if err != nil {
-		log.Fatalf("Error loading env %v", err)
+		slog.Error("failed to load env", "error", err)
+		os.Exit(1)
 	}
 
 	openAPISpec, err := api.GetSwaggerWithPrefix(conf.BasePath + "api")
 	if err != nil {
-		log.Fatalf("Error loading OpenAPI spec\n: %s", err)
+		slog.Error("failed to load OpenAPI spec", "error", err)
+		os.Exit(1)
 	}
 
 	ctx := context.Background()
@@ -52,7 +55,8 @@ func main() {
 	dbDSN := fmt.Sprintf("host=%s port=%s user=%s password=%s dbname=%s sslmode=disable", conf.DBHost, conf.DBPort, conf.DBUser, conf.DBPassword, conf.DBName)
 	connPool, err := connectDB(ctx, dbDSN)
 	if err != nil {
-		log.Fatalf("Error connecting to db %v", err)
+		slog.Error("failed to connect to db", "error", err)
+		os.Exit(1)
 	}
 	defer connPool.Close()
 
@@ -61,7 +65,27 @@ func main() {
 	e := echo.New()
 	e.Renderer = admin.NewRenderer()
 
-	e.Use(middleware.RequestLogger())
+	e.Use(middleware.RequestLoggerWithConfig(middleware.RequestLoggerConfig{
+		LogStatus:   true,
+		LogURI:      true,
+		LogMethod:   true,
+		LogLatency:  true,
+		LogError:    true,
+		HandleError: true,
+		LogValuesFunc: func(_ echo.Context, v middleware.RequestLoggerValues) error {
+			attrs := []slog.Attr{
+				slog.String("method", v.Method),
+				slog.String("uri", v.URI),
+				slog.Int("status", v.Status),
+				slog.Duration("latency", v.Latency),
+			}
+			if v.Error != nil {
+				attrs = append(attrs, slog.String("error", v.Error.Error()))
+			}
+			slog.LogAttrs(context.Background(), slog.LevelInfo, "request", attrs...)
+			return nil
+		},
+	}))
 	e.Use(middleware.Recover())
 
 	taskQueue := taskqueue.NewQueue("task-db:6379")
@@ -116,7 +140,7 @@ func main() {
 				return
 			case <-ticker.C:
 				if err := queries.DeleteExpiredSessions(sessionCleanupCtx); err != nil {
-					log.Printf("failed to delete expired sessions: %v", err)
+					slog.Error("failed to delete expired sessions", "error", err)
 				}
 			}
 		}
@@ -125,6 +149,7 @@ func main() {
 	go gameHub.Run()
 
 	if err := e.Start(":80"); err != http.ErrServerClosed {
-		log.Fatal(err)
+		slog.Error("failed to start server", "error", err)
+		os.Exit(1)
 	}
 }